An Etymological Dictionary of Astronomy and Astrophysics

The fraction of the total light or other radiation which falls on a non-luminous body, such as a → planet, → satellite, or → asteroid,
and which is reflected by it. Generally, the albedo is equal to the ratio between the light quantity reflected and the light quantity received. The albedo values range between 0.0 (0%), for a perfectly black area, which absorbs all incident light, and 1.0 (100%) for a perfect reflector. The planets or planetary satellites with dense atmospheres have greater albedos than those of transparent atmospheres or of no atmospheres. The albedo can vary from one surface point to another, so that a mean albedo is given for practical purposes. The natural surfaces reflect different light quantities in different directions and the albedo can be expressed in several ways, depending on the way in which the measurement was made: in one direction or, on the average, in all directions (M.S.: SDE). See also → Bond albedo, → geometric albedo.

Etymology (EN): Albedo, L. “whiteness,” from albus “white,” from PIE base *albho- “white”. Compare with Gk. alphos “white leprosy,” O.H.G. albig, O.E. elfet “swan, the white bird”. The idea of whiteness derives from the fact that whiter bodies have a higher reflective power, while opaque objects are more absorptive.

Etymology (PE): Sepidâ, from sepid, →, white, + -â noun-forming prefix from certain adjectives.

The fraction of the total light or other radiation which falls on a non-luminous body, such as a → planet, → satellite, or → asteroid,
and which is reflected by it. Generally, the albedo is equal to the ratio between the light quantity reflected and the light quantity received. The albedo values range between 0.0 (0%), for a perfectly black area, which absorbs all incident light, and 1.0 (100%) for a perfect reflector. The planets or planetary satellites with dense atmospheres have greater albedos than those of transparent atmospheres or of no atmospheres. The albedo can vary from one surface point to another, so that a mean albedo is given for practical purposes. The natural surfaces reflect different light quantities in different directions and the albedo can be expressed in several ways, depending on the way in which the measurement was made: in one direction or, on the average, in all directions (M.S.: SDE). See also → Bond albedo, → geometric albedo.

Etymology (EN): Albedo, L. “whiteness,” from albus “white,” from PIE base *albho- “white”. Compare with Gk. alphos “white leprosy,” O.H.G. albig, O.E. elfet “swan, the white bird”. The idea of whiteness derives from the fact that whiter bodies have a higher reflective power, while opaque objects are more absorptive.

Etymology (PE): Sepidâ, from sepid, →, white, + -â noun-forming prefix from certain adjectives.

The second brightest star of the constellation → Cygnus, with a visual magnitude of 3.0. It is a double star of strikingly different colors, with components separated by 35’’. The brighter component is a K3 giant while its partner is a main-sequence B9 star.
About 380 → light-years away, the two rotate around each other with a period of about 75,000 years. The main component is itself a binary system.

Etymology (EN): Albireo may be a corruption of the L. phrase ab ireo “from the rainbow,” as suggested by some writers on star names. It does not mean “the hen’s beak”.

Etymology (PE): Menqâr-e Dajâjé “hen’s beak,” from Ar. Minqâr al-Dajâjah, from minqâr “beak” + dajâjah “hen”.
Nok-e Mâkiyân “beak of the hen,” from Mod.Pers. nok “beak” + mâkiyân “hen”.

The second brightest star of the constellation → Cygnus, with a visual magnitude of 3.0. It is a double star of strikingly different colors, with components separated by 35’’. The brighter component is a K3 giant while its partner is a main-sequence B9 star.
About 380 → light-years away, the two rotate around each other with a period of about 75,000 years. The main component is itself a binary system.

Etymology (EN): Albireo may be a corruption of the L. phrase ab ireo “from the rainbow,” as suggested by some writers on star names. It does not mean “the hen’s beak”.

Etymology (PE): Menqâr-e Dajâjé “hen’s beak,” from Ar. Minqâr al-Dajâjah, from minqâr “beak” + dajâjah “hen”.
Nok-e Mâkiyân “beak of the hen,” from Mod.Pers. nok “beak” + mâkiyân “hen”.

An organic compound having a → hydroxyl (-OH) group attached to a carbon atom. Specifically the term is applied to ethyl alcohol or → ethanol (C₂H₅OH). Alcohol exists abundantly in the → interstellar medium in gaseous state also in the form of → methanol.

See also: The discovery of alcohol is attributed to the Iranian physician and scientist Mohammad son of Zakariyâ Râzi (864-930 AD, known in Europe as Razes or Rhazes).
He wrote in Ar., which was the scientific language of that period. However, he himself did not use a specific term for this substance as far as we know. Alcohol was first used in medicine about 1250 by two Italian physicians Valis de Furo and Thaddaeus of Florence. It was not yet called alcohol, but aqua ardens or aqua vini. The name alcohol, of Arabic origin, was introduced by the Swiss alchemist and physician Paracelsus (1493-1541) in the sixteenth century. It is composed of two parts, al-, a definite article (like “the”), plus a second component the origin of which is not clear. A broadly spread explanation for the second component is (kuHl)

darken eyelids. The powder is prepared by sublimation of the natural mineral antimony sulfide (Sb₂S₃). According to this opinion, the meaning of alkuhl would have been first extended by European alchemists to distilled substances in general, and then narrowed to ethanol. Paracelsus indeed defines the terms alcohol
and alcool as “the most subtle part of anything.” It is in that sense that he calls the substance alcool vini, that is, the most subtle part of wine. Moreover, it is always as “alcool vini” or “alcohol vini” that he uses this term, never “alcohol” alone. Later chemists dropped the “vini” and let the alcohol stand alone for the name (see M. M. Pattison Muir, Story of Alchemy and the Beginning of Chemistry, 1902, p. 192). We note that the word used in current Ar. for this substance is الکحول (alkuHul) and not الکحل (alkuHl). That word may be the Ar. rendering of the European term (probably from the older Fr. form alcohol) loaned in modern times. Alternatively, the word alcohol would originate from another Ar. word, al-ghaul (الغول), meaning “an oppression of the mind, a loss of the senses (from drunkenness), a head-ache” also “spirit, demon.” This derivation would be consistent with the use of “spirit” or “spirit of wine” as synonymous of “alcohol” in most Western languages. If this second etymology is correct, the popular etymology and the spelling “alcohol” would not be due to generalization of the meaning of al-kuhl, but rather to Western alchemists and authors confusing the two words al-kuhl and al-ghaul, because of the lack of the “gh” sound in European languages. The problem with this etymology is that no specific word is found in classical Ar. for designating “alcohol.”

An organic compound having a → hydroxyl (-OH) group attached to a carbon atom. Specifically the term is applied to ethyl alcohol or → ethanol (C₂H₅OH). Alcohol exists abundantly in the → interstellar medium in gaseous state also in the form of → methanol.

See also: The discovery of alcohol is attributed to the Iranian physician and scientist Mohammad son of Zakariyâ Râzi (864-930 AD, known in Europe as Razes or Rhazes).
He wrote in Ar., which was the scientific language of that period. However, he himself did not use a specific term for this substance as far as we know. Alcohol was first used in medicine about 1250 by two Italian physicians Valis de Furo and Thaddaeus of Florence. It was not yet called alcohol, but aqua ardens or aqua vini. The name alcohol, of Arabic origin, was introduced by the Swiss alchemist and physician Paracelsus (1493-1541) in the sixteenth century. It is composed of two parts, al-, a definite article (like “the”), plus a second component the origin of which is not clear. A broadly spread explanation for the second component is (kuHl)

darken eyelids. The powder is prepared by sublimation of the natural mineral antimony sulfide (Sb₂S₃). According to this opinion, the meaning of alkuhl would have been first extended by European alchemists to distilled substances in general, and then narrowed to ethanol. Paracelsus indeed defines the terms alcohol
and alcool as “the most subtle part of anything.” It is in that sense that he calls the substance alcool vini, that is, the most subtle part of wine. Moreover, it is always as “alcool vini” or “alcohol vini” that he uses this term, never “alcohol” alone. Later chemists dropped the “vini” and let the alcohol stand alone for the name (see M. M. Pattison Muir, Story of Alchemy and the Beginning of Chemistry, 1902, p. 192). We note that the word used in current Ar. for this substance is الکحول (alkuHul) and not الکحل (alkuHl). That word may be the Ar. rendering of the European term (probably from the older Fr. form alcohol) loaned in modern times. Alternatively, the word alcohol would originate from another Ar. word, al-ghaul (الغول), meaning “an oppression of the mind, a loss of the senses (from drunkenness), a head-ache” also “spirit, demon.” This derivation would be consistent with the use of “spirit” or “spirit of wine” as synonymous of “alcohol” in most Western languages. If this second etymology is correct, the popular etymology and the spelling “alcohol” would not be due to generalization of the meaning of al-kuhl, but rather to Western alchemists and authors confusing the two words al-kuhl and al-ghaul, because of the lack of the “gh” sound in European languages. The problem with this etymology is that no specific word is found in classical Ar. for designating “alcohol.”

A 4th magnitude star lying in the constellation → Ursa Major (also called 80 Ursae Majoris) which forms a visual pair with the brighter star → Mizar (Zeta Ursae Majoris). Alcor is separated by about 11.5 minutes of arc from Mizar. It is a → main sequence star of type A5 with a mass of 1.8 Msun. Recent observations show that Alcor is a → spectroscopic binary, whose → companion has M-band (λ = 4.8 μm) magnitude 8.8 and projected separation 1’’.11 (28 AU) from Alcor. The companion is most likely a low-mass (~ 0.3 Msun) active star which is responsible for Alcor’s → X-ray emission detected by → ROSAT (L_X ~ 10^28.3 erg/s). Alcor is a nuclear member of the → Ursa Major star cluster (distance ~ 25 pc, age ~ 0.5 Gyr). The Alcor binary is probably → gravitationally bound to the Mizar star system, making them a → sextuplet with physical separation 0.36 pc, or 74,000 → astronomical units (Mamajek et al., 2010, AJ 139, 919).

Etymology (EN): Alcor, perhaps from Ar. al-khawr “the low ground.”

Etymology (PE): Sohâ, from Ar. Suhâ.

A 4th magnitude star lying in the constellation → Ursa Major (also called 80 Ursae Majoris) which forms a visual pair with the brighter star → Mizar (Zeta Ursae Majoris). Alcor is separated by about 11.5 minutes of arc from Mizar. It is a → main sequence star of type A5 with a mass of 1.8 Msun. Recent observations show that Alcor is a → spectroscopic binary, whose → companion has M-band (λ = 4.8 μm) magnitude 8.8 and projected separation 1’’.11 (28 AU) from Alcor. The companion is most likely a low-mass (~ 0.3 Msun) active star which is responsible for Alcor’s → X-ray emission detected by → ROSAT (L_X ~ 10^28.3 erg/s). Alcor is a nuclear member of the → Ursa Major star cluster (distance ~ 25 pc, age ~ 0.5 Gyr). The Alcor binary is probably → gravitationally bound to the Mizar star system, making them a → sextuplet with physical separation 0.36 pc, or 74,000 → astronomical units (Mamajek et al., 2010, AJ 139, 919).

Etymology (EN): Alcor, perhaps from Ar. al-khawr “the low ground.”

Etymology (PE): Sohâ, from Ar. Suhâ.

The brightest star in the → Pleiades, located in the constellation → Taurus. → Apparent visual magnitude 2.87, → spectral type B7 III.

Etymology (EN): In Gk. mythology, a daughter of Aelous who, with her husband, Ceyx, was transformed into a kingfisher.

