An Etymological Dictionary of Astronomy and Astrophysics

Any of the → spectral lines arising from → singly ionized helium in the atmosphere of → O-type and → Wolf-Rayet stars. He II lines are chiefly in absorption, but some of them, such as 4686 Å occur in emission in hotter stars. The presence of He II → absorption lines separates O types from → B-type stars.

A number of these He II lines belong to the → Pickering series involving transitions with → principal quantum numbern = 4 and higher.

Although the n = 3-4 (4686 Å) transition also belongs to ionized helium and often occurs in these hot stars, it does not belong to the Pickering series because it has a lower landing level quantum number (n = 3).
The same goes for n = 2-5 (4026 Å).

See also: → helium; → line.

Any of the → spectral lines arising from → singly ionized helium in the atmosphere of → O-type and → Wolf-Rayet stars. He II lines are chiefly in absorption, but some of them, such as 4686 Å occur in emission in hotter stars. The presence of He II → absorption lines separates O types from → B-type stars.

A number of these He II lines belong to the → Pickering series involving transitions with → principal quantum numbern = 4 and higher.

Although the n = 3-4 (4686 Å) transition also belongs to ionized helium and often occurs in these hot stars, it does not belong to the Pickering series because it has a lower landing level quantum number (n = 3).
The same goes for n = 2-5 (4026 Å).

See also: → helium; → line.

An early → B-type star showing helium lines with abnormally large equivalent widths. The surface → chemical abundances of He-strong stars are influenced by the presence of a strong → magnetic field, resulting in a He overabundance that typically varies in strength over the stellar surface. Examples include HR 735, HD 184927, and CPD-62°2124.

See also: → helium; → strong; → line.

An early → B-type star showing helium lines with abnormally large equivalent widths. The surface → chemical abundances of He-strong stars are influenced by the presence of a strong → magnetic field, resulting in a He overabundance that typically varies in strength over the stellar surface. Examples include HR 735, HD 184927, and CPD-62°2124.

See also: → helium; → strong; → line.

A → chemically peculiar star with very weak helium lines. Examples include 3 Sco, HD 176582, HD 217833, HR 2949, and HD 21699.

The He-weak stars do not form a homogeneous group. Some of them display intense Si, or Ti and Sr lines, and are considered a hot extension of the magnetic → Ap/Bp stars. Others show overabundances of P and Ga, typically noted for → HgMn stars.

The star HD 139160 belongs to the non-magnetic subgroup of He-weak stars.

See also: → helium; → weak; → line.

A → chemically peculiar star with very weak helium lines. Examples include 3 Sco, HD 176582, HD 217833, HR 2949, and HD 21699.

The He-weak stars do not form a homogeneous group. Some of them display intense Si, or Ti and Sr lines, and are considered a hot extension of the magnetic → Ap/Bp stars. Others show overabundances of P and Ga, typically noted for → HgMn stars.

The star HD 139160 belongs to the non-magnetic subgroup of He-weak stars.

See also: → helium; → weak; → line.

1. That part of anything that forms or is situated at the top, summit, or upper end.
   

2. The → nucleus and surrounding → coma of a → comet.

Etymology (EN): Head, from O.E. heafod “top of the body,” also “chief person” (cf. O.S. hobid; Goth. haubiþ Ger. Haupt “head”), from PIE *kauput- “head;” cf. Skt. kaput-, kapala- “skull;” L. caput “head;” Pers. dialect Lori: kapu “head,” kapulek “skull, middle of the head;” Kurd. kapol “skull;” Pashto kaparay “skull.”

Etymology (PE): Sar “head,” soru, sorun “horn”
(karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument”);
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit.”

1. That part of anything that forms or is situated at the top, summit, or upper end.
   

2. The → nucleus and surrounding → coma of a → comet.

Etymology (EN): Head, from O.E. heafod “top of the body,” also “chief person” (cf. O.S. hobid; Goth. haubiþ Ger. Haupt “head”), from PIE *kauput- “head;” cf. Skt. kaput-, kapala- “skull;” L. caput “head;” Pers. dialect Lori: kapu “head,” kapulek “skull, middle of the head;” Kurd. kapol “skull;” Pashto kaparay “skull.”

Etymology (PE): Sar “head,” soru, sorun “horn”
(karnâ “a trumpet-like wind instrument,” variant sornâ “a wind instrument”);
Mid.Pers. sar “head,” sru “horn;” Av. sarah- “head,” srū- “horn, nail;” cf. Skt. śiras- “head, chief;” Gk. kara “head,” karena “head, top,” keras “horn;”
L. cornu “horn,” cerebrum “brain;”
P.Gmc. *khurnaz (Ger. Horn, Du. horen; cognate with E. horn, as above, from PIE *ker- “head, horn;”
O.E. horn “horn of an animal,” also “wind instrument;”
E. horn); PIE base *ker- “head, horn, top, summit.”

A member of the class of radio galaxies (→ radio galaxy) that have a strong radio emission coming from a bright “head” and a more diffuse emission from a “tail.” They are often found in clusters.

See also: → head; → tail; → galaxy.

A member of the class of radio galaxies (→ radio galaxy) that have a strong radio emission coming from a bright “head” and a more diffuse emission from a “tail.” They are often found in clusters.

See also: → head; → tail; → galaxy.

1. A hollow muscular organ that pumps the blood through the circulatory system by rhythmic contraction and dilation. In vertebrates there may be up to four chambers (as in humans), with two atria and two ventricles.
   

   2. The heart regarded as the centre of a person’s thoughts and emotions, especially love or compassion (OxfordDictionaries.com).

Etymology (EN): M.E. herte, from O.E. heorte “heart breast, soul, spirit, will, desire; courage; mind, intellect;” cf. O.Saxon herta, O.Frisian herte, O.Norse hjarta, Du. hart, O.H.G. herza, Ger. Herz; cognate with Pers. del, as below; PIE root *kerd- “heart.”

Etymology (PE): Del “heart” (Pashtu z’rrah, zyah; Baluci zirde “heart, mind, soul;” Kurd. zar; Sogd. žyâwar); Mid.Pers. dil; Av. zərəd-; cf. Skt. hrd-; Gk. kardia; L. cor “heart” (Fr. cœur; Sp. corazon, It. cuore); Russ. serdtse; Arm. sirt; E. heart, as above.

1. A hollow muscular organ that pumps the blood through the circulatory system by rhythmic contraction and dilation. In vertebrates there may be up to four chambers (as in humans), with two atria and two ventricles.
   

   2. The heart regarded as the centre of a person’s thoughts and emotions, especially love or compassion (OxfordDictionaries.com).

Etymology (EN): M.E. herte, from O.E. heorte “heart breast, soul, spirit, will, desire; courage; mind, intellect;” cf. O.Saxon herta, O.Frisian herte, O.Norse hjarta, Du. hart, O.H.G. herza, Ger. Herz; cognate with Pers. del, as below; PIE root *kerd- “heart.”

Etymology (PE): Del “heart” (Pashtu z’rrah, zyah; Baluci zirde “heart, mind, soul;” Kurd. zar; Sogd. žyâwar); Mid.Pers. dil; Av. zərəd-; cf. Skt. hrd-; Gk. kardia; L. cor “heart” (Fr. cœur; Sp. corazon, It. cuore); Russ. serdtse; Arm. sirt; E. heart, as above.

Energy possessed by a substance in the form of kinetic energy of atomic or molecular translation, rotation, or vibration.

Etymology (EN): Heat, from O.E. hætu, hæto, from P.Gmc. *khaitin- “heat,” from *khaitaz “hot” (cf. O.N. hiti, Ger. hitze “heat,” Goth. heito “fever”).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” Gk. thermos “warm;” L. formus “warm,” fornax “oven;” P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- “warm.”

Energy possessed by a substance in the form of kinetic energy of atomic or molecular translation, rotation, or vibration.

Etymology (EN): Heat, from O.E. hætu, hæto, from P.Gmc. *khaitin- “heat,” from *khaitaz “hot” (cf. O.N. hiti, Ger. hitze “heat,” Goth. heito “fever”).

Etymology (PE): Garmâ “heat, warmth,” from Mid.Pers. garmâg; O.Pers./Av. garəma- “hot, warm;” cf. Skt. gharmah “heat;” Gk. thermos “warm;” L. formus “warm,” fornax “oven;” P.Gmc. *warmaz; O.E. wearm; E. warm; O.H.G., Ger. warm; PIE *ghworm-/*ghwerm- “warm.”

The ratio of an amount of heat, dQ, transferred to a body in some process to the corresponding change in the temperature of the body: C = dQ/dT. The heat capacity depends upon the mass of the body, its chemical composition, thermodynamic state, and the kind of process employed to transfer the heat. The word “capacity” may be misleading because it suggests the essentially meaningless statement “the amount of heat a body can hold,” whereas what is meant is the heat added per unit temperature rise. → specific heat.

See also: → heat; → capacity.

The ratio of an amount of heat, dQ, transferred to a body in some process to the corresponding change in the temperature of the body: C = dQ/dT. The heat capacity depends upon the mass of the body, its chemical composition, thermodynamic state, and the kind of process employed to transfer the heat. The word “capacity” may be misleading because it suggests the essentially meaningless statement “the amount of heat a body can hold,” whereas what is meant is the heat added per unit temperature rise. → specific heat.

See also: → heat; → capacity.

A type of → heat transfer by means of molecular agitation within a material without any motion of the material as a whole.

See also: → heat; → conduction.

A type of → heat transfer by means of molecular agitation within a material without any motion of the material as a whole.

See also: → heat; → conduction.

A type of → heat transfer involving mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.

See also: → heat; → convection.

A type of → heat transfer involving mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it.

See also: → heat; → convection.

Assuming that the Universe is a thermodynamically → isolated system, a state of absolute uniformity in the Universe in which all temperature differences would reduce to zero and no energy will be available for use, according to the → second law of thermodynamics. In that condition of maximum → entropy, the Universe would be in a state of unchanging death.
First introduced by the German physicist Hermann von Helmholtz (1821-1894) in 1854, on the basis of William Thomson’s (1824-1907) idea.

See also: → heat; → death; → Universe.

Assuming that the Universe is a thermodynamically → isolated system, a state of absolute uniformity in the Universe in which all temperature differences would reduce to zero and no energy will be available for use, according to the → second law of thermodynamics. In that condition of maximum → entropy, the Universe would be in a state of unchanging death.
First introduced by the German physicist Hermann von Helmholtz (1821-1894) in 1854, on the basis of William Thomson’s (1824-1907) idea.

See also: → heat; → death; → Universe.

The amount of heat energy required to transform an amount of a substance from the liquid phase to the gas phase. → molar heat of vaporization.

See also: → heat; → vaporization.

The amount of heat energy required to transform an amount of a substance from the liquid phase to the gas phase. → molar heat of vaporization.

See also: → heat; → vaporization.

A structure that protects against excessive heat, especially that which
covers the vulnerable surfaces of a → spacecraft and protects it when re-entering the Earth’s atmosphere.

See also: → heat; → shield.

A structure that protects against excessive heat, especially that which
covers the vulnerable surfaces of a → spacecraft and protects it when re-entering the Earth’s atmosphere.

See also: → heat; → shield.

The spontaneous transportation of heat through matter, from a region of higher temperature to a region of lower temperature.

