An Etymological Dictionary of Astronomy and Astrophysics

Edwin Powell Hubble (1889-1953), the American astronomer who provided the
observational evidence of the expansion of the Universe in 1929; → Hubble-Lemaitre law.

Edwin Powell Hubble (1889-1953), the American astronomer who provided the
observational evidence of the expansion of the Universe in 1929; → Hubble-Lemaitre law.

The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories
(E0, E1, …, E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively.
Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

See also: → Hubble; → classification.

The classification of galaxies according to their visual appearance into four basic types suggested by E. Hubble: → ellipticals (E), → spirals (S), → barred spirals (SB), and → irregulars (Ir). Later on a separate class of → lenticulars (S0) was appended as an intermediate type between ellipticals and spirals. The sequence starts with round elliptical galaxies (E0). Flatter galaxies are arranged following a number which is calculated from the ratio (a - b)/a, where a and b are the major and minor axes as measured on the sky. Ellipticals are divided into eight categories
(E0, E1, …, E7). Beyond E7 a clear disk is apparent in the lenticular (S0) galaxies. The sequence then splits into two parallel branches of disk galaxies showing spiral structure: ordinary spirals, S, and barred spirals, SB. The spiral and barred types are subdivided into Sa, Sb, Sc, and SBa, SBb, SBc, respectively.
Along the sequence from Sa to Sc, the central bulge becomes smaller, while the spiral arms become more and more paramount. The original, erroneous idea that such arrangement of the galaxies might represent an evolutionary sequence led to the ellipticals being referred to as early-type galaxies, and the spirals and Irr I irregulars as late-type galaxies. See also → dwarf galaxy, → dwarf elliptical galaxy, → dwarf spheroidal galaxy.

See also: → Hubble; → classification.

→ Hubble-Lemaitre constant.

See also: → Hubble; → constant.

→ Hubble-Lemaitre constant.

See also: → Hubble; → constant.

An image of a small region in the constellation → Ursa Major, based on the results of a series of observations by the → Hubble Space Telescope. The image was assembled from 342 separate exposures taken over ten consecutive days between December 18 and December 28, 1995. It covers an area 144 arcseconds across.

See also: → Hubble; → deep;
→ field.

An image of a small region in the constellation → Ursa Major, based on the results of a series of observations by the → Hubble Space Telescope. The image was assembled from 342 separate exposures taken over ten consecutive days between December 18 and December 28, 1995. It covers an area 144 arcseconds across.

See also: → Hubble; → deep;
→ field.

A plot of the → redshift of galaxies against their distance or against their → apparent magnitude.

See also: → Hubble; → diagram.

A plot of the → redshift of galaxies against their distance or against their → apparent magnitude.

See also: → Hubble; → diagram.

The distance from the Earth to the → cosmic horizon which marks the edge of the → observable Universe. Same as
→ Hubble radius, → Hubble length, and → cosmic horizon.

See also: → Hubble; → distance.

The distance from the Earth to the → cosmic horizon which marks the edge of the → observable Universe. Same as
→ Hubble radius, → Hubble length, and → cosmic horizon.

See also: → Hubble; → distance.

→ Hubble-Lemaitre flow.

See also: → Hubble; → flow.

→ Hubble-Lemaitre flow.

See also: → Hubble; → flow.

→ Hubble-Lemaitre law.

See also: → Hubble; → law.

→ Hubble-Lemaitre law.

See also: → Hubble; → law.

The distance traveled by light along a straight → geodesic in one → Hubble time. Also called the → Hubble radius, → Hubble distance, and → cosmic horizon.

See also: → Hubble; → length.

The distance traveled by light along a straight → geodesic in one → Hubble time. Also called the → Hubble radius, → Hubble distance, and → cosmic horizon.

See also: → Hubble; → length.

→ Hubble-Lemaitre parameter.

See also: → Hubble; → parameter.

→ Hubble-Lemaitre parameter.

See also: → Hubble; → parameter.

The size of the observable Universe as derived from the ratio c/H₀, where H₀ is the → Hubble-Lemaitre constant and c the → speed of light. Same as → Hubble distance, → Hubble length, and → cosmic horizon.