Etymology (PE): Nayyer-e Sorayyâ “the bight of the Pleiades,” from Ar. nayyir “luminous” + Thorayyâ “the Pleiades”.
Raxšân-e Parvin “the bight of the Pleiades,” from Mod.Pers. raxšân “bright, luminous” + Parvin “the Pleiades”.

The brightest star in the → Pleiades, located in the constellation → Taurus. → Apparent visual magnitude 2.87, → spectral type B7 III.

Etymology (EN): In Gk. mythology, a daughter of Aelous who, with her husband, Ceyx, was transformed into a kingfisher.

Etymology (PE): Nayyer-e Sorayyâ “the bight of the Pleiades,” from Ar. nayyir “luminous” + Thorayyâ “the Pleiades”.
Raxšân-e Parvin “the bight of the Pleiades,” from Mod.Pers. raxšân “bright, luminous” + Parvin “the Pleiades”.

The brightest star in the constellation → Taurus (visual magnitude about 0.9), Aldebaran is an orange K-type giant that lies 60 → light-years away. It has a faint M2 V companion. It is slowly and irregularly variable.

Etymology (EN): Ar. Aldebaran “the follower” (of the Pleiades, which rise shortly before it does), from al “the” + dabaran “follower,” from dobur “to follow”. Gâvcašm “the bull’s eye,” from Mod.Pers. Gâv “bull, cow” + cašm “eye,” corresponding to the alternative Ar. name of the star Ayno ’s Sowr.

The brightest star in the constellation → Taurus (visual magnitude about 0.9), Aldebaran is an orange K-type giant that lies 60 → light-years away. It has a faint M2 V companion. It is slowly and irregularly variable.

Etymology (EN): Ar. Aldebaran “the follower” (of the Pleiades, which rise shortly before it does), from al “the” + dabaran “follower,” from dobur “to follow”. Gâvcašm “the bull’s eye,” from Mod.Pers. Gâv “bull, cow” + cašm “eye,” corresponding to the alternative Ar. name of the star Ayno ’s Sowr.

Any of a class of organic compounds containing the -CH=O group, that is a double-bonded oxygen and hydrogen bonded to the same terminal carbon atom.

See also: From N.L. al(cohol) dehyd(rogenatum) “alcohol deprived of hydrogen.”

Any of a class of organic compounds containing the -CH=O group, that is a double-bonded oxygen and hydrogen bonded to the same terminal carbon atom.

See also: From N.L. al(cohol) dehyd(rogenatum) “alcohol deprived of hydrogen.”

The brightest star in → Cepheus and a
→ subgiant star of apparent visual magnitude 2.44. Its → spectral type is A7 and distance 49 → light-years.

Etymology (EN): Alderamin, from Ar. al dhirâ’ al-yamin “right arm” (of Cepheus), from Ar. dhirâ’ “arm” + yamin “right”.

Etymology (PE): Zerâ’-e Yamin, from Ar. al dhira al-yamin.

The brightest star in → Cepheus and a
→ subgiant star of apparent visual magnitude 2.44. Its → spectral type is A7 and distance 49 → light-years.

Etymology (EN): Alderamin, from Ar. al dhirâ’ al-yamin “right arm” (of Cepheus), from Ar. dhirâ’ “arm” + yamin “right”.

Etymology (PE): Zerâ’-e Yamin, from Ar. al dhira al-yamin.

1. The first letter of the Hebrew alphabet (ℵ).
   

2. A → cardinal number representing an uncountable set. For example, ℵ₀ (pronounced aleph-null, aleph-nought, or aleph-zero) is the smallest infinite cardinal and ℵ₁ is the smallest cadinal larger than ℵ₀.

See also: Hebrew and Phoenician letter, from Semitic languages.

1. The first letter of the Hebrew alphabet (ℵ).
   

2. A → cardinal number representing an uncountable set. For example, ℵ₀ (pronounced aleph-null, aleph-nought, or aleph-zero) is the smallest infinite cardinal and ℵ₁ is the smallest cadinal larger than ℵ₀.

See also: Hebrew and Phoenician letter, from Semitic languages.

A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.

See also: Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.

A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.

See also: Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.

A set of tables created in Toledo, under Alfonso X, el sabio, king of Castile and Léon (1252 to 1284) to correct the anomalies in the → Toledan Tables. The starting point of the Alfonsine Tables is January 1, 1252, the year of king’s coronation (1 June). The original Spanish version of the tables is lost, but a set of canons (introductory instructions) for planetary tables are extant. They are written by Isaac ben Sid and Judah ben Moses ha-Cohen, two of the most active collaborators of Alfonso X. The Alfonsine Tables were the most widely used astronomical tables in the Middle Ages and had an enormous impact on the development of European astronomy from the 13th to 16th century. They were replaced by Erasmus Reinhold’s → Prutenic Tables, based on Copernican models, that were first published in 1551.The Latin version of the Alfonsine Tables first appeared in Paris around 1320, where a revision was undertaken by John of Lignères and John of Murs, accompanied by a number of canons for their use written by John of Saxony. There is a controversy as to the exact relationship of these tables with the work commissioned by the Spanish king.

See also: After the Spanish monarch Alfonso X (1221-1284); → table.

A set of tables created in Toledo, under Alfonso X, el sabio, king of Castile and Léon (1252 to 1284) to correct the anomalies in the → Toledan Tables. The starting point of the Alfonsine Tables is January 1, 1252, the year of king’s coronation (1 June). The original Spanish version of the tables is lost, but a set of canons (introductory instructions) for planetary tables are extant. They are written by Isaac ben Sid and Judah ben Moses ha-Cohen, two of the most active collaborators of Alfonso X. The Alfonsine Tables were the most widely used astronomical tables in the Middle Ages and had an enormous impact on the development of European astronomy from the 13th to 16th century. They were replaced by Erasmus Reinhold’s → Prutenic Tables, based on Copernican models, that were first published in 1551.The Latin version of the Alfonsine Tables first appeared in Paris around 1320, where a revision was undertaken by John of Lignères and John of Murs, accompanied by a number of canons for their use written by John of Saxony. There is a controversy as to the exact relationship of these tables with the work commissioned by the Spanish king.

See also: After the Spanish monarch Alfonso X (1221-1284); → table.

The ratio of the flow velocity to the → Alfvén speed in a medium.

See also: → Alfvén wave; → number.

The ratio of the flow velocity to the → Alfvén speed in a medium.

See also: → Alfvén wave; → number.

The ratio of the flow velocity to the → Alfvén speed in a medium.

See also: → Alfvén wave; → number.

The ratio of the flow velocity to the → Alfvén speed in a medium.

See also: → Alfvén wave; → number.

In magnetized disk models, the point where the → poloidal velocity equals the → Alfven speed. Within this point, the magnetic energy density dominates, and the gas is forced to flow along the field lines. Well beyond this point, the kinetic energy acquired by the flowing gas prevails and the field is forced to follow the flow.

See also: → Alfven wave; → point.

In magnetized disk models, the point where the → poloidal velocity equals the → Alfven speed. Within this point, the magnetic energy density dominates, and the gas is forced to flow along the field lines. Well beyond this point, the kinetic energy acquired by the flowing gas prevails and the field is forced to follow the flow.

See also: → Alfven wave; → point.

1. In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
   

2. More generally, the distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

See also: → Alfvén wave; → radius.

1. In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
   

2. More generally, the distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

See also: → Alfvén wave; → radius.

1. In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
   

2. The distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

See also: → Alfvén wave; → radius.

1. In theories of magnetized → accretion disks, the distance from a non-rotating star where the → free fall of a spherical accretion flow is stopped, which occurs where the → ram pressure of the infalling matter equals the → magnetic pressure of the star.
   

2. The distance from an accreting or wind-blowing star where the → Alfvén Mach number of the flow (→ inflow or → outflow) is unity.

See also: → Alfvén wave; → radius.

The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

See also: → Alfvén wave; → speed.

The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

See also: → Alfvén wave; → speed.

The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

See also: → Alfvén wave; → speed.

The speed at which → Alfven waves are propagated along the magnetic field. It is a characteristic velocity at which perturbations of the lines of force travel. Alfvén speed is given by: v_A = B/(μ₀.ρ)^1/2, where B is the → magnetic field strength, μ₀ is the → magnetic permeability, and ρ is the density of the plasma. Alfvén speed plays a role analogous to the sound speed in non-magnetized fluid dynamics. Same as Alfvén velocity.

See also: → Alfvén wave; → speed.

In a magnetized wind, the geometric loci of the points where the magnetic pressure equals the flow pressure. See also → Alfven point.

See also: → Alfven wave; → surface.

In a magnetized wind, the geometric loci of the points where the magnetic pressure equals the flow pressure. See also → Alfven point.

See also: → Alfven wave; → surface.

same as → Alfven speed.

See also: → Alfven wave; → velocity.

same as → Alfven speed.

See also: → Alfven wave; → velocity.

A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave
which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.

See also: Named after Hannes Alfvén (1908-1995), Swedish physicist,
who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave.

A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave
which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.

See also: Named after Hannes Alfvén (1908-1995), Swedish physicist,
who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave.

A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave
which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.

See also: Named after Hannes Alfvén (1908-1995), Swedish physicist,
who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave.

A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave
which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave.

See also: Named after Hannes Alfvén (1908-1995), Swedish physicist,
who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave.

Involving → Alfvén waves.

See also: → Alfvénic; → -ity.

Involving → Alfvén waves.

See also: → Alfvénic; → -ity.

Involving → Alfvén waves.

See also: → Alfvénic; → -ity.

Involving → Alfvén waves.

See also: → Alfvénic; → -ity.

Large amplitude fluctuations in the → solar wind with properties resembling
those of → Alfvén waves. A fluctuation is said to be Alfvénic if the following relationship between the velocity fluctuations (Δv) and magnetic field fluctuations (ΔB) is satisfied:

Δv = ± ΔB/(μ₀ρ)^1/2. Also called Alfvénicity.

See also: → Alfvén wave; → fluctuation.

Large amplitude fluctuations in the → solar wind with properties resembling
those of → Alfvén waves. A fluctuation is said to be Alfvénic if the following relationship between the velocity fluctuations (Δv) and magnetic field fluctuations (ΔB) is satisfied:

Δv = ± ΔB/(μ₀ρ)^1/2. Also called Alfvénicity.

See also: → Alfvén wave; → fluctuation.

Large amplitude fluctuations in the → solar wind with properties resembling
those of → Alfvén waves. A fluctuation is said to be Alfvénic if the following relationship between the velocity fluctuations (Δv) and magnetic field fluctuations (ΔB) is satisfied:

Δv = ± ΔB/(μ₀ρ)^1/2. Also called Alfvénicity.

See also: → Alfvénic; → fluctuation.

Large amplitude fluctuations in the → solar wind with properties resembling
those of → Alfvén waves. A fluctuation is said to be Alfvénic if the following relationship between the velocity fluctuations (Δv) and magnetic field fluctuations (ΔB) is satisfied:

Δv = ± ΔB/(μ₀ρ)^1/2. Also called Alfvénicity.

See also: → Alfvénic; → fluctuation.

→ Alfvénic fluctuation.

See also: → Alfvénic; → -ity.

→ Alfvénic fluctuation.

See also: → Alfvénic; → -ity.

→ Alfvénic fluctuation.

See also: → Alfvénic; → -ity.

→ Alfvénic fluctuation.

See also: → Alfvénic; → -ity.