See also: → heat; → transfer.

The spontaneous transportation of heat through matter, from a region of higher temperature to a region of lower temperature.

See also: → heat; → transfer.

Meteorology: A period of several successive days of abnormally hot and usually humid weather occurring in summer.

Etymology (EN): → heat; → wave.

Etymology (PE): Celle-ye tâbestân literally “the fortieth of summer,” i.e. “midsummer,” from cellé pertaining to “forty (days),” from cel, cehel, → forty, + tâbestân, → summer.

Meteorology: A period of several successive days of abnormally hot and usually humid weather occurring in summer.

Etymology (EN): → heat; → wave.

Etymology (PE): Celle-ye tâbestân literally “the fortieth of summer,” i.e. “midsummer,” from cellé pertaining to “forty (days),” from cel, cehel, → forty, + tâbestân, → summer.

1. The process whereby a system’s temperature increases. → warming.
   

2. A device or system for supplying heat, especially central heating, to a building; the heat supplied.

See also: → heat; → -ing.

1. The process whereby a system’s temperature increases. → warming.
   

2. A device or system for supplying heat, especially central heating, to a building; the heat supplied.

See also: → heat; → -ing.

The sky or Universe as seen from the Earth; the firmament. Often used in the plural.

Etymology (EN): From M.E. heven, O.E. heofon, possibly from P.Gmc. *khemina- (cf. M.L.G. heben, O.N. himinn, Goth. himins, Du. hemel, Ger. Himmel “heaven, sky”); PIE base *kem-/*kam- “to cover.”

Etymology (PE): Âsmân, from Mid.Pers. âsmân “sky, heaven;” O.Pers. asman- “heaven;” Av. asman- “stone, sling-stone; heaven;” cf. Skt. áśman- “stone, rock, thunderbolt;” Gk. akmon “heaven, meteor, anvil;” Akmon was the father of Ouranos (Uranus), god of sky; Lith. akmuo “stone;” Rus. kamen; PIE base *akmon- “stone, sky.”
The link between the “stone” and “sky” concepts indicates that the sky had once been conceived as a stone vault by prehistoric Indo-Europeans.

The sky or Universe as seen from the Earth; the firmament. Often used in the plural.

Etymology (EN): From M.E. heven, O.E. heofon, possibly from P.Gmc. *khemina- (cf. M.L.G. heben, O.N. himinn, Goth. himins, Du. hemel, Ger. Himmel “heaven, sky”); PIE base *kem-/*kam- “to cover.”

Etymology (PE): Âsmân, from Mid.Pers. âsmân “sky, heaven;” O.Pers. asman- “heaven;” Av. asman- “stone, sling-stone; heaven;” cf. Skt. áśman- “stone, rock, thunderbolt;” Gk. akmon “heaven, meteor, anvil;” Akmon was the father of Ouranos (Uranus), god of sky; Lith. akmuo “stone;” Rus. kamen; PIE base *akmon- “stone, sky.”
The link between the “stone” and “sky” concepts indicates that the sky had once been conceived as a stone vault by prehistoric Indo-Europeans.

→ astronomical object.

See also: → heaven; → -ly; → body.

→ astronomical object.

See also: → heaven; → -ly; → body.

→ Kennelly-Heaviside layer.

See also: English physicist Oliver Heaviside (1850-1925).

→ Kennelly-Heaviside layer.

See also: English physicist Oliver Heaviside (1850-1925).

Of great weight; of great amount, quantity.

Etymology (EN): M.E. hevi; O.E. hefig, from P.Gmc. *khabigas (cf. O.N. hebig).

Etymology (PE): Sangin “heavy, weighty; stony, like stone, hard,” from sang “stone, rock” (Mid.Pers. sang; O.Pers. aθanga-; Av. asenga- “stone” (related to Mod.Pers. âsmân “sky” → heaven); PIE *aken-) + -in adj. suffix.

Of great weight; of great amount, quantity.

Etymology (EN): M.E. hevi; O.E. hefig, from P.Gmc. *khabigas (cf. O.N. hebig).

Etymology (PE): Sangin “heavy, weighty; stony, like stone, hard,” from sang “stone, rock” (Mid.Pers. sang; O.Pers. aθanga-; Av. asenga- “stone” (related to Mod.Pers. âsmân “sky” → heaven); PIE *aken-) + -in adj. suffix.

In astrophysics, any → chemical element heavier than → helium. Such elements are also inappropriately referred to as “→ metals.”

See also: → heavy; → element.

In astrophysics, any → chemical element heavier than → helium. Such elements are also inappropriately referred to as “→ metals.”

See also: → heavy; → element.

→ deuterium.

See also: → heavy; → hydrogen.

→ deuterium.

See also: → heavy; → hydrogen.

Water in which the hydrogen is replaced by → deuterium. Deuterium Oxide (D₂O).

See also: → heavy; → water.

Water in which the hydrogen is replaced by → deuterium. Deuterium Oxide (D₂O).

See also: → heavy; → water.

A → lunisolar calendar used by Jews for religious purposes. The year consists of 12 months alternating between 29 and 30 days, making a year of 354 days.
In order to conform to the → solar year, a → leap month is included every third year. A month
begins the day the new moon is first seen. The years are counted from the time of “creation,” believed by Jewish theologians to have occurred in the year 3761 B.C. Also called → Jewish calendar.

Etymology (EN): Hebrew, from O.E., from O.Fr. Ebreu, from L. Hebraeus, from Gk. Hebraios, from Aramaic ‘ebhrai, corresponding to Heb. ‘ibhri “an Israelite,” literally “one from the other side,” in reference to the River Euphrates, or perhaps simply denoting “immigrant;” from ‘ebher “region on the other or opposite side;” → calendar.

Etymology (PE): Gâhšomâr, → calendar; yahud→ Jewish calendar.

A → lunisolar calendar used by Jews for religious purposes. The year consists of 12 months alternating between 29 and 30 days, making a year of 354 days.
In order to conform to the → solar year, a → leap month is included every third year. A month
begins the day the new moon is first seen. The years are counted from the time of “creation,” believed by Jewish theologians to have occurred in the year 3761 B.C. Also called → Jewish calendar.

Etymology (EN): Hebrew, from O.E., from O.Fr. Ebreu, from L. Hebraeus, from Gk. Hebraios, from Aramaic ‘ebhrai, corresponding to Heb. ‘ibhri “an Israelite,” literally “one from the other side,” in reference to the River Euphrates, or perhaps simply denoting “immigrant;” from ‘ebher “region on the other or opposite side;” → calendar.

Etymology (PE): Gâhšomâr, → calendar; yahud→ Jewish calendar.

A prefix meaning hundred (10²) used in the formation of compound words.

Etymology (EN): From Fr., from Gk. hekaton “hundred.”

Etymology (PE): Hekto-, loanword from Fr., as above.

A prefix meaning hundred (10²) used in the formation of compound words.

Etymology (EN): From Fr., from Gk. hekaton “hundred.”

Etymology (PE): Hekto-, loanword from Fr., as above.

Distance upward from a given level to a fixed point.

Etymology (EN): M.E., from O.E. hiehthu; → high + -th a suffix forming nouns of action (e.g., birth) or abstract nouns denoting quality or condition (depth; length; warmth).

Etymology (PE): Bolandi, bolandâ “height,” noun forms from boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”
Farâzâ, noun of farâz “above, up, upon, on the top, aloft,” from Mid.Pers. farâz, farâc “forward, prominent, distinguished;” Av. frānk- (adj.) “turned toward the front,” fraca (adv.) “forward, forth,” fraš (adv.) “forward, forth; before;” Proto-Iranian *frānk-.

Distance upward from a given level to a fixed point.

Etymology (EN): M.E., from O.E. hiehthu; → high + -th a suffix forming nouns of action (e.g., birth) or abstract nouns denoting quality or condition (depth; length; warmth).

Etymology (PE): Bolandi, bolandâ “height,” noun forms from boland “high,” variants bâlâ “up, above, high, elevated, height,” borz “height, magnitude” (it occurs also in the name of the mountain chain Alborz),
Lori dialect berg “hill, mountain;” Mid.Pers. buland “high;” O.Pers. baršan- “height;” Av. barəz- “high, mount,” barezan- “height;” cf. Skt. bhrant- “high;” L. fortis “strong” (Fr. & E. force); O.E. burg, burh “castle, fortified place,” from P.Gmc. *burgs “fortress;” Ger. Burg “castle,” Goth. baurgs “city,” E. burg, borough, Fr. bourgeois, bourgeoisie, faubourg); PIE base *bhergh- “high.”
Farâzâ, noun of farâz “above, up, upon, on the top, aloft,” from Mid.Pers. farâz, farâc “forward, prominent, distinguished;” Av. frānk- (adj.) “turned toward the front,” fraca (adv.) “forward, forth,” fraš (adv.) “forward, forth; before;” Proto-Iranian *frānk-.

A diffuse bright region surrounding the shadow that an observer’s head casts on an irregular surface. It can be best observed on dewy reeds or grass. This phenomenon is reminiscent of the → glory, but without its color and regular structure.

Etymology (EN): Heiligenschein, Ger., literally “saint’s shining light,” from heiligen, from heilig “holy, sacred” (P.Gmc. *khailagas; M.H.G. heilec; O.H.G. heilag; Goth. hailag; O.N. heilagr; O.E. halig; E. holy) + Schein “glow, shine” (M.H.G. schinen, O.H.G. skinan,
P.Gmc. *skinanan; E. shine; cf. Mod.Pers. sâyé “shadow;” Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;” PIE base *skai- “bright”).

Etymology (PE): Sepant foruq, from sepant “holy” (Mid.Pers. spand “holy,” Spandarmat “Holy Thought; 5-th day of the month; 12-th month of the year;”
from Av. spənta- “holy; beneficent,” spəntô.mainyav- “coming from or belonging to the holy spirit,” spəntô.təma- “holiest”)

• foruq “light, brightness” (related to rôšan “light; bright, luminous;” ruz “day,” afruxtan “to light, kindle;” Mid.Pers. payrog “light, brightness,” rošn light; bright," rôc “day;” O.Pers. raucah-; Av. raocana- “bright, shining, radiant,” raocah- “light, luminous; daylight;”
  cf. Skt. rocaná- “bright, shining, roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light” (also lumen, luna; E. light, Ger. Licht, and Fr. lumière;
  PIE base *leuk- “light, brightness”).

A diffuse bright region surrounding the shadow that an observer’s head casts on an irregular surface. It can be best observed on dewy reeds or grass. This phenomenon is reminiscent of the → glory, but without its color and regular structure.

Etymology (EN): Heiligenschein, Ger., literally “saint’s shining light,” from heiligen, from heilig “holy, sacred” (P.Gmc. *khailagas; M.H.G. heilec; O.H.G. heilag; Goth. hailag; O.N. heilagr; O.E. halig; E. holy) + Schein “glow, shine” (M.H.G. schinen, O.H.G. skinan,
P.Gmc. *skinanan; E. shine; cf. Mod.Pers. sâyé “shadow;” Mid.Pers. sâyak “shadow;” Av. a-saya- “throwing no shadow;” Skt. chāya- “shadow;” Gk. skia “shade;” Rus. sijat’ “to shine;” PIE base *skai- “bright”).

