Castor is the second-brightest object in the

zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac ...

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Gemini. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

α Geminorum, which is Latinised to Alpha Geminorum and abbreviated Alpha Gem or α Gem. With an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction of the object's light ca ...

of 1.58, it is one of the brightest stars in the

night sky The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a night sky include moonlig ...

. Castor appears singular to the naked eye, but it is actually a sextuple

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravity, gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally calle ...

organized into three binary pairs. Although it is the 'α' (alpha) member of the constellation, it is half a magnitude fainter than 'β' (beta) Geminorum, Pollux.

Stellar system

Hierarchy of orbits in the Castor system

Castor is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

made up of six individual stars; there are three visual components, all of which are

spectroscopic binaries A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...

. Appearing to the naked eye as a single star, Castor was first recorded as a

double star In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a ...

in 1718 by

James Pound James Pound (1669–1724) was an English clergyman and astronomer. Life He was the son of John Pound, of Bishops Cannings, Wiltshire, where he was born. He matriculated at St. Mary Hall, Oxford, on 16 March 1687; graduated B.A. from Hart Hall ...

, but it may have been resolved into at least two sources of light by Cassini as early as 1678. The separation between the binary systems Castor A and Castor B has increased from about 2″ (2

arcseconds A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

of angular measurement) in 1970 to about 6″ in 2017. These pairs have

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

s of 1.9 and 3.0, respectively. Castor Aa and Ba both have orbits of a few days with a much fainter companion. Castor C, or YY Geminorum, was discovered to vary in brightness with a regular period. It is an

eclipsing binary A binary star or binary star system is a Star system, system of two stars that are gravity, gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved ...

with additional variations due to areas of different brightness on the surface of one or both stars, as well as irregular flares. The Castor C components orbit in less than a day. Castor C is believed to be in orbit around Castor AB, but with an extremely long period of several thousand years. It is 73″ distant from the bright components. The combined apparent magnitude of all six stars is +1.58.

Physical properties

Castor is 49

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s away from Earth, determined from its large annual

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

. The two brightest stars are both

A-type main-sequence stars A type or type A may refer to: Science * A-type asteroid, a type of relatively uncommon inner-belt asteroids * A type blood, a type in the ABO blood group system * A-type inclusion, a type of cell inclusion * A-type potassium channel, a type of volt ...

, more massive and brighter than the Sun. The properties of their

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

companions are difficult to determine, but they are known to have masses 39% that of the Sun. YYGemLightCurve

Castor B is an

Am star An Am star or metallic-line star is a type of chemically peculiar star of spectral type A whose spectrum has strong and often variable absorption lines of metals such as zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. ...

, with particularly strong spectral lines of certain metals. Castor C is a variable star, classified as a BY Draconis type. BY Draconis variables are cool dwarf stars which vary as they rotate due to

starspot Starspots are stellar phenomena, so-named by analogy with sunspots. Spots as small as sunspots have not been detected on other stars, as they would cause undetectably small fluctuations in brightness. The commonly observed starspots are in gene ...

s or other variations in their

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

s. The two red dwarfs of Castor C are almost identical, with masses around and luminosities less than 10% of the Sun. Since 2018 it is suspected a

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

with a mass at least times the mass of Jupiter might be orbiting Castor C with a period of 50 years. If it is confirmed, Castor would turn out to be a seven-star system. All the red dwarfs in the Castor system have emissions lines in their spectra, and all are

flare star A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to magnetic reconnection, the magne ...

Etymology and culture

''α Geminorum'' ( Latinised to ''Alpha Geminorum'') is the star system's

. Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (meaning ''twins'' in Latin) its name. The name ''Castor'' refers specifically to Castor, one of the twin sons of

Zeus Zeus (, ) is the chief deity of the List of Greek deities, Greek pantheon. He is a sky father, sky and thunder god in ancient Greek religion and Greek mythology, mythology, who rules as king of the gods on Mount Olympus. Zeus is the child ...

and Leda in Greek and Roman mythology. The star was annotated by the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

description ''Al Ras al Taum al Muqadim'', which translates as ''the head of the foremost twin''. In the catalogue of stars in the ''Calendarium of al Achsasi al Mouakket'', this star was designated ''Aoul al Dzira'', which was translated into

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

as ''Prima Brachii'', meaning ''the first in the paw''. In Chinese, (), meaning '' North River'', refers to an asterism consisting of Castor, Rho Geminorum, and Pollux. Consequently, Castor itself is known as (, .) In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Castor'' for the star α Geminorum Aa. Castor C also has the variable-star designation YY Geminorum.

Notes

References

External links