A single-celled or multicellular plant living in water or moist conditions, which contain chlorophyll and other pigments but has no true root, stem, or leaf. Algae include seaweeds and pond scum.

Etymology (EN): From alga (singular), from L. alga “seaweed,” of uncertain origin.

Etymology (PE): Jolbak “alga.”

A single-celled or multicellular plant living in water or moist conditions, which contain chlorophyll and other pigments but has no true root, stem, or leaf. Algae include seaweeds and pond scum.

Etymology (EN): From alga (singular), from L. alga “seaweed,” of uncertain origin.

Etymology (PE): Jolbak “alga.”

The branch of mathematics which deals with the properties and relations of numbers using symbols (usually letters of the alphabet) to represent numbers or members of a specified set; the generalization and extension of arithmetic.

Etymology (EN): Algebra, from M.L., from Ar. al jabr “reunion of broken bones,” the first known use in the title of a book by the Persian mathematician and astronomer
Abu Ja’far Mohammad ibn Musa al-Khwarizmi (c780-c850), who worked in Baghdad under the patronage of Caliph Al-Mamun. The full title of the tratise was Hisab al-Jabr w’al-Muqabala “Arithmetic of Completion and Balancing.” → algorithm.

Etymology (PE): Jabr, from Ar. al jabr, as above.

The branch of mathematics which deals with the properties and relations of numbers using symbols (usually letters of the alphabet) to represent numbers or members of a specified set; the generalization and extension of arithmetic.

Etymology (EN): Algebra, from M.L., from Ar. al jabr “reunion of broken bones,” the first known use in the title of a book by the Persian mathematician and astronomer
Abu Ja’far Mohammad ibn Musa al-Khwarizmi (c780-c850), who worked in Baghdad under the patronage of Caliph Al-Mamun. The full title of the tratise was Hisab al-Jabr w’al-Muqabala “Arithmetic of Completion and Balancing.” → algorithm.

Etymology (PE): Jabr, from Ar. al jabr, as above.

Relating to, involving, or according to the laws of algebra.

See also: → algebra + → -ic.

Relating to, involving, or according to the laws of algebra.

See also: → algebra + → -ic.

An equation in the form of P = 0, where P is a → polynomial having a finite number of terms.

See also: → algebra; → equation.

An equation in the form of P = 0, where P is a → polynomial having a finite number of terms.

See also: → algebra; → equation.

A function expressed in terms of → polynomials and/or roots of polynomials. In other words, any function y = f(x) which satisfies an equation of the form P₀(x)yⁿ + P₁(x)y^{n - 1} + … + P_n(x) = 0, where
P₀(x), P₁(x), …, P_n(x) are polynomials in x.

See also: → algebraic + → function.

A function expressed in terms of → polynomials and/or roots of polynomials. In other words, any function y = f(x) which satisfies an equation of the form P₀(x)yⁿ + P₁(x)y^{n - 1} + … + P_n(x) = 0, where
P₀(x), P₁(x), …, P_n(x) are polynomials in x.

See also: → algebraic + → function.

A number, → real or → complex, that is a → root of a → non-zero polynomial equation
whose → coefficients are all → rational. For example, the root x of the polynomial x² - 2x + 1 = 0 is an algebraic number, because the polynomial is non-zero and the coefficients are rational numbers. The imaginary number i is algebraic, because it is the solution to x² + 1 = 0.

See also: → algebraic; → number.

A number, → real or → complex, that is a → root of a → non-zero polynomial equation
whose → coefficients are all → rational. For example, the root x of the polynomial x² - 2x + 1 = 0 is an algebraic number, because the polynomial is non-zero and the coefficients are rational numbers. The imaginary number i is algebraic, because it is the solution to x² + 1 = 0.

See also: → algebraic; → number.

A star which lies at the lower left-hand corner of → Pegasus. Its apparent magnitude varies between +2.80 and +2.86 over a period of 3.6 hours; → spectral type B2 IV.

Etymology (EN): Algenib, from Ar. Aljanb al-Faras “the horse’s flank,” from al “the” + janb “flank” + faras “horse”.

A star which lies at the lower left-hand corner of → Pegasus. Its apparent magnitude varies between +2.80 and +2.86 over a period of 3.6 hours; → spectral type B2 IV.

Etymology (EN): Algenib, from Ar. Aljanb al-Faras “the horse’s flank,” from al “the” + janb “flank” + faras “horse”.

A binary system in Leo the brighter component of which (magnitude 2.6) is a giant K star and the partner a giant G (magnitude 3.8). The angular separation of just over 4’’ means that the two stars are at least 170 AU apart, for a distance of 126 → light-years, and have an orbital period of over 500 years.

Etymology (EN): Algieba, from Ar. Al-Jabhah “the forehead” (of the Lion).

A binary system in Leo the brighter component of which (magnitude 2.6) is a giant K star and the partner a giant G (magnitude 3.8). The angular separation of just over 4’’ means that the two stars are at least 170 AU apart, for a distance of 126 → light-years, and have an orbital period of over 500 years.

Etymology (EN): Algieba, from Ar. Al-Jabhah “the forehead” (of the Lion).

A variable star in the constellation → Perseus, which was the first eclipsing binary discovered. Its brightness varies between 2.2 and 3.5 magnitudes. Lying at a distance of about 82 → light-years, it consists of at least three components. The brightest component (A)
is of spectral type B8 V, and the second one (B) a K type giant. The components A and B turn around each other with a period of about 68.8 hours.

Etymology (EN): Algol, from Ar. Ra’s al-ghul “the ghoul’s head”.

A variable star in the constellation → Perseus, which was the first eclipsing binary discovered. Its brightness varies between 2.2 and 3.5 magnitudes. Lying at a distance of about 82 → light-years, it consists of at least three components. The brightest component (A)
is of spectral type B8 V, and the second one (B) a K type giant. The components A and B turn around each other with a period of about 68.8 hours.

Etymology (EN): Algol, from Ar. Ra’s al-ghul “the ghoul’s head”.

The same as eclipsing binary stars, the prototype of which is Algol.

See also: → Algol; → variable.

The same as eclipsing binary stars, the prototype of which is Algol.

See also: → Algol; → variable.

1. A step-by-step problem-solving procedure, especially an established, recursive computational procedure for solving a problem in a finite number of steps.
   

2. Historically, the Arabic system of arithmetical notation (with the figures 1, 2, 3, etc.); the art of computation with the Arabic figures, one to nine, plus the zero; arithmetic.

Etymology (EN): From M.L. algorismus, a mangled transliteration of
al-Khwarizmi, “native of Khwarazm,” the surname of the Persian mathematician and astronomer
Abu Ja’far Mohammad ibn Musa al-Khwarizmi (c780-c850).

Etymology (PE): Xârazmik, from Xârazmi (or Xwârazmi, from Xwârazm), the name of the Persian mathematician, + Persian affix → -ik, → -ics.

1. A step-by-step problem-solving procedure, especially an established, recursive computational procedure for solving a problem in a finite number of steps.
   

2. Historically, the Arabic system of arithmetical notation (with the figures 1, 2, 3, etc.); the art of computation with the Arabic figures, one to nine, plus the zero; arithmetic.

Etymology (EN): From M.L. algorismus, a mangled transliteration of
al-Khwarizmi, “native of Khwarazm,” the surname of the Persian mathematician and astronomer
Abu Ja’far Mohammad ibn Musa al-Khwarizmi (c780-c850).

Etymology (PE): Xârazmik, from Xârazmi (or Xwârazmi, from Xwârazm), the name of the Persian mathematician, + Persian affix → -ik, → -ics.

General:An assumed name; otherwise called.
Electronics: A false signal in telecommunication links from beats between signal frequency and sampling frequency.

Etymology (EN): From L. alius “(an)other”. Compare with Skt. anya “other, different,” Av. anya-, O.Pers. aniya- “the one or the other,” Arm. ail, Gk. allos “another,” Goth. aljis “other”.
PIE *al- “beyond”.

Etymology (PE): Anyâ from Mid.Pers. anya “other, otherwise,” from Av. anya “other”. This term is used as eyni in the Modern Persian Aftari dialect: eyni sâl “other year,”
eyni vacé “other child”.

General:An assumed name; otherwise called.
Electronics: A false signal in telecommunication links from beats between signal frequency and sampling frequency.

Etymology (EN): From L. alius “(an)other”. Compare with Skt. anya “other, different,” Av. anya-, O.Pers. aniya- “the one or the other,” Arm. ail, Gk. allos “another,” Goth. aljis “other”.
PIE *al- “beyond”.

Etymology (PE): Anyâ from Mid.Pers. anya “other, otherwise,” from Av. anya “other”. This term is used as eyni in the Modern Persian Aftari dialect: eyni sâl “other year,”
eyni vacé “other child”.

The condition that two or more functions are indistinguishable because they have the same values at a finite set of points. Such functions are said to be aliases of each others. The aliasing problem often occurs in undersampled discrete Fourier transform.

Etymology (EN): Aliasing, from → alias + → -ing.

Etymology (PE): Anyâyi, from anyâ, → alias, + noun forming suffix -i.

The condition that two or more functions are indistinguishable because they have the same values at a finite set of points. Such functions are said to be aliases of each others. The aliasing problem often occurs in undersampled discrete Fourier transform.

Etymology (EN): Aliasing, from → alias + → -ing.

Etymology (PE): Anyâyi, from anyâ, → alias, + noun forming suffix -i.

A claim or piece of evidence that one was elsewhere when an act, typically a criminal one, is alleged to have taken place (OxfordDictionaries.com).

Etymology (EN): From L. alibi (adv.) “elsewhere, somewhere else,” locative of alius “another, other, different,” → alias.

Etymology (PE): Enigâh, literally “other place,” from eni, from Mid.Pers. anya “other,” → alias, +gâh “place,” → time.

A claim or piece of evidence that one was elsewhere when an act, typically a criminal one, is alleged to have taken place (OxfordDictionaries.com).

Etymology (EN): From L. alibi (adv.) “elsewhere, somewhere else,” locative of alius “another, other, different,” → alias.

Etymology (PE): Enigâh, literally “other place,” from eni, from Mid.Pers. anya “other,” → alias, +gâh “place,” → time.

1. In a → planispheric astrolabe, the small revolving rod fixed to the center of the goniometric scale plotted on the → front or → back of the instrument. Unlike the → rule, the alidade has little vanes with holes or slots at each end, called → pinnules, which are used as sights, through which the observer can aim at a particular object. An index, often consisting of the edge of the alidade itself, shows on the goniometric scales the angle between the line of sight of the targeted object and a predetermined axis, i.e. the vertical of the observation locality (online museo galileo, VirtualMuseum).
   

2. A rule having a sight at each end, used in surveying.
   

3. A topographic surveying and mapping instrument used for determining directions, consisting of a telescope and attached parts.

Etymology (EN): M.E. allidatha, alhidade, from L. alhidada, from Ar. al-‘izâda (العضاده), from al- “the” + ‘izâda “an armlet, a bracelet; a sickle-like piece of metal, used by camel drivers,
to pull down tree branches to camels.”

Etymology (PE): Ezâdé, from Ar. al-‘izâda.

1. In a → planispheric astrolabe, the small revolving rod fixed to the center of the goniometric scale plotted on the → front or → back of the instrument. Unlike the → rule, the alidade has little vanes with holes or slots at each end, called → pinnules, which are used as sights, through which the observer can aim at a particular object. An index, often consisting of the edge of the alidade itself, shows on the goniometric scales the angle between the line of sight of the targeted object and a predetermined axis, i.e. the vertical of the observation locality (online museo galileo, VirtualMuseum).
   