See also: → Hubble; → radius.

The size of the observable Universe as derived from the ratio c/H₀, where H₀ is the → Hubble-Lemaitre constant and c the → speed of light. Same as → Hubble distance, → Hubble length, and → cosmic horizon.

See also: → Hubble; → radius.

A classification scheme in which galaxies are ordered into a sequence based on their morphology.
Same as the → Hubble classification.

See also: → Hubble; → sequence.

A classification scheme in which galaxies are ordered into a sequence based on their morphology.
Same as the → Hubble classification.

See also: → Hubble; → sequence.

A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that worked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.

See also: → Hubble; → space;
→ telescope.

A telescope of 2.4 m in diameter, a joint NASA and ESA project, launched in 1990 into a low-Earth orbit 600 km above the ground. It was equipped with a collection of several science instruments that worked across the entire optical spectrum (from infrared, through the visible, to ultraviolet light). During its lifetime Hubble has become one of the most important science projects ever.

See also: → Hubble; → space;
→ telescope.

An estimate for the age of the Universe by presuming that the Universe has always expanded at the same rate as it is expanding today. It is the inverse of the → Hubble-Lemaitre constant: t_H = 1/H₀.
Also called the Hubble age or the Hubble period.

See also: → Hubble; → time.

An estimate for the age of the Universe by presuming that the Universe has always expanded at the same rate as it is expanding today. It is the inverse of the → Hubble-Lemaitre constant: t_H = 1/H₀.
Also called the Hubble age or the Hubble period.

See also: → Hubble; → time.

The → Hubble parameter for the → present epoch. It is the constant of proportionality between the → recession velocities of galaxies and their distances from each other. The latest determinations using the → Hubble Space Telescope observations of → Cepheids give H₀ = 72 ± 8 km s^-1 Mpc^-1 (W. L. Freedman et al., 2001, ApJ 553, 47, arXiv:astro-ph/0012376), the
→ WMAP observations yield
70.4 ± 1.3 km s^-1 Mpc^-1 (N. Jarosik et al., 2011, ApJS 192, 14, arXiv:1001.4744), and the → Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1 (Planck Collaboration, 2014, A&A 571, A16, arXiv:1303.5076). More recently, the Hubble constant
was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy (Adam G. Reiss et al., 2016, arXiv:1604.01424). The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The → Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.

See also: → Hubble; → Friedmann-Lemaitre Universe; → constant.

The → Hubble parameter for the → present epoch. It is the constant of proportionality between the → recession velocities of galaxies and their distances from each other. The latest determinations using the → Hubble Space Telescope observations of → Cepheids give H₀ = 72 ± 8 km s^-1 Mpc^-1 (W. L. Freedman et al., 2001, ApJ 553, 47, arXiv:astro-ph/0012376), the
→ WMAP observations yield
70.4 ± 1.3 km s^-1 Mpc^-1 (N. Jarosik et al., 2011, ApJS 192, 14, arXiv:1001.4744), and the → Planck Satellite observations give 67.3 ± 1.2 km s^-1 Mpc^-1 (Planck Collaboration, 2014, A&A 571, A16, arXiv:1303.5076). More recently, the Hubble constant
was derived by a team of astronomers, using the NASA/ESA Hubble Space Telescope, with a 2.4% accuracy (Adam G. Reiss et al., 2016, arXiv:1604.01424). The new value, 73.2 km s^-1 Mpc^-1, suggests that the Universe is expanding between five and nine percent faster than previously calculated. The → Hubble law is only applicable for large distances (> 20 Mpc), when the proper motions of galaxies in groups and clusters cannot confuse the recession due to expansion.

See also: → Hubble; → Friedmann-Lemaitre Universe; → constant.

The general outward motion of → galaxy clusters resulting from the → expansion of the Universe.

See also: → Hubble-Lemaitre law; → flow.

The general outward motion of → galaxy clusters resulting from the → expansion of the Universe.

See also: → Hubble-Lemaitre law; → flow.