Etymology (PE): Sepant foruq, from sepant “holy” (Mid.Pers. spand “holy,” Spandarmat “Holy Thought; 5-th day of the month; 12-th month of the year;”
from Av. spənta- “holy; beneficent,” spəntô.mainyav- “coming from or belonging to the holy spirit,” spəntô.təma- “holiest”)

• foruq “light, brightness” (related to rôšan “light; bright, luminous;” ruz “day,” afruxtan “to light, kindle;” Mid.Pers. payrog “light, brightness,” rošn light; bright," rôc “day;” O.Pers. raucah-; Av. raocana- “bright, shining, radiant,” raocah- “light, luminous; daylight;”
  cf. Skt. rocaná- “bright, shining, roka- “brightness, light;” Gk. leukos “white, clear;” L. lux “light” (also lumen, luna; E. light, Ger. Licht, and Fr. lumière;
  PIE base *leuk- “light, brightness”).

A person who inherits or has a right of inheritance in the property of another following the latter’s death (Dictionary.com).

See also: → heritage.

A person who inherits or has a right of inheritance in the property of another following the latter’s death (Dictionary.com).

See also: → heritage.

The uncertainty in the measurement of the position and momentum of an elementary particle. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space.

See also: Named after Werner Heisenberg (1901-1976), the German physicist who in 1927 derived the uncertainty principle. In 1932 he was awarded the Nobel Prize in Physics; uncertainty, from → un- “not” + → certainty; → principle.

The uncertainty in the measurement of the position and momentum of an elementary particle. The more precisely one quantity is known, the less certain the precision of the other. A similarly linked pair of quantities is the time and energy content in a volume of space.

See also: Named after Werner Heisenberg (1901-1976), the German physicist who in 1927 derived the uncertainty principle. In 1932 he was awarded the Nobel Prize in Physics; uncertainty, from → un- “not” + → certainty; → principle.

Of or near the → Sun, especially rising and setting with the Sun. → heliacal rising, → heliacal setting.

Etymology (EN): Heliacal “pertaining to the sun,” from Gk. heliakos “of the sun,” from helios, → sun; cognate with Pers. hur, as below.

Etymology (PE): Hurâné “sunlike,” since the star rises in the morning like the Sun, from hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun” + -âné similarity suffix.

Of or near the → Sun, especially rising and setting with the Sun. → heliacal rising, → heliacal setting.

Etymology (EN): Heliacal “pertaining to the sun,” from Gk. heliakos “of the sun,” from helios, → sun; cognate with Pers. hur, as below.

Etymology (PE): Hurâné “sunlike,” since the star rises in the morning like the Sun, from hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun” + -âné similarity suffix.

The first appearance of a star following a period of invisibility due to its conjunction with the Sun. → heliacal rising of Sirius.

See also: → heliacal; → rising.

The first appearance of a star following a period of invisibility due to its conjunction with the Sun. → heliacal rising of Sirius.

See also: → heliacal; → rising.

The first rising of → Sirius at dawn shortly before → sunrise. The heliacal rising of Sirius played a significant role in ancient Egypt by heralding the annual flooding of the Nile. The event took place some 70 days after the star had been seen for the last time in the western horizon at sunset.

The heliacal rising of Sirius and its association with the rebirth of the Nile was so important that it marked the start of the Egyptian calendar year. At the time, the heliacal rising occurred in early July, as seen from the ancient capital of Memphis. But due to the → precession of the equinoxes the star now reappears in early August in Egypt. The date depends on the latitude (assuming transparent skies), being later for higher latitudes. For latitude 48° it occurs on about August 19.

See also: → heliacal; → rising; → Sirius.

The first rising of → Sirius at dawn shortly before → sunrise. The heliacal rising of Sirius played a significant role in ancient Egypt by heralding the annual flooding of the Nile. The event took place some 70 days after the star had been seen for the last time in the western horizon at sunset.

The heliacal rising of Sirius and its association with the rebirth of the Nile was so important that it marked the start of the Egyptian calendar year. At the time, the heliacal rising occurred in early July, as seen from the ancient capital of Memphis. But due to the → precession of the equinoxes the star now reappears in early August in Egypt. The date depends on the latitude (assuming transparent skies), being later for higher latitudes. For latitude 48° it occurs on about August 19.

See also: → heliacal; → rising; → Sirius.

The last visible setting of a star below the western horizon just after sunset entering into a conjunction with the Sun.

See also: → heliacal; → setting.

The last visible setting of a star below the western horizon just after sunset entering into a conjunction with the Sun.

See also: → heliacal; → setting.

1. In particle physics, the projection of the spin of an elementary particle on the direction of momentum.
   
2. In fluid mechanics, → kinetic helicity.
   
3. In magnetohydrodynamics, → magnetic helicity.

See also: From → helix + → -ity.

1. In particle physics, the projection of the spin of an elementary particle on the direction of momentum.
   
2. In fluid mechanics, → kinetic helicity.
   
3. In magnetohydrodynamics, → magnetic helicity.

See also: From → helix + → -ity.

A combining form of Gk. helios “sun.”

See also: Helio-, combining form of from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor and xoršid (Av. hvarə-xšaēta- “shining sun”);
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”

A combining form of Gk. helios “sun.”

See also: Helio-, combining form of from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor and xoršid (Av. hvarə-xšaēta- “shining sun”);
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”

Having or representing the Sun as a center. → heliocentric cosmology, → heliocentric gravitational constant, → heliocentric Julian Day,
→ heliocentric parallax,
→ heliocentric system.

See also: → helio- + → center +
→ -ic.

Having or representing the Sun as a center. → heliocentric cosmology, → heliocentric gravitational constant, → heliocentric Julian Day,
→ heliocentric parallax,
→ heliocentric system.

See also: → helio- + → center +
→ -ic.

A model of the Universe in which the Sun was centrally located.

See also: → heliocentric; → cosmology.

A model of the Universe in which the Sun was centrally located.

See also: → heliocentric; → cosmology.

A parameter representing the product of the → gravitational constant by the → solar mass. It is 13.27 x 10¹⁹ m³ s^-2.

See also: → heliocentric; → gravitational; → constant.

A parameter representing the product of the → gravitational constant by the → solar mass. It is 13.27 x 10¹⁹ m³ s^-2.

See also: → heliocentric; → gravitational; → constant.

The → Julian Date referenced to the center of the → Sun. Since the Earth revolves around the Sun, and since light travels at a finite speed, observations of a given object taken at different positions in the Earth’s orbit are not equivalent, and so a correction for Earth’s orbit around the Sun is required. Left uncorrected, the time of an observational event measured by Earth clocks will vary by 16.6 minute over the course of a year. If not properly accounted for, this can lead to a spurious signal in a → periodogram.

See also: → heliocentric; → Julian Date.

The → Julian Date referenced to the center of the → Sun. Since the Earth revolves around the Sun, and since light travels at a finite speed, observations of a given object taken at different positions in the Earth’s orbit are not equivalent, and so a correction for Earth’s orbit around the Sun is required. Left uncorrected, the time of an observational event measured by Earth clocks will vary by 16.6 minute over the course of a year. If not properly accounted for, this can lead to a spurious signal in a → periodogram.

See also: → heliocentric; → Julian Date.

The parallax of a celestial body when viewed from two points in the Earth’s orbit around the Sun. More specifically, the angular difference in a celestial object’s position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax.

See also: → heliocentric; → parallax.

The parallax of a celestial body when viewed from two points in the Earth’s orbit around the Sun. More specifically, the angular difference in a celestial object’s position as seen from the center of the Sun and the center of the Earth. Also called → annual parallax.

See also: → heliocentric; → parallax.

A system in which the Sun is assumed to lie at its central point while the Earth and other bodies revolve around it.

See also: → heliocentric; → system.

A system in which the Sun is assumed to lie at its central point while the Earth and other bodies revolve around it.

See also: → heliocentric; → system.

A record made by a → heliograph.

See also: → helio-; + → -gram.

A record made by a → heliograph.

See also: → helio-; + → -gram.

1. An instrument for photographing the Sun, consisting of a camera and a specially adapted telescope.
   

2. A simple device using a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send coded messages. The heliograph was a highly effective instrument for instantaneous optical communication over 80km or more in the 19th century.
   

3. Meteo.: An instrument which records the duration of sunshine and gives a quantitative measure of the amount of sunshine.

See also: → helio-; + → -graph.

1. An instrument for photographing the Sun, consisting of a camera and a specially adapted telescope.
   

2. A simple device using a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send coded messages. The heliograph was a highly effective instrument for instantaneous optical communication over 80km or more in the 19th century.
   

3. Meteo.: An instrument which records the duration of sunshine and gives a quantitative measure of the amount of sunshine.

See also: → helio-; + → -graph.

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope.  The instrument was originally designed for measuring the variation of the
Sun’s diameter at different seasons of the year.

See also: → helio- + → -meter.

An instrument used to measure the angular separation of two stars that are too far apart to be included in the field of view of an ordinary telescope.  The instrument was originally designed for measuring the variation of the
Sun’s diameter at different seasons of the year.

See also: → helio- + → -meter.

The edge of the solar system where the pressure of the → solar wind balances that of the → interstellar medium plasma. In other words, the surface boundary that separates the → heliosphere from interstellar space. It is
estimated to be situated 100 → astronomical units or more from the Sun. A → bow shock likely forms as the interstellar medium wind approaches the heliosphere and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail.

Etymology (EN): From → helio- + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): Hurmarz, from hur, → helio-, + marz “frontier, border, boundary,” → frontier.

The edge of the solar system where the pressure of the → solar wind balances that of the → interstellar medium plasma. In other words, the surface boundary that separates the → heliosphere from interstellar space. It is
estimated to be situated 100 → astronomical units or more from the Sun. A → bow shock likely forms as the interstellar medium wind approaches the heliosphere and is deflected around the heliosphere, forcing it into a teardrop-shaped structure with a long, comet-like tail.

Etymology (EN): From → helio- + pause “break, cessation, stop,” from M.Fr. pause, from L. pausa “a halt, stop, cessation,” from Gk. pausis “stopping, ceasing,” from pauein “to stop, to cause to cease.”

Etymology (PE): Hurmarz, from hur, → helio-, + marz “frontier, border, boundary,” → frontier.

The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. See also → asteroseismology and → stellar pulsation. The surface of the Sun vibrates much like a bell.
A piano has 88 keys or musical tones, whereas the Sun has millions of notes.
These vibrations are the result of internal pressure waves that reflect off the → photosphere and repeatedly cross the solar interior. They are detected through the → Doppler shift of absorption lines formed in the photosphere.
Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun’s surface. The mean period of the vibrations is about five minutes, which corresponds to a speed of 0.5 km s^-1 or a frequency of about 3 mHz.
See also → p mode.

Etymology (EN): From → helio- + → seismology.

The branch of astrophysics that investigates the interior structure of the Sun by studying its surface wave oscillations. See also → asteroseismology and → stellar pulsation. The surface of the Sun vibrates much like a bell.
A piano has 88 keys or musical tones, whereas the Sun has millions of notes.
These vibrations are the result of internal pressure waves that reflect off the → photosphere and repeatedly cross the solar interior. They are detected through the → Doppler shift of absorption lines formed in the photosphere.
Because these vibrations make the solar surface move up and down, analysis of the surface patterns is used to study conditions far below the Sun’s surface. The mean period of the vibrations is about five minutes, which corresponds to a speed of 0.5 km s^-1 or a frequency of about 3 mHz.
See also → p mode.