2. A rule having a sight at each end, used in surveying.
   

3. A topographic surveying and mapping instrument used for determining directions, consisting of a telescope and attached parts.

Etymology (EN): M.E. allidatha, alhidade, from L. alhidada, from Ar. al-‘izâda (العضاده), from al- “the” + ‘izâda “an armlet, a bracelet; a sickle-like piece of metal, used by camel drivers,
to pull down tree branches to camels.”

Etymology (PE): Ezâdé, from Ar. al-‘izâda.

1. A species not native to its environment; an introduced species.
   

2. A hypothetical creature from outer space; → extraterrestrial.

Etymology (EN): M.E., from O.Fr. alien “strange, foreign; a stranger, foreigner,” from L. alienus “of or belonging to another, foreign, alien, strange,” also, as a noun, “a stranger, foreigner,” adjectival form of alius “(an)other,” → alias; meaning “not of the Earth” first recorded 1920.

Etymology (PE): Bigâné “alien, foreigner,” from Mid.Pers. bêgânag, from bêg-, bê- “out, outside, apart” (cf. Sogd. bêk “out, outside, apart, except,” bêk-dênê “heretic,” literally “out of religion”) + suffix -ânag.

1. A species not native to its environment; an introduced species.
   

2. A hypothetical creature from outer space; → extraterrestrial.

Etymology (EN): M.E., from O.Fr. alien “strange, foreign; a stranger, foreigner,” from L. alienus “of or belonging to another, foreign, alien, strange,” also, as a noun, “a stranger, foreigner,” adjectival form of alius “(an)other,” → alias; meaning “not of the Earth” first recorded 1920.

Etymology (PE): Bigâné “alien, foreigner,” from Mid.Pers. bêgânag, from bêg-, bê- “out, outside, apart” (cf. Sogd. bêk “out, outside, apart, except,” bêk-dênê “heretic,” literally “out of religion”) + suffix -ânag.

To arrange in a line or so as to be parallel; to adjust to produce a proper relationship or orientation.

Etymology (EN): M.Fr. aligner, from O.Fr. aligner, from à “to” + ligner “to line,” from L. lineare, from linea “linen thread, string, line;” → line.

Etymology (PE): Âxatidan, from â- intesive and nuance prefix

• xat, → line, + infinitive suffix -idan.

To arrange in a line or so as to be parallel; to adjust to produce a proper relationship or orientation.

Etymology (EN): M.Fr. aligner, from O.Fr. aligner, from à “to” + ligner “to line,” from L. lineare, from linea “linen thread, string, line;” → line.

Etymology (PE): Âxatidan, from â- intesive and nuance prefix

• xat, → line, + infinitive suffix -idan.

Setting the axis of a telescope parallel to prime directions. In equatorial mounting, they are made parallel with the Earth’s axis of rotation and the equator respectively. → collimation.

See also: → align; → telescope.

Setting the axis of a telescope parallel to prime directions. In equatorial mounting, they are made parallel with the Earth’s axis of rotation and the equator respectively. → collimation.

See also: → align; → telescope.

Arranged in a → straight line.

See also: Past participle of → align.

Arranged in a → straight line.

See also: Past participle of → align.

A magnetic field whose lines of force are oriented along a particular direction or by a particular manner (axially, vertically; randomly, properly, etc.)

See also: → aligned; → magnetic field.

A magnetic field whose lines of force are oriented along a particular direction or by a particular manner (axially, vertically; randomly, properly, etc.)

See also: → aligned; → magnetic field.

The fact of being in line or bringing into line.

See also: Verbal noun of → align; → -ment.

The fact of being in line or bringing into line.

See also: Verbal noun of → align; → -ment.

The brightest of the seven stars that make up the → Big Dipper → asterism. Alioth shines at magnitude +1.77 from a distance of about 80 → light-years. It is a white star of → spectral type A0pCr. The spectrum of the star is characterized by abnormally strong lines of → chromium and → europium.

Etymology (EN): Alioth, from Aliot, from Ar. Alyat (الیه)
“tail, buttocks”.

Etymology (PE): Jown, from Ar. Jawn “black camel or horse”.

The brightest of the seven stars that make up the → Big Dipper → asterism. Alioth shines at magnitude +1.77 from a distance of about 80 → light-years. It is a white star of → spectral type A0pCr. The spectrum of the star is characterized by abnormally strong lines of → chromium and → europium.

Etymology (EN): Alioth, from Aliot, from Ar. Alyat (الیه)
“tail, buttocks”.

Etymology (PE): Jown, from Ar. Jawn “black camel or horse”.

The second brightest star in → Ursa Major and the end star in the handle of the → Big Dipper. Alkaid is a blue B3V main sequence star of apparent magnitude of 1.86 and lies at about 100 → light-years.

Etymology (EN): Alkaid “leader, chief,” from Al-Qa’id al-Banat an-Na’ash “the leader of the daughters of the bier,” from Banat “daughters”

• Na’ash “bier”. Banat an-Na’ash is the Ar. name of the constellation.

Etymology (PE): Qâed from Ar. Al-Qa’id.

The second brightest star in → Ursa Major and the end star in the handle of the → Big Dipper. Alkaid is a blue B3V main sequence star of apparent magnitude of 1.86 and lies at about 100 → light-years.

Etymology (EN): Alkaid “leader, chief,” from Al-Qa’id al-Banat an-Na’ash “the leader of the daughters of the bier,” from Banat “daughters”

• Na’ash “bier”. Banat an-Na’ash is the Ar. name of the constellation.

Etymology (PE): Qâed from Ar. Al-Qa’id.

A substance that dissolves in water to give hydroxide ions. A generic name for → bases.

Etymology (EN): M.E. alkaly, from M.fr. alcali, M.L. alkali, from Ar. al-qily (القلی) “salt wort,” a plant growing in alkaline soils.

Etymology (PE): Qalyâ, loan from Ar., as above.

A substance that dissolves in water to give hydroxide ions. A generic name for → bases.

Etymology (EN): M.E. alkaly, from M.fr. alcali, M.L. alkali, from Ar. al-qily (القلی) “salt wort,” a plant growing in alkaline soils.

Etymology (PE): Qalyâ, loan from Ar., as above.

A spectral line produced by an → alkali metal.

See also: → alkali; → line.

A spectral line produced by an → alkali metal.

See also: → alkali; → line.

Any of the chemical elements belonging to group A of the → periodic table, which burn vigorously in air; i.e. → lithium, → sodium, → potassium, → rubidium, → cesium, and → francium. Alkali metals have a → valence of one and are softer and less dense than other metals.

See also: → alkali; → metal.

Any of the chemical elements belonging to group A of the → periodic table, which burn vigorously in air; i.e. → lithium, → sodium, → potassium, → rubidium, → cesium, and → francium. Alkali metals have a → valence of one and are softer and less dense than other metals.

See also: → alkali; → metal.

Having the properties of an → alkali; having a → pH greater than 7.0.

Etymology (EN): From alkal(i) + -ine a suffix denoting “of or pertaining to,” “of the nature of,” “like.”

Etymology (PE): From qalyâ, → alkali, + -yi adj. suffix.

Having the properties of an → alkali; having a → pH greater than 7.0.

Etymology (EN): From alkal(i) + -ine a suffix denoting “of or pertaining to,” “of the nature of,” “like.”

Etymology (PE): From qalyâ, → alkali, + -yi adj. suffix.

Any of the metallic chemical elements belonging to group 2 of the → periodic table; i.e. → beryllium, → magnesium, → calcium, → strontium, → barium, and → radium. They are not found free in the nature because they are highly reactive.

Etymology (EN): → alkaline; → earth; → metal.

Etymology (PE): Felez, → metal; qalyâyi, → alkaline; xâki “of or pertaining to soil,” from xâk, → soil.

Any of the metallic chemical elements belonging to group 2 of the → periodic table; i.e. → beryllium, → magnesium, → calcium, → strontium, → barium, and → radium. They are not found free in the nature because they are highly reactive.

Etymology (EN): → alkaline; → earth; → metal.

Etymology (PE): Felez, → metal; qalyâyi, → alkaline; xâki “of or pertaining to soil,” from xâk, → soil.

The whole quantity or amount.

Etymology (EN): M.E. al, plural alle; O.E. eall “all, every, entire;”
cf. O.Fris., O.H.G. al, O.N. allr, Goth. alls.

Etymology (PE): Hamé- “all,” variant hami “all the time, always;” Mid.Pers. hamâg “all,” hamê “all the time, always;” Av. hama- “any;” cf. Skt. sama-“any, every, whichever;” Gk. amo-then “whichever;”
Goth. sums “any;” O.N. sumr “any;” O.E. sum “some;” E. some.

The whole quantity or amount.

Etymology (EN): M.E. al, plural alle; O.E. eall “all, every, entire;”
cf. O.Fris., O.H.G. al, O.N. allr, Goth. alls.

Etymology (PE): Hamé- “all,” variant hami “all the time, always;” Mid.Pers. hamâg “all,” hamê “all the time, always;” Av. hama- “any;” cf. Skt. sama-“any, every, whichever;” Gk. amo-then “whichever;”
Goth. sums “any;” O.N. sumr “any;” O.E. sum “some;” E. some.

A → survey that collects data on the whole sky. For example the infrared → Two Micron All Sky Survey (2MASS) and the X-ray → ROSAT All-Sky Survey.

See also: → all; → sky; → survey.

A → survey that collects data on the whole sky. For example the infrared → Two Micron All Sky Survey (2MASS) and the X-ray → ROSAT All-Sky Survey.

See also: → all; → sky; → survey.

A “Large Number of Small Dishes” (LNSD) array designed to be sensitive for → commensal surveys of conventional → radio astronomy

projects and → SETI targets at centimeter wavelengths.

The ATA will consist of 350 6m-diameter → dishes when completed, which will provide an outstanding survey speed and sensitivity. In addition, the many → antennas and → baseline pairs provide a rich → sampling of the → interferometer  → uv plane, so that a single pointing snapshot of the array of 350 antennas yields an image in a single field with about 15,000 independent → pixels. Other important features of the ATA include continuous frequency coverage over 0.5 GHz to 10 GHz and four simultaneously available 600-MHz bands at the → back-end which can be tuned to different frequencies in the overall band.

The ATA is a joint project of the Radio Astronomy Laboratory of the University of California, Berkeley, and the SETI Institute in Mountain View, CA.

The ATA is now complete to 42 antennas. Highlights of the system are the frequency agility, the low background and → side lobes of the antennas, the wideband feed and input receiver, the analog fiber optical system, the large spatial dynamic range, the back-end processing systems and the overall low cost (see, e.g., Backer et al., 2009, arXiv:0908.1175.pdf).

See also: Named after Paul G. Allen (1953-2018), an American business magnate, computer programmer, researcher, investor, and philanthropist. A donation of $11.5 million by his foundation in 2004 contributed to the development of the project.

A “Large Number of Small Dishes” (LNSD) array designed to be sensitive for → commensal surveys of conventional → radio astronomy

projects and → SETI targets at centimeter wavelengths.

The ATA will consist of 350 6m-diameter → dishes when completed, which will provide an outstanding survey speed and sensitivity. In addition, the many → antennas and → baseline pairs provide a rich → sampling of the → interferometer  → uv plane, so that a single pointing snapshot of the array of 350 antennas yields an image in a single field with about 15,000 independent → pixels. Other important features of the ATA include continuous frequency coverage over 0.5 GHz to 10 GHz and four simultaneously available 600-MHz bands at the → back-end which can be tuned to different frequencies in the overall band.