The speed with which a → galaxy cluster recedes from us is directly proportional to its distance. It can be stated as v = H₀d, where v is the recessional velocity, H₀ the → Hubble-Lamaitre constant, and d the distance. See also → Hubble-Lemaitre flow.

It should be underlined that Hubble was not the first to discover the → velocity-distance relation. Two years before Hubble, in 1927, Georges Lemaître (1894-1966) had derived the relation and published it in a paper in French which remained neglected (→ Friedmann-Lemaitre Universe).

See also: The International Astronomical Union (IAU) at its 30th Meeting approved the Resolution B4 proposed by the IAU Executive Committee recommending the use of Hubble-Lemaitre law instead of Hubble’s law, after Edwin Hubble (1889-1953), the American astronomer who published his results in 1929 and Georges Lemaître, Belgian priest and astronomer, who published a paper on the expansion of the Universe in 1927; → law.

The speed with which a → galaxy cluster recedes from us is directly proportional to its distance. It can be stated as v = H₀d, where v is the recessional velocity, H₀ the → Hubble-Lamaitre constant, and d the distance. See also → Hubble-Lemaitre flow.

It should be underlined that Hubble was not the first to discover the → velocity-distance relation. Two years before Hubble, in 1927, Georges Lemaître (1894-1966) had derived the relation and published it in a paper in French which remained neglected (→ Friedmann-Lemaitre Universe).

See also: The International Astronomical Union (IAU) at its 30th Meeting approved the Resolution B4 proposed by the IAU Executive Committee recommending the use of Hubble-Lemaitre law instead of Hubble’s law, after Edwin Hubble (1889-1953), the American astronomer who published his results in 1929 and Georges Lemaître, Belgian priest and astronomer, who published a paper on the expansion of the Universe in 1927; → law.

The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.

See also: → Hubble-Lemaitre law; → parameter.

The rate pf change of the → cosmic scale factor: H(t) = (dR/dt)/R. The Hubble parameter is a time-dependent quantity and therefore is not constant. The → Hubble-Lemaitre constant is the Hubble-Lemaître parameter measured today.

See also: → Hubble-Lemaitre law; → parameter.

The discrepancy between the value of the → Hubble-Lemaitre constant inferred from a ΛCDM fit (→ Lambda cold dark matter model) to the → cosmic microwave background (CMB) and local measurements. The Universe appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents. Based on the → Hubble Space Telescope observations, the Hubble-Lemaitre constant is very recently estimated to be 74.03 km s^-1 Mpc^-1. This value indicates that the Universe is expanding at a rate about 9% faster than that implied by the → Planck satellite’s observations of the → early Universe, which give a value for the Hubble constant of 67.4 km s^-1 Mpc^-1. For discussion, see D’Arcy Kenworthy et al. (2019, ApJ 875, 145).

See also: → Hubble-Lamaitre constant; → tension.

The discrepancy between the value of the → Hubble-Lemaitre constant inferred from a ΛCDM fit (→ Lambda cold dark matter model) to the → cosmic microwave background (CMB) and local measurements. The Universe appears to be expanding much faster now than predicted even with our latest understanding of its initial conditions and contents. Based on the → Hubble Space Telescope observations, the Hubble-Lemaitre constant is very recently estimated to be 74.03 km s^-1 Mpc^-1. This value indicates that the Universe is expanding at a rate about 9% faster than that implied by the → Planck satellite’s observations of the → early Universe, which give a value for the Hubble constant of 67.4 km s^-1 Mpc^-1. For discussion, see D’Arcy Kenworthy et al. (2019, ApJ 875, 145).

See also: → Hubble-Lamaitre constant; → tension.

Same as the → Hubble classification.

See also: → Hubble; → Hubble-Sandage variable; → sequence.

Same as the → Hubble classification.

See also: → Hubble; → Hubble-Sandage variable; → sequence.

A type of highly luminous → blue supergiant star with variable light, first discovered in the M31 and M33 galaxies; also called
→ S Doradus stars. They are now believed
to be part of the class of → Luminous Blue Variable stars.

See also: → Hubble; Allan Rex Sandage (1926-2010), American cosmologist.