Etymology (EN): From → helio- + → seismology.

The region located between the → termination shock and the → heliopause where the turbulent and hot → solar wind is compressed as it passes outward against the interstellar wind.

Etymology (EN): Heliosheath, from → helio- + sheath, from O.E. sceað, scæð; cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide “scabbard.”

Etymology (PE): Hurniyâm, from hur- “sun,” → helio-,

• niyâm “sheath,” from Proto-Iranian *nigāma-, from ni- “down; into,” → ni- (PIE),
• gāma- “to go, to come” (Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
  “step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”); cf. Skt. nigamá- “insertion, incorporation.”

The region located between the → termination shock and the → heliopause where the turbulent and hot → solar wind is compressed as it passes outward against the interstellar wind.

Etymology (EN): Heliosheath, from → helio- + sheath, from O.E. sceað, scæð; cf. M.Du. schede, Du. schede, O.H.G. skaida, Ger. Scheide “scabbard.”

Etymology (PE): Hurniyâm, from hur- “sun,” → helio-,

• niyâm “sheath,” from Proto-Iranian *nigāma-, from ni- “down; into,” → ni- (PIE),
• gāma- “to go, to come” (Av. gam- “to come; to go,” jamaiti “goes;” O.Pers. gam- “to come; to go;” Mod./Mid.Pers. gâm
  “step, pace,” âmadan “to come;” cf. Skt. gamati “goes;” Gk. bainein “to go, walk, step;” L. venire “to come;” Tocharian A käm- “to come;” O.H.G. queman “to come;” E. come; PIE root *gwem- “to go, come”); cf. Skt. nigamá- “insertion, incorporation.”

The vast, three-dimensional region of space around the Sun filled with the → solar wind and the remnant of the → solar magnetic field
carried in it. It is bounded by the → heliopause, which is estimated to be 100 → astronomical units or more from the Sun. The radius of the heliosphere is expected to vary with the → solar cycle. The heliosphere may be very elongated owing to the presence of an interstellar wind of neutral hydrogen flowing from the direction of the Galactic center.

Etymology (EN): From → helio- + → sphere.

The vast, three-dimensional region of space around the Sun filled with the → solar wind and the remnant of the → solar magnetic field
carried in it. It is bounded by the → heliopause, which is estimated to be 100 → astronomical units or more from the Sun. The radius of the heliosphere is expected to vary with the → solar cycle. The heliosphere may be very elongated owing to the presence of an interstellar wind of neutral hydrogen flowing from the direction of the Galactic center.

Etymology (EN): From → helio- + → sphere.

An instrument consisting of a mirror moved by clockwork for tracking the movement of the Sun and reflecting the sunlight into a stationary solar telescope. A heliostat is similar to a → coelostat.

Etymology (EN): Heliostat, from → helio- + -stat
prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Hurdâštâr, from hur-, → helio- + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess;” Mid.Pers. dâštan; O.Pers./Av. dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support.”

An instrument consisting of a mirror moved by clockwork for tracking the movement of the Sun and reflecting the sunlight into a stationary solar telescope. A heliostat is similar to a → coelostat.

Etymology (EN): Heliostat, from → helio- + -stat
prefix denoting something that stabilizes, keeps, fixes, from -stata, from Gk. -states “one that causes to stand,” or statos “standing,” from *sta- “to stand.”

Etymology (PE): Hurdâštâr, from hur-, → helio- + dâštâr “holder, maintainer,” from dâštan “to hold, maintain; to have; to possess;” Mid.Pers. dâštan; O.Pers./Av. dar- “to hold, keep back, maintain, keep in mind;” cf.
Skt. dhr-, dharma- “law;” Gk. thronos “elevated seat, throne;” L. firmus “firm, stable;” Lith. daryti “to make;” PIE base *dher- “to hold, support.”

Chemical element; symbol He; atomic number 2; atomic weight 4.0026; melting point below -272°C at 26 atmospheres pressure; boiling point -268.934°C at 1 atmosphere pressure.

See also: Helium, from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”
The element was discovered by spectroscopy during a solar eclipse in the Sun’s chromosphere in 1868 by the French astronomer Pierre-Jules-Cesar Janssen (1824-1907).

Chemical element; symbol He; atomic number 2; atomic weight 4.0026; melting point below -272°C at 26 atmospheres pressure; boiling point -268.934°C at 1 atmosphere pressure.

See also: Helium, from Gk. helios “sun;” cognate with
Persian hur “sun”, variant xor;
Mid.Pers. xvar “sun;” Av. hû-, hvar- “sun;” Skt.
surya-; L. sol; O.H.G. sunna; Ger. Sonne; E. sun;
PIE *sawel- “sun.”
The element was discovered by spectroscopy during a solar eclipse in the Sun’s chromosphere in 1868 by the French astronomer Pierre-Jules-Cesar Janssen (1824-1907).

The relative amount of helium with respect to another → chemical species, usually → hydrogen, in an astronomical object.

See also: → helium; → abundance.

The relative amount of helium with respect to another → chemical species, usually → hydrogen, in an astronomical object.

See also: → helium; → abundance.

The stage in the evolution of a star, after the exhaustion of hydrogen, when
the star produces its energy by the fusion of helium into carbon and oxygen.

See also: → helium; → burning.

The stage in the evolution of a star, after the exhaustion of hydrogen, when
the star produces its energy by the fusion of helium into carbon and oxygen.

See also: → helium; → burning.

The sudden onset of → helium burning in the core of an → intermediate-mass star that has exhausted its hydrogen and has become a → red giant. With a → degenerate core, the temperature increases but the pressure does not. Therefore, the core cannot expand and cool, so the temperature continues to rise. When it approaches 100,000,000 K, helium will begin to fuse into carbon in the → triple alpha process. The helium flash ends the giant star’s ascent of the → red giant branch. However, the violent ignition of helium in the core does not increase the star’s luminosity. On the contrary, the energy released in the helium flash expands and cools the core and ultimately results in a reduction in the energy output. On the → H-R diagram the star moves down
from red giant branch to the → horizontal branch, a stable state with steady helium burning in the core.

See also: → helium; → flash.

The sudden onset of → helium burning in the core of an → intermediate-mass star that has exhausted its hydrogen and has become a → red giant. With a → degenerate core, the temperature increases but the pressure does not. Therefore, the core cannot expand and cool, so the temperature continues to rise. When it approaches 100,000,000 K, helium will begin to fuse into carbon in the → triple alpha process. The helium flash ends the giant star’s ascent of the → red giant branch. However, the violent ignition of helium in the core does not increase the star’s luminosity. On the contrary, the energy released in the helium flash expands and cools the core and ultimately results in a reduction in the energy output. On the → H-R diagram the star moves down
from red giant branch to the → horizontal branch, a stable state with steady helium burning in the core.

See also: → helium; → flash.

1. The normal component of → liquid helium (⁴He) existing between the superfluid transition point (→ lambda point about 2.17 K) at 1 atmosphere of pressure and its boiling point of 4.2 K.
   

2. In stellar and interstellar plasma spectroscopy, neutral helium.

See also: → helium.

1. The normal component of → liquid helium (⁴He) existing between the superfluid transition point (→ lambda point about 2.17 K) at 1 atmosphere of pressure and its boiling point of 4.2 K.
   

2. In stellar and interstellar plasma spectroscopy, neutral helium.

See also: → helium.

1. A → superfluid, colder component of → liquid helium (⁴He), occurring when → helium I is cooled below the → lambda point.
   

2. 2. In stellar and interstellar plasma spectroscopy, ionized helium.

See also: → helium.

1. A → superfluid, colder component of → liquid helium (⁴He), occurring when → helium I is cooled below the → lambda point.
   

2. 2. In stellar and interstellar plasma spectroscopy, ionized helium.

See also: → helium.

A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.

See also: → helium; → shell; → burning.

A stage in the evolution of an → asymptotic giant branch star, when all the helium in the core is fused into carbon and oxygen. No more fusion takes place in the core, and as a result the core contracts. The core contraction generates a sufficient temperature for fusing the surrounding layers of helium. Since helium shell burning is unstable, it causes → helium shell flashes.

See also: → helium; → shell; → burning.

A violent outburst of energy that occurs periodically in an
→ asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.

See also: → helium; → shell; → flash.

A violent outburst of energy that occurs periodically in an
→ asymptotic giant branch star. It occurs when helium is being burnt in a thin shell surrounding the inner dense core of carbon and oxygen. → Helium shell burning is unstable, producing energy mainly in short intense flashes. The shell flash causes considerable expansion of the star followed by collapse, thus setting up deep convection. As a consequence, the → convective zone in the outer part of the star goes deeper and may → dredge-up carbon to the surface. See also → late thermal pulse; → very late thermal pulse; → AGB final thermal pulse.

See also: → helium; → shell; → flash.

An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.

See also: → helium; → star.

An → evolved star which has lost most or all of its hydrogen-rich envelope, leaving just a core of helium.

See also: → helium; → star.

A wavelength calibration of astronomical spectra using a helium-argon light source.

See also: → helium; → argon;
→ calibration.

A wavelength calibration of astronomical spectra using a helium-argon light source.

See also: → helium; → argon;
→ calibration.

A comparison light source containing the known spectral lines of helium and Argon.

See also: → helium; → argon;
→ lamp.

A comparison light source containing the known spectral lines of helium and Argon.

See also: → helium; → argon;
→ lamp.

A curve which lies on a cylinder or cone, so that its angle to a plane perpendicular to the axis is constant.

Etymology (EN): From L. helix “spiral,” from Gk. helix (genitive helikos), related to eilein “to turn, twist, roll.”

Etymology (PE): Picâr “that which twists,” from
pic, present stem of picidan “to twist, entwine, coil” + -ar agent noun suffix (on the model of parastâr, padidâr, dustâr, xâstâr).

A curve which lies on a cylinder or cone, so that its angle to a plane perpendicular to the axis is constant.

Etymology (EN): From L. helix “spiral,” from Gk. helix (genitive helikos), related to eilein “to turn, twist, roll.”

Etymology (PE): Picâr “that which twists,” from
pic, present stem of picidan “to twist, entwine, coil” + -ar agent noun suffix (on the model of parastâr, padidâr, dustâr, xâstâr).

A large and bright → planetary nebula in the constellation → Aquarius. Its apparent diameter is about half the size of the full Moon, corresponding to about 2.5 → light-years for a distance of about 700 light-years. It is the nearest bright planetary nebulae to Earth and one of the most spectacular examples of such objects. The Helix Nebula possibly consists of at least two separate disks with outer rings and filaments. The brighter inner disk seems to be expanding at about 100,000 km/h and to have taken about 12,000 years to form. High-resolution observations of the inner edge of the Helix’s main ring have revealed thousands of cometary knots of gas with faint tails extending away from the central star. The knots have masses similar to the Earth, but are typically the size of our Solar system. The comet-like shape of the knots results from the steady evaporation of gas from the knots, produced by the strong winds and ultraviolet radiation from the central star of the nebula.The origin of the knots is currently not well understood.