The ATA is a joint project of the Radio Astronomy Laboratory of the University of California, Berkeley, and the SETI Institute in Mountain View, CA.

The ATA is now complete to 42 antennas. Highlights of the system are the frequency agility, the low background and → side lobes of the antennas, the wideband feed and input receiver, the analog fiber optical system, the large spatial dynamic range, the back-end processing systems and the overall low cost (see, e.g., Backer et al., 2009, arXiv:0908.1175.pdf).

See also: Named after Paul G. Allen (1953-2018), an American business magnate, computer programmer, researcher, investor, and philanthropist. A donation of $11.5 million by his foundation in 2004 contributed to the development of the project.

1. The act of allying or state of being allied; the result of this action.
   

2. A union to advance the common interests of the members.

Etymology (EN): M.E., from O.Fr. aliance, from al(ier) “to ally,” → alloy, + → -ance.

Etymology (PE): Hamdasti, literally “joining hand,” from ham-, → com-,

• dast, → hand, + -i noun suffix.

1. The act of allying or state of being allied; the result of this action.
   

2. A union to advance the common interests of the members.

Etymology (EN): M.E., from O.Fr. aliance, from al(ier) “to ally,” → alloy, + → -ance.

Etymology (PE): Hamdasti, literally “joining hand,” from ham-, → com-,

• dast, → hand, + -i noun suffix.

1. General: To assign or allot for a particular purpose.
   

2. Computers: To assign an amount of resource (disk, memory, CPU time, etc.) to a defined user.

Etymology (EN): From M.L. allocate imperative plural of allocare “allocate,” from → ad- “to” + locare “to place,” from locus “a place.”

Etymology (PE): Teskidan, from tesk “portion, share, part, lot; a tax upon lands, tribute extracted,” variants tešk, toxs (kardan) “distribute, divide;” loaned in Ar. tisq, tasq; tasu “a weight of four barley corns; the twenty-forth part of a weight;” Mid.Pers.
tasû “the fourth part,” loaned in Ar. tassûj, in Syriac tassûgâ “the fourth part; a measure;” ultimately Proto-Ir. *caçû-ka-; cf. Av. caθwarô, catur-, → four.

1. General: To assign or allot for a particular purpose.
   

2. Computers: To assign an amount of resource (disk, memory, CPU time, etc.) to a defined user.

Etymology (EN): From M.L. allocate imperative plural of allocare “allocate,” from → ad- “to” + locare “to place,” from locus “a place.”

Etymology (PE): Teskidan, from tesk “portion, share, part, lot; a tax upon lands, tribute extracted,” variants tešk, toxs (kardan) “distribute, divide;” loaned in Ar. tisq, tasq; tasu “a weight of four barley corns; the twenty-forth part of a weight;” Mid.Pers.
tasû “the fourth part,” loaned in Ar. tassûj, in Syriac tassûgâ “the fourth part; a measure;” ultimately Proto-Ir. *caçû-ka-; cf. Av. caθwarô, catur-, → four.

The act of allocating; the state of being allocated.

See also: Verbal noun of → allocate.

The act of allocating; the state of being allocated.

See also: Verbal noun of → allocate.

One of two or more forms in which a → chemical element occurs, each differing in physical properties; e.g. → diamond and → graphite are allotropes of → carbon.

Etymology (EN): From allo-, combining form of Gk. allos “other, different;” cf. L. alius “else;” → alias + trope, from Gk. -tropos “a turn, way, manner,” from tropein “to turn;” PIE base *trep- “to turn” (cf. L. trepit “he turns”).

Etymology (PE): Degarvâr, from degar “other, another” (Mid.Pers. dit, ditikar “the other, the second;” O.Pers. duvitiya- “second;” Av. daibitya-, bitya- “second;” Skt. dvitiya- “second;” PIE *duitiio- “second”) + -vâr denoting “resembling, like;” Mid.Pers. -wâr; Av. -vara, -var; cf. Skt. -vara.

One of two or more forms in which a → chemical element occurs, each differing in physical properties; e.g. → diamond and → graphite are allotropes of → carbon.

Etymology (EN): From allo-, combining form of Gk. allos “other, different;” cf. L. alius “else;” → alias + trope, from Gk. -tropos “a turn, way, manner,” from tropein “to turn;” PIE base *trep- “to turn” (cf. L. trepit “he turns”).

Etymology (PE): Degarvâr, from degar “other, another” (Mid.Pers. dit, ditikar “the other, the second;” O.Pers. duvitiya- “second;” Av. daibitya-, bitya- “second;” Skt. dvitiya- “second;” PIE *duitiio- “second”) + -vâr denoting “resembling, like;” Mid.Pers. -wâr; Av. -vara, -var; cf. Skt. -vara.

A property of certain → chemical elements, as → carbon, → sulfur, and → phosphorus
of existing in two or more distinct forms, known as → allotropes.

See also: → allotrope.

A property of certain → chemical elements, as → carbon, → sulfur, and → phosphorus
of existing in two or more distinct forms, known as → allotropes.

See also: → allotrope.

In solid-state physics, the range of energies which electrons can attain in a material.

Etymology (EN): P.p. of v. allow, from O.Fr. alouer “approve,” from L. allaudare , compound of → ad- “to” + laudare “to praise.”

Etymology (PE): Bând, → band; parzâmidé, p.p. of parzâmidan “to send through, permit, allow,” from parzâm “permission,” from par- “through”

• zâm stem of zâmidan, Mid.Pers. zâmenidan
  “to send, lead;” → permit

In solid-state physics, the range of energies which electrons can attain in a material.

Etymology (EN): P.p. of v. allow, from O.Fr. alouer “approve,” from L. allaudare , compound of → ad- “to” + laudare “to praise.”

Etymology (PE): Bând, → band; parzâmidé, p.p. of parzâmidan “to send through, permit, allow,” from parzâm “permission,” from par- “through”

• zâm stem of zâmidan, Mid.Pers. zâmenidan
  “to send, lead;” → permit

A material composed of two or more → metals, or of a metal or metals with a non-metal, exhibiting characteristic metallic properties. Some examples: → bronze is an alloy of → copper and → tin, brass is an alloy of → zinc and copper, and → steel is an alloy of → iron and → carbon. Alloys have properties which differ from those of their components. Moreover, different component proportions yield alloys with different properties.

Etymology (EN): From M.F. aloi, from O.Fr. alei, from aleier “to mix, combine,” from L. alligare “to bind up,” from → ad- “to” + → ligare “to bind.”

Etymology (PE): Âlyâž, loanword from Fr.

A material composed of two or more → metals, or of a metal or metals with a non-metal, exhibiting characteristic metallic properties. Some examples: → bronze is an alloy of → copper and → tin, brass is an alloy of → zinc and copper, and → steel is an alloy of → iron and → carbon. Alloys have properties which differ from those of their components. Moreover, different component proportions yield alloys with different properties.

Etymology (EN): From M.F. aloi, from O.Fr. alei, from aleier “to mix, combine,” from L. alligare “to bind up,” from → ad- “to” + → ligare “to bind.”

Etymology (PE): Âlyâž, loanword from Fr.

The third brightest star in Andromeda and one of the most beautiful double stars in the sky. The brighter star of the pair appears golden yellow or slightly orange; it
is a bright (of second magnitude) giant K star. The fainter companion, which appears greenish-blue, is also double.

Etymology (EN): This star is also known as Almaak, Alamak, Almak, or Almaach, from Ar. Al-‘Anaq al-‘Ardh “a small animal of Arabia similar to a badger.”

The third brightest star in Andromeda and one of the most beautiful double stars in the sky. The brighter star of the pair appears golden yellow or slightly orange; it
is a bright (of second magnitude) giant K star. The fainter companion, which appears greenish-blue, is also double.

Etymology (EN): This star is also known as Almaak, Alamak, Almak, or Almaach, from Ar. Al-‘Anaq al-‘Ardh “a small animal of Arabia similar to a badger.”

A comprehensive treatise, compiled by Claudius Ptolemy of Alexandria, around A.D. 140,
that summarized the astronomy, geography, and mathematics of antiquity, and included a star catalogue with data for 1,022 stars.

Etymology (EN): Almagest, from Ar. Al-majisti, from al “the” + Gk. megiste (suntaxis) “the greatest (composition),” from femenine of megistos, superlative of megas “great.”

A comprehensive treatise, compiled by Claudius Ptolemy of Alexandria, around A.D. 140,
that summarized the astronomy, geography, and mathematics of antiquity, and included a star catalogue with data for 1,022 stars.

Etymology (EN): Almagest, from Ar. Al-majisti, from al “the” + Gk. megiste (suntaxis) “the greatest (composition),” from femenine of megistos, superlative of megas “great.”

A book of tables, usually covering a period of one calendar year, that lists the future positions of the Moon, planets, and other prominent celestial objects, together with other useful astronomical data.

Etymology (EN): M.E. almenak, from M.L. almanach, perhaps from late Gk. almenikhiaka “ephemeris,” perhaps of Coptic origin.

Etymology (PE): Axtar sâlnâmé, from axtar, → star, + sâlnâmé “calendar,” from sâl, → year,

• nâmé “book.”

A book of tables, usually covering a period of one calendar year, that lists the future positions of the Moon, planets, and other prominent celestial objects, together with other useful astronomical data.

Etymology (EN): M.E. almenak, from M.L. almanach, perhaps from late Gk. almenikhiaka “ephemeris,” perhaps of Coptic origin.

Etymology (PE): Axtar sâlnâmé, from axtar, → star, + sâlnâmé “calendar,” from sâl, → year,

• nâmé “book.”

A small circle on the celestial sphere parallel to the horizon. The locus of all points of a given altitude. Also called altitude circle, circle of altitude, parallel of altitude.

Etymology (EN): Almucantar, from L. almucantarath, from Ar. almuqantarât, from al- “the” + muqantarât “sundial,” from qantarah “arch”.

Etymology (PE): Moqantar, from Ar., as above. Parhun-e farâzâ from parhun, → circle, + farâzâ→ altitude.

A small circle on the celestial sphere parallel to the horizon. The locus of all points of a given altitude. Also called altitude circle, circle of altitude, parallel of altitude.

Etymology (EN): Almucantar, from L. almucantarath, from Ar. almuqantarât, from al- “the” + muqantarât “sundial,” from qantarah “arch”.

Etymology (PE): Moqantar, from Ar., as above. Parhun-e farâzâ from parhun, → circle, + farâzâ→ altitude.

A blue star of visual magnitude 1.65 in the constellation → Taurus. Alnath is a giant star of type B7 lying at a distance of about 10.95 → light-years.

Etymology (EN): Alnath, from Ar. An-nâteh “the butting” (horn), from nath “to butt”.

Etymology (PE): Šâxzan “the butting,” from Mod.Pers. šâx zadan “to butt or push with the horns,” from šâx “horn” + zadan “to strike, to butt”.

A blue star of visual magnitude 1.65 in the constellation → Taurus. Alnath is a giant star of type B7 lying at a distance of about 10.95 → light-years.

Etymology (EN): Alnath, from Ar. An-nâteh “the butting” (horn), from nath “to butt”.

Etymology (PE): Šâxzan “the butting,” from Mod.Pers. šâx zadan “to butt or push with the horns,” from šâx “horn” + zadan “to strike, to butt”.

The central and brightest of the three stars in → Orion’s Belt and the fourth brightest in the whole of → Orion. Alnilam is a blue-white → supergiant of → spectral type B0 Iae with a → visual magnitude of 1.70 and a → luminosity of 375,000 times the → solar luminosity. It lies at about 1,340 → light-years.