A type of highly luminous → blue supergiant star with variable light, first discovered in the M31 and M33 galaxies; also called
→ S Doradus stars. They are now believed
to be part of the class of → Luminous Blue Variable stars.

See also: → Hubble; Allan Rex Sandage (1926-2010), American cosmologist.

A region of the Universe, nearly a billion light-years across, mostly devoid of stars, gas, other normal matter, and also
→ dark matter. Situated at about 6 billion light-years from us, in projection on the the constellation → Eridanus, it shows up as a particularly cold region in the map of the → cosmic microwave background (CMB) radiation. Observations made using the → Very Large Array (VLA) radio telescope show a relative absence of matter in that area.

Etymology (EN): Huge, from M.E. huge, hoge, from O.F. ahuge, ahoge “enormous,” from a variant of → ad- + hoge “height,” → high; → hole.

Etymology (PE): Surâx, → hole; kalân “great, large, bulky.”

A region of the Universe, nearly a billion light-years across, mostly devoid of stars, gas, other normal matter, and also
→ dark matter. Situated at about 6 billion light-years from us, in projection on the the constellation → Eridanus, it shows up as a particularly cold region in the map of the → cosmic microwave background (CMB) radiation. Observations made using the → Very Large Array (VLA) radio telescope show a relative absence of matter in that area.

Etymology (EN): Huge, from M.E. huge, hoge, from O.F. ahuge, ahoge “enormous,” from a variant of → ad- + hoge “height,” → high; → hole.

Etymology (PE): Surâx, → hole; kalân “great, large, bulky.”

A band in the → absorption spectrum of → ozone (O₃) extending in the → ultraviolet from 310 nm to 370 nm. It is located at the red end of the strong → Hartley band.

See also: W. Huggins and M. Huggins, Proc. R. Soc. London 48, 216 (1890).

A band in the → absorption spectrum of → ozone (O₃) extending in the → ultraviolet from 310 nm to 370 nm. It is located at the red end of the strong → Hartley band.

See also: W. Huggins and M. Huggins, Proc. R. Soc. London 48, 216 (1890).

A curve, on the pressure versus specific volume plane,
representing the locus of all the possible states that can be reached by a substance immediately after the passage of a single → shock wave. For each initial condition there is a different curve. No combustion occurs in the process and, therefore, the chemical composition of the medium does not change. See also → Rayleigh line; → Crussard curve.

See also: Named after the French physicist Pierre Henri Hugoniot (1851-1887), who worked on fluid mechanics, especially flow properties before and after shock waves; → curve.

A curve, on the pressure versus specific volume plane,
representing the locus of all the possible states that can be reached by a substance immediately after the passage of a single → shock wave. For each initial condition there is a different curve. No combustion occurs in the process and, therefore, the chemical composition of the medium does not change. See also → Rayleigh line; → Crussard curve.

See also: Named after the French physicist Pierre Henri Hugoniot (1851-1887), who worked on fluid mechanics, especially flow properties before and after shock waves; → curve.

A → pulsar with a period of 59 milliseconds (17 pulses per second) moving around a compact companion in an elongated orbit (period 7.75 hours). It is thought that the companion is probably also a → neutron star with the same mass as the pulsar (1.4 solar masses). The orbit is gradually shrinking because of → gravitational radiation, as predicted by the theory of → general relativity. See also → binary pulsar, → millisecond pulsar.

Etymology (EN): Named after the American physicists Russell Hulse and Joseph Taylor of Princeton University, who discovered the pulsar in 1974, for which they shared the 1993 Nobel prize in physics; → pulsar.

A → pulsar with a period of 59 milliseconds (17 pulses per second) moving around a compact companion in an elongated orbit (period 7.75 hours). It is thought that the companion is probably also a → neutron star with the same mass as the pulsar (1.4 solar masses). The orbit is gradually shrinking because of → gravitational radiation, as predicted by the theory of → general relativity. See also → binary pulsar, → millisecond pulsar.

Etymology (EN): Named after the American physicists Russell Hulse and Joseph Taylor of Princeton University, who discovered the pulsar in 1974, for which they shared the 1993 Nobel prize in physics; → pulsar.