See also: → helix; → nebula.

A large and bright → planetary nebula in the constellation → Aquarius. Its apparent diameter is about half the size of the full Moon, corresponding to about 2.5 → light-years for a distance of about 700 light-years. It is the nearest bright planetary nebulae to Earth and one of the most spectacular examples of such objects. The Helix Nebula possibly consists of at least two separate disks with outer rings and filaments. The brighter inner disk seems to be expanding at about 100,000 km/h and to have taken about 12,000 years to form. High-resolution observations of the inner edge of the Helix’s main ring have revealed thousands of cometary knots of gas with faint tails extending away from the central star. The knots have masses similar to the Earth, but are typically the size of our Solar system. The comet-like shape of the knots results from the steady evaporation of gas from the knots, produced by the strong winds and ultraviolet radiation from the central star of the nebula.The origin of the knots is currently not well understood.

See also: → helix; → nebula.

One of the largest identified → impact craters both on → Mars and within the → Solar System. Hellas spans more than 2000 km across in the → southern hemisphere, a region that is much more heavily cratered and higher in average elevation than the northern hemisphere. The depth of Hellas from its bottom to its inner rim is more than 4 km. In comparison, the depth of the Grand Canyon in the United States is roughly 1.6 km, that is 2.5 times smaller! The western part of the Hellas basin contains the lowest point on Mars, about 8.2 km below the Mars datum or Martian “sea level.” The formation of the impact structure is believed to have taken place in the early Noachian epoch, between 3.9 and 4.6 billion years ago (Planetary Science Institute webpage).

See also: Hellas refers to the classical name for Greece; → basin.

One of the largest identified → impact craters both on → Mars and within the → Solar System. Hellas spans more than 2000 km across in the → southern hemisphere, a region that is much more heavily cratered and higher in average elevation than the northern hemisphere. The depth of Hellas from its bottom to its inner rim is more than 4 km. In comparison, the depth of the Grand Canyon in the United States is roughly 1.6 km, that is 2.5 times smaller! The western part of the Hellas basin contains the lowest point on Mars, about 8.2 km below the Mars datum or Martian “sea level.” The formation of the impact structure is believed to have taken place in the early Noachian epoch, between 3.9 and 4.6 billion years ago (Planetary Science Institute webpage).

See also: Hellas refers to the classical name for Greece; → basin.

1. Any of various protective head coverings worn by soldiers, policemen, firemen, etc.
   

2. Something resembling a helmet in form or position. → helmet streamer.

Etymology (EN): From M.Fr. helmet, diminutive of helme “helmet,” from Frank. *helm (cf. O.H.G. helm “helmet”);
PIE base *kel- “to cover, to hide;” cf. Av. sar- “shelter;” Laki šârd “hidden, hiddenly,” šârden “to hide;” Kurd. šâr-, šârdinawa “to hide;” Skt. śárman- “cover, protection, refuge;” L. celare “to conceal;” Goth. huljan “to cover, conceal;” O.H.G. helan “to hide.”

Etymology (PE): Xud “helmet,” from O.Pers. xaudā- “hat, cap,” tigra-xauda- “wearing the pointed cap” (as is shown in the sculpture of Skunkha the Scythian at Behistan); Av. xaoδa- “hat, cap, helmet;” Ossetic xodä; Arm. (borrowed) xoir “headband.”

1. Any of various protective head coverings worn by soldiers, policemen, firemen, etc.
   

2. Something resembling a helmet in form or position. → helmet streamer.

Etymology (EN): From M.Fr. helmet, diminutive of helme “helmet,” from Frank. *helm (cf. O.H.G. helm “helmet”);
PIE base *kel- “to cover, to hide;” cf. Av. sar- “shelter;” Laki šârd “hidden, hiddenly,” šârden “to hide;” Kurd. šâr-, šârdinawa “to hide;” Skt. śárman- “cover, protection, refuge;” L. celare “to conceal;” Goth. huljan “to cover, conceal;” O.H.G. helan “to hide.”

Etymology (PE): Xud “helmet,” from O.Pers. xaudā- “hat, cap,” tigra-xauda- “wearing the pointed cap” (as is shown in the sculpture of Skunkha the Scythian at Behistan); Av. xaoδa- “hat, cap, helmet;” Ossetic xodä; Arm. (borrowed) xoir “headband.”

A large-scale → coronal feature with apparent → cusp, seen during a → solar eclipse. They usually arise from → sunspots and → active regions, so at the base of a helmet streamer one will often find a → prominence.  They form magnetic loops that connect the sunspots and suspend material above the surface of the Sun.  The magnetic field lines trap the material to form the streamers. The action of the → solar wind is at the origin of the peak feature.

See also: → helmet; → streamer.

A large-scale → coronal feature with apparent → cusp, seen during a → solar eclipse. They usually arise from → sunspots and → active regions, so at the base of a helmet streamer one will often find a → prominence.  They form magnetic loops that connect the sunspots and suspend material above the surface of the Sun.  The magnetic field lines trap the material to form the streamers. The action of the → solar wind is at the origin of the peak feature.

See also: → helmet; → streamer.

Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.

See also: After the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → free; → energy.

Of a system, the quantity whose decrease gives the maximum amount of external work which is performed when any physical or chemical process is carried out reversibly at constant temperature. It is defined by F = U - TS, where U is the → internal energy, T the → absolute temperature, and S the final → entropy.

See also: After the German physicist and physician Hermann Ludwig Ferdinand von Helmholtz (1821-1894), who made important contributions to the thermodynamics of gaseous systems; → free; → energy.

A → decomposition theorem, whereby a continuous → vector field, F, can be broken down into the sum of a → gradient and a → curl term:
F = -∇φ + ∇ xA, where φ is called the → scalar potential and A the → vector potential.

See also: → Helmholtz free energy; → theorem.

A → decomposition theorem, whereby a continuous → vector field, F, can be broken down into the sum of a → gradient and a → curl term:
F = -∇φ + ∇ xA, where φ is called the → scalar potential and A the → vector potential.

See also: → Helmholtz free energy; → theorem.

A systematic trend in the motion of some → Galactic halo→ old stars thought to be a relic of the → merging of a dwarf satellite galaxy devoured by our Milky Way. Using kinematic data from the → Hipparcos satellite, Helmi et al. (1999, Nature 402, 53) found two halo star streams which share a common progenitor: a single coherent object disrupted during or soon after the Milky Way’s formation, and which probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.

See also: See also Helmi & White 1999, MNRAS 307, 495; → stream.

A systematic trend in the motion of some → Galactic halo→ old stars thought to be a relic of the → merging of a dwarf satellite galaxy devoured by our Milky Way. Using kinematic data from the → Hipparcos satellite, Helmi et al. (1999, Nature 402, 53) found two halo star streams which share a common progenitor: a single coherent object disrupted during or soon after the Milky Way’s formation, and which probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.

See also: See also Helmi & White 1999, MNRAS 307, 495; → stream.

A service in an organization or computer system where users are directed for technical support or assistance.

Etymology (EN): M.E., O.E. help (m.), helpe (f.) “assistance, succor,” helpan “o help” (cf. O.N. hjialp, hjalpa, M.Du., Du. hulp, helpen, O.H.G. helfa, helfan, Ger. Hilfe, helfen); PIE base *kelb- “to help” (cf. Lith. selpiu “to support, help”); desk, from M.E. deske; M.L. desca, descus “table to write on,” from L. discus “quoit, platter, dish,” from Gk. diskos “disk, dish.”

Etymology (PE): Yâigâh, from yâri “help, assistance,” → gravity assist,

• -gâh “place; time,” → station.

A service in an organization or computer system where users are directed for technical support or assistance.

Etymology (EN): M.E., O.E. help (m.), helpe (f.) “assistance, succor,” helpan “o help” (cf. O.N. hjialp, hjalpa, M.Du., Du. hulp, helpen, O.H.G. helfa, helfan, Ger. Hilfe, helfen); PIE base *kelb- “to help” (cf. Lith. selpiu “to support, help”); desk, from M.E. deske; M.L. desca, descus “table to write on,” from L. discus “quoit, platter, dish,” from Gk. diskos “disk, dish.”

Etymology (PE): Yâigâh, from yâri “help, assistance,” → gravity assist,

• -gâh “place; time,” → station.

A mineral that is often found in meteorites. It is an oxide of iron (Fe₂O₃) that is similar to magnetite. It does not attract a magnet. When it is rubbed against an object harder than itself, it leaves a reddish-brown stain. Hematite is also sometimes called bloodstone.

See also: From M.Fr. hematite, from L. hæmatites, from Gk. haimatites lithos “bloodlike stone,” from haima (genitive haimatos) “blood”

• -ites, → -ite, + lithos “stone.”

A mineral that is often found in meteorites. It is an oxide of iron (Fe₂O₃) that is similar to magnetite. It does not attract a magnet. When it is rubbed against an object harder than itself, it leaves a reddish-brown stain. Hematite is also sometimes called bloodstone.

See also: From M.Fr. hematite, from L. hæmatites, from Gk. haimatites lithos “bloodlike stone,” from haima (genitive haimatos) “blood”

• -ites, → -ite, + lithos “stone.”

A defect of the eyes in which sight is normal in the night or in a dim light but is abnormally poor or wholly absent in the day or in a bright light. Also called day blindness. Opposite of → nyctalopia

Etymology (EN): From N.L., from Gk hemeralop- (stem of hemeralops having such a condition, from hemer(a) “day” + al(aos) “blind”

• -ops having such an appearance) + -ia a noun suffix.

Etymology (PE): Ruzkuri, from ruz, → day, + kuri “blindness,” from kur, → blind.

A defect of the eyes in which sight is normal in the night or in a dim light but is abnormally poor or wholly absent in the day or in a bright light. Also called day blindness. Opposite of → nyctalopia

Etymology (EN): From N.L., from Gk hemeralop- (stem of hemeralops having such a condition, from hemer(a) “day” + al(aos) “blind”

• -ops having such an appearance) + -ia a noun suffix.

Etymology (PE): Ruzkuri, from ruz, → day, + kuri “blindness,” from kur, → blind.

Half of a sphere bounded by a great circle, especially one of the halves into which the earth or the celestial sphere is divided.

Etymology (EN): From L. hemisphærium, from Gk. hemisphairion, from hemi- “half,” (from PIE base *semi-; cf. Skt. sami, L. semi-, O.H.G. sami- “half,” and O.E. sam-) + sphaira, → sphere.

Etymology (PE): From nim-, → half + sepehr, koré,
→ sphere.

Half of a sphere bounded by a great circle, especially one of the halves into which the earth or the celestial sphere is divided.

Etymology (EN): From L. hemisphærium, from Gk. hemisphairion, from hemi- “half,” (from PIE base *semi-; cf. Skt. sami, L. semi-, O.H.G. sami- “half,” and O.E. sam-) + sphaira, → sphere.

Etymology (PE): From nim-, → half + sepehr, koré,
→ sphere.

A catalog of stars in which every star is classified by its stellar spectrum. This system is named for the astronomer Henry Draper, but was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall.

See also: Henry Draper (1837-1882), an American pioneer of astronomical spectroscopy who established the observing techniques and program for the work that would bear his name when published, seven years after his early death; → system.