Etymology (EN): Alnilam, from Ar. An-Nizam al-Jawza’ (النظام‌الجوزاء)
“the Orion’s arrangement (of pearls, beads),” from nizam “arrangement” + Jawza’ “Orion”.

The central and brightest of the three stars in → Orion’s Belt and the fourth brightest in the whole of → Orion. Alnilam is a blue-white → supergiant of → spectral type B0 Iae with a → visual magnitude of 1.70 and a → luminosity of 375,000 times the → solar luminosity. It lies at about 1,340 → light-years.

Etymology (EN): Alnilam, from Ar. An-Nizam al-Jawza’ (النظام‌الجوزاء)
“the Orion’s arrangement (of pearls, beads),” from nizam “arrangement” + Jawza’ “Orion”.

The left hand or easternmost star in → Orion’s Belt, which is the fifth brightest in the whole of → Orion with a → visual magnitude of 1.79. Alnitak is a wide visual binary system consisting of components ζ Ori A (HR 1948) and ζ Ori B (HR 1949), currently separated by ~ 2’’.4. ζ Ori A is a → close binary system comprising Alnitak Aa and Alnitak Ab. Aa is a hot → blue supergiant of → spectral type O9.5 Iab with an → absolute magnitude of -6.0 and an → apparent magnitude of 2.08. Its mass is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 250,000 times more luminous than the Sun, with a surface temperature of about 30,000 K. It is the brightest star of class O in the night sky. Alnitak Ab is a blue → subgiant of spectral type B1 IV with an absolute magnitude of -3.9, an apparent magnitude of 4.3, and a mass of 14 Msun. Ab revolves around Ab with a period of 2,687 days. The system has a 4th magnitude companion, Alnitak B, nearly 3 arc-seconds distant. It is a B0 III type star which orbits Alnitak A every 1,500 years. Alnitak is associated with the → emission nebula  → IC 434 containing the → Horsehead Nebula.A much fainter fourth component, ζ Ori C, is located about 57’’ away from ζ Ori Aa
(C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).

Etymology (EN): Alnitak, from Ar. An-Nitaq al-Jawza’ (النطاق‌الجوزاء) “Orion’s Belt,” from nitaq “belt” + Jawza “Orion.”

The left hand or easternmost star in → Orion’s Belt, which is the fifth brightest in the whole of → Orion with a → visual magnitude of 1.79. Alnitak is a wide visual binary system consisting of components ζ Ori A (HR 1948) and ζ Ori B (HR 1949), currently separated by ~ 2’’.4. ζ Ori A is a → close binary system comprising Alnitak Aa and Alnitak Ab. Aa is a hot → blue supergiant of → spectral type O9.5 Iab with an → absolute magnitude of -6.0 and an → apparent magnitude of 2.08. Its mass is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 250,000 times more luminous than the Sun, with a surface temperature of about 30,000 K. It is the brightest star of class O in the night sky. Alnitak Ab is a blue → subgiant of spectral type B1 IV with an absolute magnitude of -3.9, an apparent magnitude of 4.3, and a mass of 14 Msun. Ab revolves around Ab with a period of 2,687 days. The system has a 4th magnitude companion, Alnitak B, nearly 3 arc-seconds distant. It is a B0 III type star which orbits Alnitak A every 1,500 years. Alnitak is associated with the → emission nebula  → IC 434 containing the → Horsehead Nebula.A much fainter fourth component, ζ Ori C, is located about 57’’ away from ζ Ori Aa
(C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).

Etymology (EN): Alnitak, from Ar. An-Nitaq al-Jawza’ (النطاق‌الجوزاء) “Orion’s Belt,” from nitaq “belt” + Jawza “Orion.”

The first letter of the Greek alphabet (A, α).

See also: Gk. alpha, from Hebrew or Phoenician → aleph.

The first letter of the Greek alphabet (A, α).

See also: Gk. alpha, from Hebrew or Phoenician → aleph.

An annual → meteor shower that takes place within the boundaries constellation → Capricornus near the star named Alpha. The meteor shower is visible between July 03 and August 15 with the peak occurring on July 30. Alpha Capricornids meteors are bright and often include spectacular colorful → fireballs.

See also: → alpha; → Capricornus.

An annual → meteor shower that takes place within the boundaries constellation → Capricornus near the star named Alpha. The meteor shower is visible between July 03 and August 15 with the peak occurring on July 30. Alpha Capricornids meteors are bright and often include spectacular colorful → fireballs.

See also: → alpha; → Capricornus.

Brightest star in the constellation → Centaurus (V = -0.01 magnitude) and third brightest star in the sky; also known as → Rigil Kent. It is a main-sequence star of the same spectral class (G2 V) as the Sun. Actually, Alpha Centauri is a triple-star system, the components being designated A, B, and C. The component C is also called → Proxima Centauri because it is the closest star to the Earth (other than the Sun), at a distance of 4.22 → light-years, but it is too dim to be seen with the naked eye. Components A and B are currently about 4.36 light-years away.

See also: Alpha (α), a Gk. letter of alphabet used in the → Bayer designation; Centauri, genitive of → Centaurus.

Brightest star in the constellation → Centaurus (V = -0.01 magnitude) and third brightest star in the sky; also known as → Rigil Kent. It is a main-sequence star of the same spectral class (G2 V) as the Sun. Actually, Alpha Centauri is a triple-star system, the components being designated A, B, and C. The component C is also called → Proxima Centauri because it is the closest star to the Earth (other than the Sun), at a distance of 4.22 → light-years, but it is too dim to be seen with the naked eye. Components A and B are currently about 4.36 light-years away.

See also: Alpha (α), a Gk. letter of alphabet used in the → Bayer designation; Centauri, genitive of → Centaurus.

A system of three stars, the → close binary Alpha Centauri A (→ spectral type G2 V) and Alpha Centauri B (K1 V), and a small and faint → red dwarf, Alpha Centauri C (M6 Ve), better known as → Proxima Centauri. To the unaided eye, the two main components (AB) appear as a single object with an → apparent visual magnitude of -0.27, forming the brightest star in the southern constellation → Centaurus
and the third brightest star in the night sky, after → Sirius and → Canopus. The individual visual magnitudes of the components A, B, and Proxima are +0.01, +1.33, and +11.05, respectively. The masses of A and B are 1.100 and 0.907 Msun, respectively. Their → effective temperatures are (A) 5,790 K and (B) 5,260 K; their luminosities (A) 1.519 Lsun and (B) 0.500 Lsun. The binary members are separated in average by only 23 → astronomical units.
They revolve around a common center of mass with a period of about 80 years. Both have a distance of 4.37 → light-years. Proxima Centauri, lying about 15,000 AU apart from AB, is → gravitationally bound to them. It has a mass of 0.1 Msun, a radius of 0.1 Rsun, a luminosity of about 0.001 Lsun, and an → effective temperature of ~ 3,000 K.

See also: → alpha; → Centaurus; → system.

A system of three stars, the → close binary Alpha Centauri A (→ spectral type G2 V) and Alpha Centauri B (K1 V), and a small and faint → red dwarf, Alpha Centauri C (M6 Ve), better known as → Proxima Centauri. To the unaided eye, the two main components (AB) appear as a single object with an → apparent visual magnitude of -0.27, forming the brightest star in the southern constellation → Centaurus
and the third brightest star in the night sky, after → Sirius and → Canopus. The individual visual magnitudes of the components A, B, and Proxima are +0.01, +1.33, and +11.05, respectively. The masses of A and B are 1.100 and 0.907 Msun, respectively. Their → effective temperatures are (A) 5,790 K and (B) 5,260 K; their luminosities (A) 1.519 Lsun and (B) 0.500 Lsun. The binary members are separated in average by only 23 → astronomical units.
They revolve around a common center of mass with a period of about 80 years. Both have a distance of 4.37 → light-years. Proxima Centauri, lying about 15,000 AU apart from AB, is → gravitationally bound to them. It has a mass of 0.1 Msun, a radius of 0.1 Rsun, a luminosity of about 0.001 Lsun, and an → effective temperature of ~ 3,000 K.

See also: → alpha; → Centaurus; → system.

The radioactive transformation of a nuclide by alpha-particle emission. Also called alpha disintegration.

See also: → alpha; → decay.

The radioactive transformation of a nuclide by alpha-particle emission. Also called alpha disintegration.

See also: → alpha; → decay.

A simple → accretion disk model in which the → angular momentum is transported outward by action of some kind of → viscosity. In this model, first proposed by Shakura & Sunyaev (1973), the turbulent kinematic viscosity is given by ν = α c_s H, where α is a parameter, c_s the sound speed in the medium, and H → scale height. The α parameter controls the amount of → turbulence in the medium whose H and c_s are upper limits for
→ mixing length and turbulent speed, respectively. Values of α = 10^-3 to 10^-2 yield evolution → time scales
that are broadly consistent with the ages inferred for → T Tauri stars. A weak point of this model is the arbitrariness of the choice of the parameter α, which reflects the lack of a rigorous theory of turbulence.

See also: → alpha, the parameter name; → disk; → model.

A simple → accretion disk model in which the → angular momentum is transported outward by action of some kind of → viscosity. In this model, first proposed by Shakura & Sunyaev (1973), the turbulent kinematic viscosity is given by ν = α c_s H, where α is a parameter, c_s the sound speed in the medium, and H → scale height. The α parameter controls the amount of → turbulence in the medium whose H and c_s are upper limits for
→ mixing length and turbulent speed, respectively. Values of α = 10^-3 to 10^-2 yield evolution → time scales
that are broadly consistent with the ages inferred for → T Tauri stars. A weak point of this model is the arbitrariness of the choice of the parameter α, which reflects the lack of a rigorous theory of turbulence.

See also: → alpha, the parameter name; → disk; → model.

The → mechanism that imparts or transfers power to a → telescope so that it can move along the → right ascension direction. See also → tacking.

See also: → alpha, → right ascension; → drive.

The → mechanism that imparts or transfers power to a → telescope so that it can move along the → right ascension direction. See also → tacking.

See also: → alpha, → right ascension; → drive.

In the → solar dynamo model, the process whereby
the → azimuthal magnetic field transforms into a → meridional magnetic field due to the interaction of → convection and → differential rotation. See also → omega effect.

See also: → alpha; → effect.

In the → solar dynamo model, the process whereby
the → azimuthal magnetic field transforms into a → meridional magnetic field due to the interaction of → convection and → differential rotation. See also → omega effect.

See also: → alpha; → effect.

A → chemical element synthesized in → massive stars by → alpha particle capture leading to iron before the advent of a
→ type II supernova. Stable alpha elements are: C, O, Ne, Mg, Si, S, Ar, Ca.

See also: → alpha; → element.

A → chemical element synthesized in → massive stars by → alpha particle capture leading to iron before the advent of a
→ type II supernova. Stable alpha elements are: C, O, Ne, Mg, Si, S, Ar, Ca.

See also: → alpha; → element.

The point in the plot showing → alpha element abundances ([α/Fe]) of a galaxy as a function of the → metallicity ([Fe/H]) where the α-element abundance drops.
The metallicity of the turn-over in α-element abundances is linked to the → star formation rate during the early stage of star formation in a galaxy and therefore also depends on the total mass of the system. Higher star formation efficiency leads to higher overall metallicity before the onset of → Type Ia supernova → enrichment, and thus to a knee that is located at higher [Fe/H] values.

See also: → alpha; → element; → knee.