1. (adj.) Belonging or pertaining to or of the nature of man or mankind, contrasted with animals.
   

2. (n.) A human being, → man. See also → anthropo-.

Etymology (EN): M.E. from M.F. humain, from L. humanus “of man, human,” also “humane, kind, gentle, polite,” probably related to homo “man,” and to humus “earth,” on notion of “earthly beings.”

Etymology (PE): Martu, → man, + -gân a suffix forming nouns or adjectives denoting relation and plurality.

1. (adj.) Belonging or pertaining to or of the nature of man or mankind, contrasted with animals.
   

2. (n.) A human being, → man. See also → anthropo-.

Etymology (EN): M.E. from M.F. humain, from L. humanus “of man, human,” also “humane, kind, gentle, polite,” probably related to homo “man,” and to humus “earth,” on notion of “earthly beings.”

Etymology (PE): Martu, → man, + -gân a suffix forming nouns or adjectives denoting relation and plurality.

1. Any system or mode of thought or action in which human interests, values, and dignity predominate (dictionary.com).
   

2. Philo.: A variety of ethical theory and practice that emphasizes reason, scientific inquiry, and human fulfillment in the natural world and often rejects the importance of belief in God (dictionary.com).

See also: → human + → -ism.

1. Any system or mode of thought or action in which human interests, values, and dignity predominate (dictionary.com).
   

2. Philo.: A variety of ethical theory and practice that emphasizes reason, scientific inquiry, and human fulfillment in the natural world and often rejects the importance of belief in God (dictionary.com).

See also: → human + → -ism.

A person having a strong interest in or concern for human welfare, values, and dignity (dictionary.com).

See also: → human + → -ist.

A person having a strong interest in or concern for human welfare, values, and dignity (dictionary.com).

See also: → human + → -ist.

Having concern for or helping to improve the welfare and happiness of people
(dictionary.com).

Etymology (EN): → humanity + -arian a suffix forming nouns and adjectives, from -ari(us) or -ary + -an.

Etymology (PE): Martugândust, literally “friend/lover of humanity,” from martugân, → humanity, + dust “friend,” Mid.Pers. dôst “friend,” dôšidan
“to love, like, choose;” O.Pers. dauštā- “friend;” Av. zuš- “to take pleasure;” PIE root *geus- “to taste, like, choose;” cf. Skt. jos- “to like, enjoy;” Gk. geuomai, L. gustus “taste, enjoyment” (Cheung 2007).

Having concern for or helping to improve the welfare and happiness of people
(dictionary.com).

Etymology (EN): → humanity + -arian a suffix forming nouns and adjectives, from -ari(us) or -ary + -an.

Etymology (PE): Martugândust, literally “friend/lover of humanity,” from martugân, → humanity, + dust “friend,” Mid.Pers. dôst “friend,” dôšidan
“to love, like, choose;” O.Pers. dauštā- “friend;” Av. zuš- “to take pleasure;” PIE root *geus- “to taste, like, choose;” cf. Skt. jos- “to like, enjoy;” Gk. geuomai, L. gustus “taste, enjoyment” (Cheung 2007).

The study of classical languages and classical literature.

Etymology (EN): Plural of → humanity.

Etymology (PE): Martugânik, from martugân, → human, + → -ik, → -ics.

The study of classical languages and classical literature.

Etymology (EN): Plural of → humanity.

Etymology (PE): Martugânik, from martugân, → human, + → -ik, → -ics.

1. All human beings collectively; the human race; humankind.
   

2. The quality of being human; human nature.

See also: → human + → -ity.

1. All human beings collectively; the human race; humankind.
   

2. The quality of being human; human nature.

See also: → human + → -ity.

1. To render humane, kind, or gentle.
   

2. To make human.

See also: → human; → -ize.

1. To render humane, kind, or gentle.
   

2. To make human.

See also: → human; → -ize.

A cold ocean current that flows northward along the western side of South America, offshore Chile and Peru. Dominate weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the weather pattern known as El Niño.

See also: Named after the German naturalist and explorer Alexander von Humboldt (1769-1859). → current.