A catalog of stars in which every star is classified by its stellar spectrum. This system is named for the astronomer Henry Draper, but was cataloged by Annie J. Cannon (225,300 stars), and later extended by Margaret W. Mayall.

See also: Henry Draper (1837-1882), an American pioneer of astronomical spectroscopy who established the observing techniques and program for the work that would bear his name when published, seven years after his early death; → system.

A powerful numerical technique to solve the stellar structure equations where the star is sub-divided in a finite number of grid cells for which the local conditions are evaluated and computed from the surface inwards to the center by utilizing a Newton-Raphson solver. Relevant physical quantities are either defined at the cell boundaries or as mean values over the complete cell.

See also: Henyey, L. G.; Forbes, J. E.; Gould, N. L., 1964, ApJ 139, 306; → method.

A powerful numerical technique to solve the stellar structure equations where the star is sub-divided in a finite number of grid cells for which the local conditions are evaluated and computed from the surface inwards to the center by utilizing a Newton-Raphson solver. Relevant physical quantities are either defined at the cell boundaries or as mean values over the complete cell.

See also: Henyey, L. G.; Forbes, J. E.; Gould, N. L., 1964, ApJ 139, 306; → method.

A nearly horizontal path on the → Hertzsprung-Russell diagram that a → pre-main sequence star of small mass follows in an early stage of evolution after leaving the → Hayashi track and before reaching the → main sequence. During this stage the pre-main sequence star remains almost wholly in radiative equilibrium.

See also: After Louis George Henyey (1910-1970), American astronomer. Henyey et al. (1955, PASP 67, 154).

A nearly horizontal path on the → Hertzsprung-Russell diagram that a → pre-main sequence star of small mass follows in an early stage of evolution after leaving the → Hayashi track and before reaching the → main sequence. During this stage the pre-main sequence star remains almost wholly in radiative equilibrium.

See also: After Louis George Henyey (1910-1970), American astronomer. Henyey et al. (1955, PASP 67, 154).

A combining form meaning “seven.”

Etymology (EN): From Gk. hepta “seven;” cognate with L. septem; Pers. haft, as below; Du. zeven, O.H.G. sibun, Ger. sieben, E. seven.

Etymology (PE): Haft-, from haft “seven;” Mid.Pers. haft; Av. hapta; cf. Skt. sapta; Gk. hepta, L. septem; PIE *septm.

A combining form meaning “seven.”

Etymology (EN): From Gk. hepta “seven;” cognate with L. septem; Pers. haft, as below; Du. zeven, O.H.G. sibun, Ger. sieben, E. seven.

Etymology (PE): Haft-, from haft “seven;” Mid.Pers. haft; Av. hapta; cf. Skt. sapta; Gk. hepta, L. septem; PIE *septm.

A → polygon with seven → angles and seven → sides.

See also: → hepta-; → -gon.

A → polygon with seven → angles and seven → sides.

See also: → hepta-; → -gon.

A young → A-type or → B-type star showing → emission lines in its spectrum. Herbig AeBe stars are → pre-main sequence stars of
→ intermediate mass (→ intermediate-mass star). They are often called the higher mass counterparts of → T Tauri stars.

See also: Named after George H. Herbig (1920-2013), who first classified them (Herbig 1960, ApJS 4, 337); → A star; → B star; e indicating
→ emission.

A young → A-type or → B-type star showing → emission lines in its spectrum. Herbig AeBe stars are → pre-main sequence stars of
→ intermediate mass (→ intermediate-mass star). They are often called the higher mass counterparts of → T Tauri stars.

See also: Named after George H. Herbig (1920-2013), who first classified them (Herbig 1960, ApJS 4, 337); → A star; → B star; e indicating
→ emission.

A small patch of → nebulosity in a → star-forming region, created when fast-moving → jets of material (with speeds up to about 1000 km per sec) from a newborn star collide with the → interstellar medium.

See also: → Herbig AeBe star; Guillermo Haro (1913-1988), who first in 1940s studied these objects in detail and recognized that they were a by-product of the star formation process; → object.

A small patch of → nebulosity in a → star-forming region, created when fast-moving → jets of material (with speeds up to about 1000 km per sec) from a newborn star collide with the → interstellar medium.

See also: → Herbig AeBe star; Guillermo Haro (1913-1988), who first in 1940s studied these objects in detail and recognized that they were a by-product of the star formation process; → object.

An ancient → constellation (right ascension about 17h, declination 30° north), one of the largest in the sky, which is located between → Lyra and → Corona Borealis. It is traditionally depicted as the hero Hercules in a kneeling position. There are no very bright stars in Hercules, the brightest one is → Rasalgethi, a variable → red supergiant of magnitude about 3.5. Abbreviation: Her; Genitive: Herculis.
See also: → Hercules cluster.

Etymology (EN): L. Hercules, from Gk. Heracles “glory of Hera,” the most popular hero of Gk. mythology, son of Zeus and the woman Alcmena, who the god seduced in the shape of her husband Amphitryon, king of Thebes.

Etymology (PE): Herâkles, as above; Herkul, from Fr. Hercule, as above; Arabicized name of the constellation:

(الجاثی، الجاثی علی‌رکبتیه) “the kneeling one,” râqes (راقص) “the dancing one.”
Bar zânu nešasté “the kneeling one,” Pers. descriptive rendering of the Gk. mythological figure, by the famous 11-th century astronomer Biruni.

An ancient → constellation (right ascension about 17h, declination 30° north), one of the largest in the sky, which is located between → Lyra and → Corona Borealis. It is traditionally depicted as the hero Hercules in a kneeling position. There are no very bright stars in Hercules, the brightest one is → Rasalgethi, a variable → red supergiant of magnitude about 3.5. Abbreviation: Her; Genitive: Herculis.
See also: → Hercules cluster.

Etymology (EN): L. Hercules, from Gk. Heracles “glory of Hera,” the most popular hero of Gk. mythology, son of Zeus and the woman Alcmena, who the god seduced in the shape of her husband Amphitryon, king of Thebes.

Etymology (PE): Herâkles, as above; Herkul, from Fr. Hercule, as above; Arabicized name of the constellation:

(الجاثی، الجاثی علی‌رکبتیه) “the kneeling one,” râqes (راقص) “the dancing one.”
Bar zânu nešasté “the kneeling one,” Pers. descriptive rendering of the Gk. mythological figure, by the famous 11-th century astronomer Biruni.

A small, irregular → cluster of galaxies with fewer than 100 galaxies in its core.
It has no strongly dominant central galaxy and is notable for the high proportion of spirals. It lies some 500 million → light-years away in the constellation → Hercules; also known as Abell 2151.

See also: → Hercules; → cluster.

A small, irregular → cluster of galaxies with fewer than 100 galaxies in its core.
It has no strongly dominant central galaxy and is notable for the high proportion of spirals. It lies some 500 million → light-years away in the constellation → Hercules; also known as Abell 2151.

See also: → Hercules; → cluster.

→ inheritable.

See also: → inherit; → -able.

→ inheritable.

See also: → inherit; → -able.

1. Passing, or capable of passing, naturally from parent to offspring through the genes: Blue eyes are hereditary in our family.
   

2. Of or relating to inheritance or heredity (Dictionary.com).

See also: Of or relating to → heredity.

1. Passing, or capable of passing, naturally from parent to offspring through the genes: Blue eyes are hereditary in our family.
   

2. Of or relating to inheritance or heredity (Dictionary.com).

See also: Of or relating to → heredity.

The passing on of physical or mental characteristics genetically from one generation to another (OxfordDictionaries.com).

Etymology (EN): M.E., from M.Fr. hérédité, from O.Fr. eredite “inheritance, legacy,” from L. hereditatem (nominative hereditas) “heirship, inheritance,” → heritage.

Etymology (PE): Rigandâšt, literally “possessing heritage,” from rigan, → heritage, + dâšt past stem of dâštan “to have, hold, possess, maintain,” → property.

The passing on of physical or mental characteristics genetically from one generation to another (OxfordDictionaries.com).

Etymology (EN): M.E., from M.Fr. hérédité, from O.Fr. eredite “inheritance, legacy,” from L. hereditatem (nominative hereditas) “heirship, inheritance,” → heritage.

Etymology (PE): Rigandâšt, literally “possessing heritage,” from rigan, → heritage, + dâšt past stem of dâštan “to have, hold, possess, maintain,” → property.

→ inheritable.

See also: → inherit; → -able.

→ inheritable.

See also: → inherit; → -able.

1. Something inherited at birth, such as personal characteristics, status, and possessions.
   

2. Anything that has been transmitted from the past or handed down by tradition (Dictionary.com).

Etymology (EN): M.E. from M.Fr., from O.Fr. iritage, eritage, heritage “heir; inheritance, ancestral estate, heirloom,” from heriter “inherit,” from L.L. hereditare, ultimately from L. heres (genitive heredis) “heir, heiress,” from PIE root *ghe- “to be empty, left behind” (related Gk. word khera “widow”).

Etymology (PE): Rigan from rig “left, abandoned” (in mordé rig “heritage, effects of a dead person, anything hereditary, heirloom”) + noun suffix -an (as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.); ultimately from Proto-Ir. *raic- “to leave, abandon;” cf. Av. raēc- “to leave, let;” Mid.Pers. (+ *pati-) phryz-, Mod.Pers. parhêz, parhiz “to keep away from, abstain, avoid;” Khotanese (+ *fra-) hars- “to be left, remain;” Mod.Pers. rištan “to set at liberty, absolve;” Mid.Pers. (+ *ui-) wirēz-, Mod.Pers. gurēz, goriz, gurēxtan, gorixtan “to flee, run away;” Gk. leipein “to leave;” L. linquere “to leave;” PIE *leik^w- “to leave, let” (Cheung 2006).

1. Something inherited at birth, such as personal characteristics, status, and possessions.
   

2. Anything that has been transmitted from the past or handed down by tradition (Dictionary.com).

Etymology (EN): M.E. from M.Fr., from O.Fr. iritage, eritage, heritage “heir; inheritance, ancestral estate, heirloom,” from heriter “inherit,” from L.L. hereditare, ultimately from L. heres (genitive heredis) “heir, heiress,” from PIE root *ghe- “to be empty, left behind” (related Gk. word khera “widow”).

Etymology (PE): Rigan from rig “left, abandoned” (in mordé rig “heritage, effects of a dead person, anything hereditary, heirloom”) + noun suffix -an (as in rowzan, rowšan, suzan, rasan, zaqan, hâvan, etc.); ultimately from Proto-Ir. *raic- “to leave, abandon;” cf. Av. raēc- “to leave, let;” Mid.Pers. (+ *pati-) phryz-, Mod.Pers. parhêz, parhiz “to keep away from, abstain, avoid;” Khotanese (+ *fra-) hars- “to be left, remain;” Mod.Pers. rištan “to set at liberty, absolve;” Mid.Pers. (+ *ui-) wirēz-, Mod.Pers. gurēz, goriz, gurēxtan, gorixtan “to flee, run away;” Gk. leipein “to leave;” L. linquere “to leave;” PIE *leik^w- “to leave, let” (Cheung 2006).

Biology: An individual, animal, or plant possessing both male and female reproductive organs.