The point in the plot showing → alpha element abundances ([α/Fe]) of a galaxy as a function of the → metallicity ([Fe/H]) where the α-element abundance drops.
The metallicity of the turn-over in α-element abundances is linked to the → star formation rate during the early stage of star formation in a galaxy and therefore also depends on the total mass of the system. Higher star formation efficiency leads to higher overall metallicity before the onset of → Type Ia supernova → enrichment, and thus to a knee that is located at higher [Fe/H] values.

See also: → alpha; → element; → knee.

The release of → alpha particles at high velocity from an atom’s nucleus as it undergoes radioactive transformation.

See also: → alpha; → emission.

The release of → alpha particles at high velocity from an atom’s nucleus as it undergoes radioactive transformation.

See also: → alpha; → emission.

An atomic nucleus decaying by an → alpha particle emission.

See also: → alpha; → emitter.

An atomic nucleus decaying by an → alpha particle emission.

See also: → alpha; → emitter.

→ alpha process.

See also: → alpha; → ladder.

→ alpha process.

See also: → alpha; → ladder.

A short distance from the target, in right ascension, where the telescope is pointed for various purposes.

See also: → alpha; → offset.

A short distance from the target, in right ascension, where the telescope is pointed for various purposes.

See also: → alpha; → offset.

A positively charged particle emitted from the nuclei of certain atoms during radioactive disintegration. The alpha particle has an atomic weight of 4 and a positive charge equal in magnitude to 2 electronic charges; hence it is essentially a helium nucleus.

See also: → alpha; → particle.

A positively charged particle emitted from the nuclei of certain atoms during radioactive disintegration. The alpha particle has an atomic weight of 4 and a positive charge equal in magnitude to 2 electronic charges; hence it is essentially a helium nucleus.

See also: → alpha; → particle.

A class of → nuclear fusion reactions by which stars convert → helium into → heavy elements.

Once carbon has been created, through → triple alpha process,
in a star’s interior, it can then continue to fuse with further → alpha particles to produce progressively heavier elements called → alpha particles. The first stage produces oxygen, followed by neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium and iron. This is known as the → alpha ladder, with energy released as a photon at each stage.

See also: → alpha;→ process.

A class of → nuclear fusion reactions by which stars convert → helium into → heavy elements.

Once carbon has been created, through → triple alpha process,
in a star’s interior, it can then continue to fuse with further → alpha particles to produce progressively heavier elements called → alpha particles. The first stage produces oxygen, followed by neon, magnesium, silicon, sulfur, argon, calcium, titanium, chromium and iron. This is known as the → alpha ladder, with energy released as a photon at each stage.

See also: → alpha;→ process.

The only bright star in the constellation → Hydra, that has a magnitude of about 2 and a reddish color. Alphard is a giant of spectral type K3, and has a → white dwarf→ companion. Alphard is mild barium star probably contaminated by its companion before becoming a white dwarf.

Etymology (EN): Alphard, from Ar. Al-Frad ash-Shuja’ “the solitary of the Serpent,” from Frad “solitary” + Shuja’ “a species of serpent”.

The only bright star in the constellation → Hydra, that has a magnitude of about 2 and a reddish color. Alphard is a giant of spectral type K3, and has a → white dwarf→ companion. Alphard is mild barium star probably contaminated by its companion before becoming a white dwarf.

Etymology (EN): Alphard, from Ar. Al-Frad ash-Shuja’ “the solitary of the Serpent,” from Frad “solitary” + Shuja’ “a species of serpent”.

Also known as Gemma, the brightest star in Corona Borealis (visual magnitude 2.23). Alphekka is an A type dwarf lying at about 7 → light-years. Actually it has a faint Sun-like (G5 V) companion, that produces an eclipse of the primary every 17.4 days.

Etymology (EN): Alphekka, from Ar. Nayyir al-Fakkah “the bright of the broken” (ring of star), from Nayyir “bright” + fakkah “broken,” from fakk “to disjoin, unloose”.

Also known as Gemma, the brightest star in Corona Borealis (visual magnitude 2.23). Alphekka is an A type dwarf lying at about 7 → light-years. Actually it has a faint Sun-like (G5 V) companion, that produces an eclipse of the primary every 17.4 days.

Etymology (EN): Alphekka, from Ar. Nayyir al-Fakkah “the bright of the broken” (ring of star), from Nayyir “bright” + fakkah “broken,” from fakk “to disjoin, unloose”.

The brightest star in → Andromeda with a visual magnitude of 2.07. Alpheratz is a blue → subgiant star of spectral type B8 IV lying at a distance of about 97 → light-years. It is particularly remarkable because of the unusual strength of mercury and manganese absorption lines in its spectrum.

Etymology (EN): Other names for this star are Alpherat, Sirrah, or Sirah. These names derive from Ar. As-Surrat al-Faras (السره‌الفرس) “The Horse’s Navel,” from surrat (سره) “navel” + faras (فرس) “horse”. The name refers to the location of the star in the figure of → Pegasus. However, the star is now considered to belong to the constellation → Andromeda.

Ra’s-ol-Mosalsalé, from Ar. Ar-Ra’s al-Mar’ah al-Musalsalah “The head of the chained woman,” from Ra’s “head” + Mar’ah “woman” + Musalsalah “chained”.

The brightest star in → Andromeda with a visual magnitude of 2.07. Alpheratz is a blue → subgiant star of spectral type B8 IV lying at a distance of about 97 → light-years. It is particularly remarkable because of the unusual strength of mercury and manganese absorption lines in its spectrum.

Etymology (EN): Other names for this star are Alpherat, Sirrah, or Sirah. These names derive from Ar. As-Surrat al-Faras (السره‌الفرس) “The Horse’s Navel,” from surrat (سره) “navel” + faras (فرس) “horse”. The name refers to the location of the star in the figure of → Pegasus. However, the star is now considered to belong to the constellation → Andromeda.

Ra’s-ol-Mosalsalé, from Ar. Ar-Ra’s al-Mar’ah al-Musalsalah “The head of the chained woman,” from Ra’s “head” + Mar’ah “woman” + Musalsalah “chained”.

1. By this or that time; previously; prior to or at some specified or implied time.
   

2. Now; so soon; so early (Dictionary.com).

Etymology (EN): From M.E. al redy, literally “fully ready,” → all, + M.E. redy “ready,” from rædig, from O.E. ræde “prompt” + -ig “-y.”

Etymology (PE): Pišnun, literally “prior to now,” from piš-, → pre-, + Mid./Mod.Pers. nun “now, at present” (variants aknun, konun, ultimately from Proto-Ir.
*hak-nun); Av. nū- “now,” nūrəm “now;” O.Pers.
nūram “now;” cf. Skt. nú- “now, just, but,” nūnám “now, at present, indeed;” Gk. nun “now;” L. nu- “now” (in nu-dis “the day after tomorrow”); Goth. nu “now;” O.E. nu; E. now; PIE base *nu- “now.”

1. By this or that time; previously; prior to or at some specified or implied time.
   

2. Now; so soon; so early (Dictionary.com).

Etymology (EN): From M.E. al redy, literally “fully ready,” → all, + M.E. redy “ready,” from rædig, from O.E. ræde “prompt” + -ig “-y.”

Etymology (PE): Pišnun, literally “prior to now,” from piš-, → pre-, + Mid./Mod.Pers. nun “now, at present” (variants aknun, konun, ultimately from Proto-Ir.
*hak-nun); Av. nū- “now,” nūrəm “now;” O.Pers.
nūram “now;” cf. Skt. nú- “now, just, but,” nūnám “now, at present, indeed;” Gk. nun “now;” L. nu- “now” (in nu-dis “the day after tomorrow”); Goth. nu “now;” O.E. nu; E. now; PIE base *nu- “now.”

The brightest star in → Aquila (apparent visual magnitude 0.77), and the twelfth brightest star in the sky. Altair is a whitish A7 → main sequence star. It has one of the fastest known rotational speeds, 242 km/s at the equator, compared with the Sun’s about 2 km/s.

Etymology (EN): Altair, from Ar. An Nasr at-Taiir “The Flying Vulture,” from Nasr “vulture” + Ta’ir “flying.”

Etymology (PE): Karkas-e Parandé “The Flying Vulture” coined by Biruni (around A.D. 1000), from karkas “vulture,” Av. kahrkâsa- “devourer hen,” from *kahrka “hen”
(Mod.Pers. kark “hen,” karak “quail”)

• *âsa “to eat” (Mod.Pers. âš “food, soup”) + Parandé “flying,” from paridan “to fly” (from Mid./Mod.Pers. par(r) “feather, wing,” Av. parəna- “feather, wing;” cp. Skt. parna “feather,” E. fern; PIE *porno- “feather”).

The brightest star in → Aquila (apparent visual magnitude 0.77), and the twelfth brightest star in the sky. Altair is a whitish A7 → main sequence star. It has one of the fastest known rotational speeds, 242 km/s at the equator, compared with the Sun’s about 2 km/s.

Etymology (EN): Altair, from Ar. An Nasr at-Taiir “The Flying Vulture,” from Nasr “vulture” + Ta’ir “flying.”

Etymology (PE): Karkas-e Parandé “The Flying Vulture” coined by Biruni (around A.D. 1000), from karkas “vulture,” Av. kahrkâsa- “devourer hen,” from *kahrka “hen”
(Mod.Pers. kark “hen,” karak “quail”)

• *âsa “to eat” (Mod.Pers. âš “food, soup”) + Parandé “flying,” from paridan “to fly” (from Mid./Mod.Pers. par(r) “feather, wing,” Av. parəna- “feather, wing;” cp. Skt. parna “feather,” E. fern; PIE *porno- “feather”).

1. A → mounting for → telescopes that permits both → vertical and
   → horizontal → rotation.
   

2. A telescope having such a mounting.
   

See also:

→ altazimuth coordinate system, → altazimuth instrument, → altazimuth mounting.

Etymology (EN): Altazimuth, from alt(itude) + → azimuth.

Etymology (PE): Farâzâ-sugân, from farâzâ, → altitude, + sugân, → azimuth.

1. A → mounting for → telescopes that permits both → vertical and
   → horizontal → rotation.
   

2. A telescope having such a mounting.
   

See also:

→ altazimuth coordinate system, → altazimuth instrument, → altazimuth mounting.

Etymology (EN): Altazimuth, from alt(itude) + → azimuth.

Etymology (PE): Farâzâ-sugân, from farâzâ, → altitude, + sugân, → azimuth.

The coordinate system in which the position of a body on the → celestial sphere is described with respect to an observer’s → celestial horizon and → zenith. The coordinates of a point in this system are its → altitude on the → vertical circle, and its → azimuth westward (clockwise) along the celestial horizon from the observer’s south. Same as → horizon coordinate system.

See also: → altazimuth; → coordinate; → system.

The coordinate system in which the position of a body on the → celestial sphere is described with respect to an observer’s → celestial horizon and → zenith. The coordinates of a point in this system are its → altitude on the → vertical circle, and its → azimuth westward (clockwise) along the celestial horizon from the observer’s south. Same as → horizon coordinate system.

See also: → altazimuth; → coordinate; → system.

A telescope that moves vertically along the → altitude circle of a celestial body and horizontally along its → azimuth circle.

See also: → altazimuth; → instrument.

A telescope that moves vertically along the → altitude circle of a celestial body and horizontally along its → azimuth circle.

See also: → altazimuth; → instrument.

A → telescope mounting which has its two axes of movement aligned with the → horizon and the → zenith.

See also: → mounting; → mounting.