A cold ocean current that flows northward along the western side of South America, offshore Chile and Peru. Dominate weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the weather pattern known as El Niño.

See also: Named after the German naturalist and explorer Alexander von Humboldt (1769-1859). → current.

Containing or characterized by a high amount of water or water vapor; moist. → humidity.

Etymology (EN): Adj. of → humidity.

Etymology (PE): Namnâk, namur, from nam, → humidity + adj. suffixes -nâk and -ur, variant -var (Mid.Pers. -uwar, -war, from O.Pers. -bara, from bar- “to bear, carry”), as in ranjur, ganjur, dastur.

Containing or characterized by a high amount of water or water vapor; moist. → humidity.

Etymology (EN): Adj. of → humidity.

Etymology (PE): Namnâk, namur, from nam, → humidity + adj. suffixes -nâk and -ur, variant -var (Mid.Pers. -uwar, -war, from O.Pers. -bara, from bar- “to bear, carry”), as in ranjur, ganjur, dastur.

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with → relative humidity. → absolute humidity, → dew point, → mixing ratio, → specific humidity.

Etymology (EN): Humidity, from O.Fr. humide, from L. humidus “moist, wet,” variant (by influence of humus “earth”) of umidus, from umere “be moist.”

Etymology (PE): Nam “humidity, moisture,” from Mid.Pers. nam, namb “moisture;” Av. napta- “moist,” nabās-câ- “cloud,” nabah- “sky;” cf. Skt. nábhas- “moisture, cloud, mist;” Gk. nephos “cloud, mass of clouds,” nephele “cloud;” L. nebula “mist,” nimbus “rainstorm, rain cloud;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” from PIE *nebh- “cloud, vapor, fog, moist, sky.”

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with → relative humidity. → absolute humidity, → dew point, → mixing ratio, → specific humidity.

Etymology (EN): Humidity, from O.Fr. humide, from L. humidus “moist, wet,” variant (by influence of humus “earth”) of umidus, from umere “be moist.”

Etymology (PE): Nam “humidity, moisture,” from Mid.Pers. nam, namb “moisture;” Av. napta- “moist,” nabās-câ- “cloud,” nabah- “sky;” cf. Skt. nábhas- “moisture, cloud, mist;” Gk. nephos “cloud, mass of clouds,” nephele “cloud;” L. nebula “mist,” nimbus “rainstorm, rain cloud;” O.H.G. nebul; Ger. Nebel “fog;” O.E. nifol “dark;” from PIE *nebh- “cloud, vapor, fog, moist, sky.”

A series of → spectral lines in the → infrared spectrum of → neutral hydrogen emitted by electrons in → excited states transitioning to the level described by the → principal quantum number  n = 6. It begins at 12368 nm (Hu α 12.37 microns) and has been traced to 3281.4 nm (3.28 microns).

See also: Named after Curtis J. Humphreys (1898-1986), American physicist; → series.

A series of → spectral lines in the → infrared spectrum of → neutral hydrogen emitted by electrons in → excited states transitioning to the level described by the → principal quantum number  n = 6. It begins at 12368 nm (Hu α 12.37 microns) and has been traced to 3281.4 nm (3.28 microns).

See also: Named after Curtis J. Humphreys (1898-1986), American physicist; → series.

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type
→ hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

See also: Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

An empirical upper → luminosity boundary in the → H-R diagram. It consists of two sections, a sloping part and a horizontal part. The sloping part, which decreases with decreasing → effective temperature, corresponds roughly to the → Eddington limit. The horizontal part is the temperature-independent upper luminosity limit for late-type
→ hypergiants. It is thought that → massive stars above the Humphreys-Davidson limit encounter an → instability, possibly due to the opacity-modified Eddington limit, and experience high → mass loss episodes which prevent their evolution to cooler temperatures. → Luminous Blue Variable stars are examples of this high mass loss phase.

See also: Named after Roberta M. Humphreys and Kris Davidson, who first dealt with this limit (1979, ApJ 232, 409); → limit.

An empirical rule stating that all orbitals of a given sublevel must be occupied by single electrons before pairing begins.