Etymology (EN): From L. hermaphroditus, from Gk. hermaphroditos the mythical son of Hermes and Aphrodite who merged bodies with a naiad and thereafter possessed both male and female qualities.

Etymology (PE): Narmâde, literally “male-female,” → male, → female.

Biology: An individual, animal, or plant possessing both male and female reproductive organs.

Etymology (EN): From L. hermaphroditus, from Gk. hermaphroditos the mythical son of Hermes and Aphrodite who merged bodies with a naiad and thereafter possessed both male and female qualities.

Etymology (PE): Narmâde, literally “male-female,” → male, → female.

Biology: For an animal or plant, the condition of having both male and female reproductive tissue or organs.

See also: → hermaphrodite; → -ism.

Biology: For an animal or plant, the condition of having both male and female reproductive tissue or organs.

See also: → hermaphrodite; → -ism.

Of or related to hermeneutics, interpretative; explanatory. Also hermeneutical.

See also: → hermeneutics.

Of or related to hermeneutics, interpretative; explanatory. Also hermeneutical.

See also: → hermeneutics.

The science or art of → interpretation. Originally the term was limited to the interpretation of the Scriptures, but since the nineteenth century it has developed into a general theory of human understanding through the work of Friedrich Schleiermacher (1768-1834), Wilhelm Dilthey (1833-1911), and others. The comprehension of any written text requires hermeneutics. Many different hermeneutic theorists have proposed many different methodologies.

Etymology (EN): From Gk. hermeneutikos “interpreting,” from hermeneutes “interpreter,” from hermeneuein “to interpret,” of unknown origin. It was formerly thought to derive from Hermes, the tutelary divinity of speech, writing, and eloquence.

Etymology (PE): Âzand-pardâzik, from âzand, → interpretation,

• pardâz, present stem of pardâxtan “to accomplish, bring to perfection; to care,” → theoretician, + -ik, → ics.

The science or art of → interpretation. Originally the term was limited to the interpretation of the Scriptures, but since the nineteenth century it has developed into a general theory of human understanding through the work of Friedrich Schleiermacher (1768-1834), Wilhelm Dilthey (1833-1911), and others. The comprehension of any written text requires hermeneutics. Many different hermeneutic theorists have proposed many different methodologies.

Etymology (EN): From Gk. hermeneutikos “interpreting,” from hermeneutes “interpreter,” from hermeneuein “to interpret,” of unknown origin. It was formerly thought to derive from Hermes, the tutelary divinity of speech, writing, and eloquence.

Etymology (PE): Âzand-pardâzik, from âzand, → interpretation,

• pardâz, present stem of pardâxtan “to accomplish, bring to perfection; to care,” → theoretician, + -ik, → ics.

Someone who interprets literary or scriptural texts.

See also: Agent noun from → hermeneutics

Someone who interprets literary or scriptural texts.

See also: Agent noun from → hermeneutics

Math.: The Hermitian conjugate of an m by n matrix A is the n by m matrix A^* obtained from A by taking the → transpose and then taking the complex conjugate of each entry. Also called adjoint matrix, conjugate transpose. → Hermitian operator.

See also: Hermitian, named in honor of the Fr. mathematician Charles Hermite (1822-1901), who made important contributions to number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra. One of his students was Henri Poincaré; → conjugate.

Math.: The Hermitian conjugate of an m by n matrix A is the n by m matrix A^* obtained from A by taking the → transpose and then taking the complex conjugate of each entry. Also called adjoint matrix, conjugate transpose. → Hermitian operator.

See also: Hermitian, named in honor of the Fr. mathematician Charles Hermite (1822-1901), who made important contributions to number theory, quadratic forms, invariant theory, orthogonal polynomials, elliptic functions, and algebra. One of his students was Henri Poincaré; → conjugate.

An operator A that satisfies the relation A = A, where
A is the adjoint of A. → Hermitian conjugate.

See also: → Hermitian conjugate; → operator.

An operator A that satisfies the relation A = A, where
A is the adjoint of A. → Hermitian conjugate.

See also: → Hermitian conjugate; → operator.

Sir William Herschel (1738-1822), German-born English astronomer, the discoverer of the → infrared radiation and planet → Uranus.
→ Herschelian telescope, → Herschel Satellite

Sir William Herschel (1738-1822), German-born English astronomer, the discoverer of the → infrared radiation and planet → Uranus.
→ Herschelian telescope, → Herschel Satellite

A European Space Agency (ESA) mission to perform imaging photometry and spectroscopy in the → far infrared and → submillimeter regions of the electromagnetic spectrum, covering approximately the 55-672 µm range. In fact Herschel is the first space facility dedicated to these wavelength ranges. It carries a 3.5 m diameter passively cooled mirror. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution → superheterodyne spectrometer (HIFI) - are housed in a superfluid helium cryostat. Herschel was launched on 14 May 2009, together with the → Planck Satellite. Its observing position lies at the L2 → Lagrangian point, some 1.5 million km from Earth. Herschel is designed, among other things, to study the formation of galaxies in the early Universe, and to investigate the formation of stars and their interaction with the → interstellar medium.

See also: → Herschel; → satellite.

A European Space Agency (ESA) mission to perform imaging photometry and spectroscopy in the → far infrared and → submillimeter regions of the electromagnetic spectrum, covering approximately the 55-672 µm range. In fact Herschel is the first space facility dedicated to these wavelength ranges. It carries a 3.5 m diameter passively cooled mirror. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution → superheterodyne spectrometer (HIFI) - are housed in a superfluid helium cryostat. Herschel was launched on 14 May 2009, together with the → Planck Satellite. Its observing position lies at the L2 → Lagrangian point, some 1.5 million km from Earth. Herschel is designed, among other things, to study the formation of galaxies in the early Universe, and to investigate the formation of stars and their interaction with the → interstellar medium.

See also: → Herschel; → satellite.

A → reflecting telescope in which the → primary mirror is tilted so that light is focused near one side of the open end of the tube. The → eyepiece then picks up this light directly, avoiding light loss from reflection by a → secondary mirror. The drawback is → astigmatism, unless the → focal ratio is large. Herschel used this design in his giant 48-inch instrument.

See also: → Herschel; → telescope.

A → reflecting telescope in which the → primary mirror is tilted so that light is focused near one side of the open end of the tube. The → eyepiece then picks up this light directly, avoiding light loss from reflection by a → secondary mirror. The drawback is → astigmatism, unless the → focal ratio is large. Herschel used this design in his giant 48-inch instrument.

See also: → Herschel; → telescope.

The SI unit of frequency, defined as a frequency of 1 cycle per second.

See also: After Heinrich Rudolf Hertz (1857-1894), the German physicist, who made several important contributions to the study of electromagnetism.

The SI unit of frequency, defined as a frequency of 1 cycle per second.

See also: After Heinrich Rudolf Hertz (1857-1894), the German physicist, who made several important contributions to the study of electromagnetism.

A laboratory experiment carried out by Heinrich Hertz in 1888 to generate and detect
→ electromagnetic waves for the first time. It involved a
high voltage power source, consisting of two → capacitors,
each provided with a conducting rod. The rods were separated by a small → spark gap and connected to an → induction coil. When the electrodes were raised to a sufficiently high → potential difference, a spark passed across the gap,
and an oscillating discharge took place. A group of waves with a wavelength of a few meters were emitted at each discharge. A wire loop provided with a detecting spark gap, held away from the oscillating sparks, produced sparks upon arrival of the oscillating electric and magnetic fields.

See also: → hertz (Hz); → experiment.

A laboratory experiment carried out by Heinrich Hertz in 1888 to generate and detect
→ electromagnetic waves for the first time. It involved a
high voltage power source, consisting of two → capacitors,
each provided with a conducting rod. The rods were separated by a small → spark gap and connected to an → induction coil. When the electrodes were raised to a sufficiently high → potential difference, a spark passed across the gap,
and an oscillating discharge took place. A group of waves with a wavelength of a few meters were emitted at each discharge. A wire loop provided with a detecting spark gap, held away from the oscillating sparks, produced sparks upon arrival of the oscillating electric and magnetic fields.

See also: → hertz (Hz); → experiment.

→ frequency to wavelength conversion.

See also: → hertz; → meter; → conversion.

→ frequency to wavelength conversion.

See also: → hertz; → meter; → conversion.

An electrical system used for the production of → electromagnetic waves. It consists of two equal → capacitors connected to two electrodes with a → spark gap between the electrodes. The system is connected to an → induction coil. When the induction coil is activated, electromagnetic waves are generated across the spark gap. See also → Hertz experiment.

See also: → hertz (Hz); → oscillator.

An electrical system used for the production of → electromagnetic waves. It consists of two equal → capacitors connected to two electrodes with a → spark gap between the electrodes. The system is connected to an → induction coil. When the induction coil is activated, electromagnetic waves are generated across the spark gap. See also → Hertz experiment.

See also: → hertz (Hz); → oscillator.

A region of the → Hertzsprung-Russell diagram, between the → main sequence and the
→ giant branch, occupied by very few stars. It corresponds to a very short period in stellar evolution.

See also: Named after the Danish astronomer Ejnar Hertzsprung (1873-1967), who first noticed this phenomenon; → gap

A region of the → Hertzsprung-Russell diagram, between the → main sequence and the
→ giant branch, occupied by very few stars. It corresponds to a very short period in stellar evolution.

See also: Named after the Danish astronomer Ejnar Hertzsprung (1873-1967), who first noticed this phenomenon; → gap

A display of stellar properties using a plot of
→ effective temperature (or instead → color or → spectral type) along the abscissa versus
→ luminosity (or → absolute magnitude). The temperature is plotted in the inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity → massive stars to low temperature-low luminosity → low-mass stars. This feature is known as the → main sequence. This is the locus of stars burning hydrogen in their cores (→ proton-proton chain). The lower edge of this strip, known as the → zero age main sequence (ZAMS), designates the positions
where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of → white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (→ white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called → red giants, they are still burning helium in their cores (→ helium burning, → carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the → supergiants. They can be hot or cool, hence blue or red in color. Same as → H-R diagram.

See also:
→ asymptotic giant branch, → blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → Hayashi track, → horizontal branch, → post-asymptotic giant branch star, → red giant branch, → supra-horizontal branch star, → zero age horizontal branch star, → Humphreys-Davidson limit.

See also: Named after the Danish Ejnar Hertzsprung (1873-1967) and the American Henry Norris Russell (1877-1957). However,
the first H-R diagram was published not by Hertzpurung neither Russell, but by a PhD student of Karl Schwarzschild at Göttingen. The student was Hans Rosenberg (1879-1940), who in 1910 published the diagram for stars in the → Pleiades (Astronomische Nachrichten, Vol. 186 (4445), p. 71, 1910). Although Hertzpurung had a very preliminary diagram in 1908, his first proper diagram was published in 1911. Likewise, Russell published his version only in 1915 with the better and more numerous data then available (Nielsen, A.V., 1969, Centaurus 9, 219; Valls-Gabaud, D., 2002, Observed HR diagrams and stellar evolution, ASP Conf. Proceedings, Vol. 274. Edited by Thibault Lejeune and João Fernandes);
→ diagram.