A → telescope mounting which has its two axes of movement aligned with the → horizon and the → zenith.

See also: → mounting; → mounting.

(v.tr.) To change or make different.
(v.intr.) To change or become different.

Etymology (EN): M.E. alteren, from O.Fr. altérer, from M.L. alterre, from L. alter “other,” from PIE *al- “beyond” + comp. suffix -ter.

Etymology (PE): Degargun kardan, degargunidan, from degargun, from degar, → change, + -gun “manner, fashion,” → elliptical, + kardan, -idan, → -ize.

(v.tr.) To change or make different.
(v.intr.) To change or become different.

Etymology (EN): M.E. alteren, from O.Fr. altérer, from M.L. alterre, from L. alter “other,” from PIE *al- “beyond” + comp. suffix -ter.

Etymology (PE): Degargun kardan, degargunidan, from degargun, from degar, → change, + -gun “manner, fashion,” → elliptical, + kardan, -idan, → -ize.

1. The act or process of altering; the state of being altered.
   

2. The result of altering.

See also: Verbal noun of → alter.

1. The act or process of altering; the state of being altered.
   

2. The result of altering.

See also: Verbal noun of → alter.

1. Happening or following in turns; succeeding each other continuously.
   

2a) (v.intr.) To occur in a successive manner (day alternates with night); To change back and forth from one state, action, or place to another.

2b) Electricity: To reverse direction at regular intervals in a → circuit.

3. (v.tr.) To do or execute in succession or one after another (to alternate work and rest).

Etymology (EN): From L. alternus “one after the other,” p.p. of alternare “to do first one thing, then the other,” from alternus “every other,” from alter “the other”.

Etymology (PE): 1) Peyvâr from pey “step; after” (as in peyâpey “successively, repeatedly”) + Pers. vâr “turn (succession), time (repetition), alternation”. Compare with Skt. vâra “one’s turn, appointed time, alteration, succession”.

2, 3) Peyvâr šodan, peyvâr kardan, peyvâridan, verbs from peyvâr “alternate,” as above.

1. Happening or following in turns; succeeding each other continuously.
   

2a) (v.intr.) To occur in a successive manner (day alternates with night); To change back and forth from one state, action, or place to another.

2b) Electricity: To reverse direction at regular intervals in a → circuit.

3. (v.tr.) To do or execute in succession or one after another (to alternate work and rest).

Etymology (EN): From L. alternus “one after the other,” p.p. of alternare “to do first one thing, then the other,” from alternus “every other,” from alter “the other”.

Etymology (PE): 1) Peyvâr from pey “step; after” (as in peyâpey “successively, repeatedly”) + Pers. vâr “turn (succession), time (repetition), alternation”. Compare with Skt. vâra “one’s turn, appointed time, alteration, succession”.

2, 3) Peyvâr šodan, peyvâr kardan, peyvâridan, verbs from peyvâr “alternate,” as above.

Occuring in a successive manner. Acting or proceeding by turns. → alternating current.

See also: Verbal adj. from → alternate; → -ing.

Occuring in a successive manner. Acting or proceeding by turns. → alternating current.

See also: Verbal adj. from → alternate; → -ing.

An → electric current that reverses direction of flow at regular intervals. The typical alternating current is → sinusoidal in shape. Alternating current has the advantage over → direct current in that its → voltage magnitude can be changed easily through a → transformer. Moreover, it
is safer to transfer over the longer city distances and can provide more → power. The frequency of alternating current is 50 Hz (Europe) or 60 Hz (USA).

See also: → alternating; → current.

An → electric current that reverses direction of flow at regular intervals. The typical alternating current is → sinusoidal in shape. Alternating current has the advantage over → direct current in that its → voltage magnitude can be changed easily through a → transformer. Moreover, it
is safer to transfer over the longer city distances and can provide more → power. The frequency of alternating current is 50 Hz (Europe) or 60 Hz (USA).

See also: → alternating; → current.

General: Successive change from one thing or state to another and back again.
The process of periodically varying a voltage from zero to a maximum, back to zero, to a minimum, and then to zero.

Etymology (EN): Alternation, noun from → alternate.

General: Successive change from one thing or state to another and back again.
The process of periodically varying a voltage from zero to a maximum, back to zero, to a minimum, and then to zero.

Etymology (EN): Alternation, noun from → alternate.

1. (n) A proposition or situation offering a choice between two or more things only one of which may be chosen.
   
2. (adj) Offering or expressing a choice. → alternative energy, → alternatively.

Etymology (EN): Alternative, from → alternate.

Etymology (PE): Degarine, from degar “other,” variant digar, from Mid.Pers. dit, ditikar “the other, the second,” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second,” Skt. dvitiya- “second,” PIE *duitiio- “second” + suffix -ine.

1. (n) A proposition or situation offering a choice between two or more things only one of which may be chosen.
   
2. (adj) Offering or expressing a choice. → alternative energy, → alternatively.

Etymology (EN): Alternative, from → alternate.

Etymology (PE): Degarine, from degar “other,” variant digar, from Mid.Pers. dit, ditikar “the other, the second,” O.Pers. duvitiya- “second,” Av. daibitya-, bitya- “second,” Skt. dvitiya- “second,” PIE *duitiio- “second” + suffix -ine.

Energy from a source other than the conventional fossil fuel sources.

See also: → alternative; → energy.

Energy from a source other than the conventional fossil fuel sources.

See also: → alternative; → energy.

Statistics: In → significance testing, any hypothesis which differs from the one being tested. A
hypothesis alternative to the → null hypothesis is denoted by H₁.

See also: → alternative; → hypothesis.

Statistics: In → significance testing, any hypothesis which differs from the one being tested. A
hypothesis alternative to the → null hypothesis is denoted by H₁.

See also: → alternative; → hypothesis.

In place of, or as an alternative to.

Etymology (EN): Adverb from → alternative.

In place of, or as an alternative to.

Etymology (EN): Adverb from → alternative.

A machine or device that produces alternating current.

Etymology (EN): Alternator, noun from → alternate + → -or.

A machine or device that produces alternating current.

Etymology (EN): Alternator, noun from → alternate + → -or.

An instrument which determines the altitude of an object with respect to a fixed level, such as sea level.

Etymology (EN): L. altus “high” + → -meter.

Etymology (PE): Farâzsanj, from farâz “above, over, aloft” + -sanj, → -meter; farâzyâb, from farâz + yâb “finder,” from yâbidan “to find, discover, obtain.”

An instrument which determines the altitude of an object with respect to a fixed level, such as sea level.

Etymology (EN): L. altus “high” + → -meter.

Etymology (PE): Farâzsanj, from farâz “above, over, aloft” + -sanj, → -meter; farâzyâb, from farâz + yâb “finder,” from yâbidan “to find, discover, obtain.”

The measurement of heights in the atmosphere (altitude) by an → altimeter.

See also: → altimeter; → -metry.

The measurement of heights in the atmosphere (altitude) by an → altimeter.

See also: → altimeter; → -metry.

1. The height above sea level.
   

2. The angle between an object’s position on the celestial sphere and the horizon.
   

3. Of a triangle, the perpendicular from the base to the opposite vertex.

Etymology (EN): Altitude, from L. altitudo, from altus “high”.

Etymology (PE): Farâzâ, from farâz “above, up, upon,” → height, + noun making suffix -a.

1. The height above sea level.
   

2. The angle between an object’s position on the celestial sphere and the horizon.
   

3. Of a triangle, the perpendicular from the base to the opposite vertex.

Etymology (EN): Altitude, from L. altitudo, from altus “high”.

Etymology (PE): Farâzâ, from farâz “above, up, upon,” → height, + noun making suffix -a.

A circle on the celestial sphere that has equal altitude over the Earth’s surface and lies parallel to the horizon. Also called almucantar, circle of altitude, parallel of altitude.

See also: → altitude; → circle.

A circle on the celestial sphere that has equal altitude over the Earth’s surface and lies parallel to the horizon. Also called almucantar, circle of altitude, parallel of altitude.

See also: → altitude; → circle.

Same as → altazimuth mounting.

See also: → altitude; → azimuth; → mounting.

Same as → altazimuth mounting.

See also: → altitude; → azimuth; → mounting.

A silver-white, malleable and ductile metal, symbol Al.
→ Atomic number 13; → atomic weight 26.98154; → melting point 660.37°C; → boiling point 2,467°C; → specific gravity 2.6989 at 20°C. Its electric → conductivity is comparable with that of copper, so that being much lighter it is used extensively for transmission lines. The metal and its → alloys have strength with lightness. The → reflectivity of aluminium is high and it is therefore used broadly
for coating → mirrors (→ aluminization). Aluminium occurs widely in clays; it is extracted mainly from bauxite. It has several → radioactive isotopes with half-lives from 2.3 sec (²³Al) to 6.56 min (²⁹Al). When aluminium is bombarded with → alpha particles, its atoms first turn into radioactive → phosphorus, then into → silicon. This occurs naturally in → massive stars.

See also: The name of the chemical element, was coined by Sir Humphry Davy (1778-1829), from L. alumen “alum;
bitter salt,” akin to Gk. aludoimos “bitter” and Eng. ale. Davy originally called it alumium (1808), then modified this to aluminum, which remains the U.S. word, but British editors in 1812 further amended it to aluminium. In 1825, the Danish physicist Hans Christian Oersted (1777-1851) isolated impure aluminium. The pure metal was first isolated by the German chemist Friedrich Wöhler (1800-1882) in 1827.

A silver-white, malleable and ductile metal, symbol Al.
→ Atomic number 13; → atomic weight 26.98154; → melting point 660.37°C; → boiling point 2,467°C; → specific gravity 2.6989 at 20°C. Its electric → conductivity is comparable with that of copper, so that being much lighter it is used extensively for transmission lines. The metal and its → alloys have strength with lightness. The → reflectivity of aluminium is high and it is therefore used broadly
for coating → mirrors (→ aluminization). Aluminium occurs widely in clays; it is extracted mainly from bauxite. It has several → radioactive isotopes with half-lives from 2.3 sec (²³Al) to 6.56 min (²⁹Al). When aluminium is bombarded with → alpha particles, its atoms first turn into radioactive → phosphorus, then into → silicon. This occurs naturally in → massive stars.

See also: The name of the chemical element, was coined by Sir Humphry Davy (1778-1829), from L. alumen “alum;
bitter salt,” akin to Gk. aludoimos “bitter” and Eng. ale. Davy originally called it alumium (1808), then modified this to aluminum, which remains the U.S. word, but British editors in 1812 further amended it to aluminium. In 1825, the Danish physicist Hans Christian Oersted (1777-1851) isolated impure aluminium. The pure metal was first isolated by the German chemist Friedrich Wöhler (1800-1882) in 1827.

To coat a telescope → mirror with a very thin but perfectly uniform layer of → aluminium to make it reflective.

Etymology (EN): Aluminize, v. from alumin(um), → aluminium,

• → -ize.

Etymology (PE): Âluminiom andudan, from âluminiom + andudan “to incrustate, plaster, cover over.”

To coat a telescope → mirror with a very thin but perfectly uniform layer of → aluminium to make it reflective.

Etymology (EN): Aluminize, v. from alumin(um), → aluminium,

• → -ize.

Etymology (PE): Âluminiom andudan, from âluminiom + andudan “to incrustate, plaster, cover over.”

The process by which the coating of aluminium is deposited on a telescope mirror.

See also: Verbal noun of → aluminize.

The process by which the coating of aluminium is deposited on a telescope mirror.

See also: Verbal noun of → aluminize.