See also: After the German physicist Friedrich Hund (1896-1997), known for his work on atoms and molecules. → rule.

An empirical rule stating that all orbitals of a given sublevel must be occupied by single electrons before pairing begins.

See also: After the German physicist Friedrich Hund (1896-1997), known for his work on atoms and molecules. → rule.

The smallest three digit number in the decimal system and the smallest square of a two-digit number (10).

Etymology (EN): Hundred, from O.E. hundred “a counting of 100,” from P.Gmc. *hunda- “hundred,” as below, + *rath “reckoning, number.”

Etymology (PE): Sad “hundred,” from Mid.Pers. sad, sat, Av. sata- “hundred,” satô.raocana- “with a hundred windows,” satô.təmô.sata- “hundreds of hundred;” cf. Skt. śatá- “hundred;” Gk. hekaton; L. centum; Lith. simtas; P.Gmc. *hunda- “hundred” (Goth. hund; O.H.G. hunt); PIE *kmtom “hundred.”

The smallest three digit number in the decimal system and the smallest square of a two-digit number (10).

Etymology (EN): Hundred, from O.E. hundred “a counting of 100,” from P.Gmc. *hunda- “hundred,” as below, + *rath “reckoning, number.”

Etymology (PE): Sad “hundred,” from Mid.Pers. sad, sat, Av. sata- “hundred,” satô.raocana- “with a hundred windows,” satô.təmô.sata- “hundreds of hundred;” cf. Skt. śatá- “hundred;” Gk. hekaton; L. centum; Lith. simtas; P.Gmc. *hunda- “hundred” (Goth. hund; O.H.G. hunt); PIE *kmtom “hundred.”

To chase or search for (game or other wild animals) for the purpose of catching or killing (Dictionary.com). See also → poach, → poaching.

Etymology (EN): M.E, hunten, from O.E. huntian “chase game,” from hunta “hunter,” and related to hentan “to pursue.”

Etymology (PE): Šekâridan, šekâr kardan, from šekâr “hunt;” variant bešgar(d) “hunter, fowler; chase; game; place for hunting;” Parthian Mid.Pers. škr “to hunt, pursuit;”
Sogdian škr-, (’)škr- “to lead, take; pursue, persecute,” prefixed ‘pškr- “to chase;” Proto-Ir. *skar- “to pursue, drive, look for (the cattle);” + -gar, → -or.

To chase or search for (game or other wild animals) for the purpose of catching or killing (Dictionary.com). See also → poach, → poaching.

Etymology (EN): M.E, hunten, from O.E. huntian “chase game,” from hunta “hunter,” and related to hentan “to pursue.”

Etymology (PE): Šekâridan, šekâr kardan, from šekâr “hunt;” variant bešgar(d) “hunter, fowler; chase; game; place for hunting;” Parthian Mid.Pers. škr “to hunt, pursuit;”
Sogdian škr-, (’)škr- “to lead, take; pursue, persecute,” prefixed ‘pškr- “to chase;” Proto-Ir. *skar- “to pursue, drive, look for (the cattle);” + -gar, → -or.

A person who hunts game or other wild animals for food or in sport (Dictionary.com).

See also: → hunt; → -er.

A person who hunts game or other wild animals for food or in sport (Dictionary.com).

See also: → hunt; → -er.

A diagram where the surface → nitrogen  → chemical abundance of stars is plotted against their → projected rotational velocity (v sini).

See also: I. Hunter et al., 2009, A&A, 496, 841; → diagram.

A diagram where the surface → nitrogen  → chemical abundance of stars is plotted against their → projected rotational velocity (v sini).

See also: I. Hunter et al., 2009, A&A, 496, 841; → diagram.

An intense warm-core oceanic cyclone that originates in tropical latitudes; called a typhoon in the western Pacific Ocean. Sustained winds are 120 km per hr or higher.

Etymology (EN): Hurricane, from Sp. huracán, from Taino (the language of an extinct Arawakan Indian tribe of the West Indies) hurakán.

Etymology (PE): Tufand, from tufidan “to roar, to raise a tumult,” tufân “storm, the roaring of the sea, the confused hum of men or animals.” Is this Persian word related to Gk. typhon “whirlwind,” personified as a giant, father of the winds?