A display of stellar properties using a plot of
→ effective temperature (or instead → color or → spectral type) along the abscissa versus
→ luminosity (or → absolute magnitude). The temperature is plotted in the inverse direction, with high temperatures on the left and low temperatures on the right. On the diagram the majority of stars are concentrated in a diagonal strip running from upper left to lower right, i.e. from high temperature-high luminosity → massive stars to low temperature-low luminosity → low-mass stars. This feature is known as the → main sequence. This is the locus of stars burning hydrogen in their cores (→ proton-proton chain). The lower edge of this strip, known as the → zero age main sequence (ZAMS), designates the positions
where stars of different mass first begin to burn hydrogen in their cores. Well below the main sequence there is a group of stars that, despite being very hot, are so small that their luminosity is very small as a consequence. These are the class of → white dwarfs. These objects represent old and very evolved stars that have shed their outer layers to reveal a very small but extremely hot inner core. They are no longer generating energy but are merely emitting light as they cool (→ white dwarf cooling track). Stars with high luminosities but relatively low temperatures occupy a wide region above the main sequence. The majority of them have used up all the hydrogen in their cores and have expanded and cooled as a result of internal readjustment. Called → red giants, they are still burning helium in their cores (→ helium burning, → carbon burning). There are also stars with very high luminosities, resulting from their enormous outputs of energy, because they are burning their fuel at a prodigious rate. These are the → supergiants. They can be hot or cool, hence blue or red in color. Same as → H-R diagram.

See also:
→ asymptotic giant branch, → blue horizontal branch star, → extreme horizontal branch star, → field horizontal branch star, → Hayashi track, → horizontal branch, → post-asymptotic giant branch star, → red giant branch, → supra-horizontal branch star, → zero age horizontal branch star, → Humphreys-Davidson limit.

See also: Named after the Danish Ejnar Hertzsprung (1873-1967) and the American Henry Norris Russell (1877-1957). However,
the first H-R diagram was published not by Hertzpurung neither Russell, but by a PhD student of Karl Schwarzschild at Göttingen. The student was Hans Rosenberg (1879-1940), who in 1910 published the diagram for stars in the → Pleiades (Astronomische Nachrichten, Vol. 186 (4445), p. 71, 1910). Although Hertzpurung had a very preliminary diagram in 1908, his first proper diagram was published in 1911. Likewise, Russell published his version only in 1915 with the better and more numerous data then available (Nielsen, A.V., 1969, Centaurus 9, 219; Valls-Gabaud, D., 2002, Observed HR diagrams and stellar evolution, ASP Conf. Proceedings, Vol. 274. Edited by Thibault Lejeune and João Fernandes);
→ diagram.

The Martian geologic era after the Noachian Era which lasted from about 3500 million to 2500 million years ago. During this period Martian climate began to change to drier, dustier conditions. Water that flowed on the Martian surface during the Noachian Era may have frozen as underground ice deposits, and most river channels probably experienced their final flow episodes during this era. → Noachian era; → Amazonian era.

See also: Named after the Martian plains of Hesperis; → era.

The Martian geologic era after the Noachian Era which lasted from about 3500 million to 2500 million years ago. During this period Martian climate began to change to drier, dustier conditions. Water that flowed on the Martian surface during the Noachian Era may have frozen as underground ice deposits, and most river channels probably experienced their final flow episodes during this era. → Noachian era; → Amazonian era.

See also: Named after the Martian plains of Hesperis; → era.

An → evening star, especially the planet Venus in its appearance as the evening star.

Etymology (EN): M.E., from L., from Gk. hesperos “evening, western;” → west.

Etymology (PE): Setâre-ye šâmgâh “evening star,” from setâré→ star + šâmgâh “evening,” from šâm “evening, evening meal” + gâh “time.” The first component, šâm, from Mid.Pers. šâm “evening meal, supper,” from Av. xšāfnya- “evening meal,” from Av. xšap-, xšapā-, xšapan-, xšafn- “night” (O.Pers. xšap- “night,” Mid.Pers. šap, Mod.Pers. šab “night”); cf. Skt. ksap- “nigh, darkness;” Hittite ispant- “night.” The second component gâh “time,” Mid.Pers. gâh, gâs “time,”
O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”

An → evening star, especially the planet Venus in its appearance as the evening star.

Etymology (EN): M.E., from L., from Gk. hesperos “evening, western;” → west.

Etymology (PE): Setâre-ye šâmgâh “evening star,” from setâré→ star + šâmgâh “evening,” from šâm “evening, evening meal” + gâh “time.” The first component, šâm, from Mid.Pers. šâm “evening meal, supper,” from Av. xšāfnya- “evening meal,” from Av. xšap-, xšapā-, xšapan-, xšafn- “night” (O.Pers. xšap- “night,” Mid.Pers. šap, Mod.Pers. šab “night”); cf. Skt. ksap- “nigh, darkness;” Hittite ispant- “night.” The second component gâh “time,” Mid.Pers. gâh, gâs “time,”
O.Pers. gāθu-, Av. gātav-, gātu- “place, throne, spot;” cf. Skt. gâtu- “going, motion; free space for moving; place of abode;” PIE *gwem- “to go, come.”

→ High Energy Stereoscopic System (H.E.S.S.).

See also: → H.E.S.S.; → collaboration.

→ High Energy Stereoscopic System (H.E.S.S.).

See also: → H.E.S.S.; → collaboration.

A diagram showing the relative density of occurrence of stars at various → color-magnitude positions of the → Hertzsprung-Russell diagram for a given → galaxy.

See also: Named after R. Hess who originated it in 1924: “Die Verteilungsfunktion der absoluten Helligkeiten in ihrer Abhängigkeit vom Spektrum”. Probleme der Astronomie. Festschrift fur Hugo v. Seeliger. Springer, Berlin. p. 265; → diagram.

A diagram showing the relative density of occurrence of stars at various → color-magnitude positions of the → Hertzsprung-Russell diagram for a given → galaxy.

See also: Named after R. Hess who originated it in 1924: “Die Verteilungsfunktion der absoluten Helligkeiten in ihrer Abhängigkeit vom Spektrum”. Probleme der Astronomie. Festschrift fur Hugo v. Seeliger. Springer, Berlin. p. 265; → diagram.

Prefix denoting “other, different.”

Etymology (EN): From Gk. heteros “the other (of two), another, different.”

Etymology (PE): Degar “another, other;” from Mid.Pers. dit, ditikar “the other, the second;” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second;” Skt. dvitiya- “second,” PIE *duitiio- “second.”

Prefix denoting “other, different.”

Etymology (EN): From Gk. heteros “the other (of two), another, different.”

Etymology (PE): Degar “another, other;” from Mid.Pers. dit, ditikar “the other, the second;” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second;” Skt. dvitiya- “second,” PIE *duitiio- “second.”

1. Denoting a device or method of combining two → electromagnetic waves of different → frequency (a locally generated wave and an incoming wave)
   in a → nonlinear device to produce two frequencies which are
   equal to the → sum and → difference of the first two. The phenomenon is the counterpart of → beats produced by → sound waves. For example, heterodyning a 100-kHz and a 10-kHz signal will produce a 110-KHz and a 90-kHz signal. See also → homodyne.
   

2. The term heterodyne is often loosely used instead of
   → superheterodyne in the radio frequency field.

See also: Heterodyne, from → hetero- + -dyne, from Gk. dynamics→ dynamics; → receiver.

1. Denoting a device or method of combining two → electromagnetic waves of different → frequency (a locally generated wave and an incoming wave)
   in a → nonlinear device to produce two frequencies which are
   equal to the → sum and → difference of the first two. The phenomenon is the counterpart of → beats produced by → sound waves. For example, heterodyning a 100-kHz and a 10-kHz signal will produce a 110-KHz and a 90-kHz signal. See also → homodyne.
   

2. The term heterodyne is often loosely used instead of
   → superheterodyne in the radio frequency field.

See also: Heterodyne, from → hetero- + -dyne, from Gk. dynamics→ dynamics; → receiver.

An → interferometer using a technique that involves introducing a small → frequency shift between the optical frequencies of the two interfering light beams. This results in an intensity modulation at the → beat frequency of the two beams for any given point of the → interference pattern. A convenient way of introducing such a frequency shift is by means of an acousto-optic modulator.

See also: → heterodyne; → interferometer.

An → interferometer using a technique that involves introducing a small → frequency shift between the optical frequencies of the two interfering light beams. This results in an intensity modulation at the → beat frequency of the two beams for any given point of the → interference pattern. A convenient way of introducing such a frequency shift is by means of an acousto-optic modulator.

See also: → heterodyne; → interferometer.

→ superheterodyne receiver.

See also: → heterodyne; → receiver.

→ superheterodyne receiver.

See also: → heterodyne; → receiver.

→ superheterodyne technique.

See also: → heterodyne; → technique.

→ superheterodyne technique.

See also: → heterodyne; → technique.

The quality or state of being → heterogeneous. See also → homogeneity, → inhomogeneity.

See also: Noun from → heterogeneous.

The quality or state of being → heterogeneous. See also → homogeneity, → inhomogeneity.

See also: Noun from → heterogeneous.

1. Composed of parts of different kinds; having widely dissimilar elements or constituents. See also → homogeneous, → inhomogeneous.
   

2. Chemistry: A mixture that does not have uniform composition and properties throughout; composed of different substances or the same substance in different phases.

See also: → hetero- + -genous, → homogeneous.

1. Composed of parts of different kinds; having widely dissimilar elements or constituents. See also → homogeneous, → inhomogeneous.
   

2. Chemistry: A mixture that does not have uniform composition and properties throughout; composed of different substances or the same substance in different phases.

See also: → hetero- + -genous, → homogeneous.

Based on chemical composition, the atmosphere is divided into two broad layers: the → homosphere and the heterosphere. The heterosphere has heterogeneous chemical composition, with layered structure, of nitrogen, oxygen, helium, and hydrogen, respectively.
The heterosphere begins from about 90 km from the Earth’s surface and extends to space.

See also: → hetero-; → shere.

Based on chemical composition, the atmosphere is divided into two broad layers: the → homosphere and the heterosphere. The heterosphere has heterogeneous chemical composition, with layered structure, of nitrogen, oxygen, helium, and hydrogen, respectively.
The heterosphere begins from about 90 km from the Earth’s surface and extends to space.

See also: → hetero-; → shere.

Methodology, Math.: Pertaining to a method of analyzing outcome through comparison to previously recognized patterns in the absence of an → algorithm for formal proof.

Etymology (EN): From L. heuristicus (from Gk. heuretikos “inventive,” related to heuriskein “to find,” from heur-) + -isticus, → -ic.

Etymology (PE): Yâftik, from yâft past tense of yâftan, yâb- “to → find” + -ik, → -ic.

Methodology, Math.: Pertaining to a method of analyzing outcome through comparison to previously recognized patterns in the absence of an → algorithm for formal proof.

Etymology (EN): From L. heuristicus (from Gk. heuretikos “inventive,” related to heuriskein “to find,” from heur-) + -isticus, → -ic.

Etymology (PE): Yâftik, from yâft past tense of yâftan, yâb- “to → find” + -ik, → -ic.

A prefix meaning → six. → hexagon.

See also: → six.

A prefix meaning → six. → hexagon.

See also: → six.

A six-sided → polygon.

See also: → hexa-; → -gon;.

A six-sided → polygon.

See also: → hexa-; → -gon;.