An intense warm-core oceanic cyclone that originates in tropical latitudes; called a typhoon in the western Pacific Ocean. Sustained winds are 120 km per hr or higher.

Etymology (EN): Hurricane, from Sp. huracán, from Taino (the language of an extinct Arawakan Indian tribe of the West Indies) hurakán.

Etymology (PE): Tufand, from tufidan “to roar, to raise a tumult,” tufân “storm, the roaring of the sea, the confused hum of men or animals.” Is this Persian word related to Gk. typhon “whirlwind,” personified as a giant, father of the winds?

1. A state of urgency or eagerness; haste
   

   2. (often followed by up) To move, proceed, or act with haste (Dictionary.com).

Etymology (EN): Of uncertain origin.

Etymology (PE): Tâveš, from Tâleši tâveš “hurry, haste,” tâvisté “to hurry up;” variant Kurd. (Mahâbâd) tus “hasty.”

1. A state of urgency or eagerness; haste
   

   2. (often followed by up) To move, proceed, or act with haste (Dictionary.com).

Etymology (EN): Of uncertain origin.

Etymology (PE): Tâveš, from Tâleši tâveš “hurry, haste,” tâvisté “to hurry up;” variant Kurd. (Mahâbâd) tus “hasty.”

Christiaan Huygens (1629-1695), an outstanding Dutch mathematician, astronomer, physicist, and horologist. → Huygens Division, → Huygens Region, → Huygens’ principle, → Huygens-Fresnel principle.

Christiaan Huygens (1629-1695), an outstanding Dutch mathematician, astronomer, physicist, and horologist. → Huygens Division, → Huygens Region, → Huygens’ principle, → Huygens-Fresnel principle.

In the system of → Saturn’s rings, the gap at the inner edge of the → Cassini division at a distance of 117,680 km from the center of the planet with a width of 285-400 km.

See also: → Huygens; → division.

In the system of → Saturn’s rings, the gap at the inner edge of the → Cassini division at a distance of 117,680 km from the center of the planet with a width of 285-400 km.

See also: → Huygens; → division.

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5’ across corresponding to 0.7 pc (for a distance of 440 pc). See O’Dell (2001, ARAA 39, 99).

See also: Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

The inner bright part of the → Orion Nebula, from which most of the radiation is emitted. It is about 5’ across corresponding to 0.7 pc (for a distance of 440 pc). See O’Dell (2001, ARAA 39, 99).

See also: Named after the Dutch astronomer Christiaan → Huygens (1629-1695), who sketched the appearance of the Orion Nebula. His drawing, the first such known sketch, was published in Systema Saturnium in 1659. First named such by O. Gingerich (1982, Ann. NY Acad. Sci. 395, 308); → region.

A development of → Huygens’ principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront
is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.

See also: → Huygens; → Fresnel diffraction; → principle.

A development of → Huygens’ principle stating that every point on a → wavefront acts, at a given instant, as a source of outgoing secondary spherical waves. The secondary wavelets mutually interfere and the resulting net light amplitude at any position in the outgoing wavefront
is the vector sum of the amplitudes of all the individual wavelets. Using this principle, Fresnel calculated with a high accuracy the distribution of light in → diffraction patterns. The Huygens-Fresnel principle was put on a firm theoretical basis by Kirchhoff and expressed as an integral derived from the → wave equation.

See also: → Huygens; → Fresnel diffraction; → principle.

Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens’ principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.

See also: → Huygens; → principle.

Every point of a → wavefront may be considered as a center of a secondary disturbance which gives rise to spherical wavelets, and the wavefront at any later instant may be regarded as the envelope of these wavelets. This statement suffices to account for the laws of → reflection and → refraction, and the approximately straight line propagation of light through large apertures, but it fails to account for → diffraction, the deviations from exact straight line propagation of light. Huygens’ principle was later extended by Fresnel and led to the formulation of → Huygens-Fresnel principle, which is of great importance in the theory of diffraction.

See also: → Huygens; → principle.