Star Facts: Castor

Pollux and Castor
Image Credit: The stars Pollux (left) and Castor (right) by Rogelio Bernal Andreo

Castor (Alpha Geminorum) is the “other” star that marks out the “heads” of the Heavenly Twins in the constellation Gemini, the other being Pollux (Beta Geminorum). Even though Castor is the least luminous of the pair and only the 23rd brightest star in the night sky, compared to the 17th brightest for Pollux, it still revels in being assigned the constellation’s “alpha” designation . Another distinction between the two stars is that while Pollux is a single star, Castor, on the other hand, is a multiple star system consisting of three spectroscopic binary pairs of stars.

Quick Facts

• Constellation: Gemini
• Distance: 51 light years
• Star Type(s): A (A1V + dM1e), B (Am + dM1e), C (dM1e + dM1e)
• Apparent Magnitudes: A (+1.93), B (+2.97), C (+9.83)
• Mass: (A): 2.76 / (B): 2.98 (solar masses)
• Coordinates:
A – RA 07h 34m 35.863s | Dec. +31° 53′ 17.79″
B – RA 07h 34m 36.100s | Dec. +31° 53′ 18.57″
C – RA 07h 34m 37.584s | Dec. +31° 53′ 17.8160″
• Age: A and B (370m years), C (30 to 85m years)
• Other Designations: a Gem, 66 Gem, FK5 287, Gliese 278, HIP 36850, SAO 60198


The constellation of Gemini can be seen from between latitudes of +90° and -60°, where it occupies an area of 514 square degrees of the northern skies. Observers from mid-northern latitudes can observe Gemini, and by extension, both Castor and Pollux, from January to around the beginning of May relatively high in the sky. By late May, or the beginning of June, Gemini has sunk low over the west-north-western horizon just after sunset, and at around the time of the summer solstice on June 21st, both Castor and Pollux become difficult to spot after sunset. The best time to view Gemini and its two brightest stars is in January and February, when the Twins stand virtually upright at around 5 A.M. Local Time. Note however that the constellation sets two hours earlier every month after February.

Physical Properties


While the British astronomer James Pound is credited with first resolving Castor as a double star in 1718, it is possible that Giovanni Cassini had done the same as long ago as 1678, although there is some debate about this. Over the years, however, it is becoming easier resolve the primary pair of stars as their angular separation has increased from 2″ in 1907 to as much as 7″ in 1997.

All three visual components of the Castor system are in fact close spectroscopic binaries, with Castor A and B separated by an average of 104 AU, or 15 light years, and orbiting each other every 445 years, while Castor C is separated from the principal Castor AB pair by a distance of about 1000 AU, and orbits them over a 14,000 year period. In terms of the three individual systems, Castor A and B have fainter companions which orbit them over a few days, while Castor C has a companion with an orbital period of less than a day.

Interestingly, the regular variations in the luminosity of Castor C was at first thought to be the result of the two stars eclipsing each other, but recent studies are suggesting that these variations are caused by areas on the surface of one (or perhaps both stars) that have different luminosities. As a result, Castor C now has the variable star designation YY Geminorum.


The two principal pairs of stars each consist of a hot A-type main sequence star, that are both more massive and luminous than the Sun, and a cool red dwarf, whose physical properties are difficult to determine given their close proximity to their primaries, although the dwarfs are thought to be only about 50% as massive as the Sun. By contrast, the two red dwarfs that make up Castor C are almost identical, each having about 50% of the Sun’s mass, giving them luminosities of around 10% (or less) that of the Sun. It is worth noting though that all four red dwarfs in the Castor system show strong emission lines in their spectra, and that all four dwarfs are flare stars, which are a class of variable stars that exhibit sudden and dramatic fluctuations in brightness over periods of only a few minutes.


The stars Castor and Pollux in Gemini, the zodiac’s most northerly constellation, present a striking sight in the night sky, and have been included in many legends across the world. In ancient Greece, for instance, they represented the twin son’s of Queen Leda of Sparta, with Pollux being the immortal offspring of the adulterous Zeus, while his brother Castor was the son of King Tyndareus, and therefore moral. Together the twins were inseparable, and their many adventures were legendary, including joining Jason and the Argonauts on their quest to find the Golden Fleece.

Elsewhere around the world, Castor was known to the Arabs as Al-Ras al-Taum al-Muqadim, meaning “the head of the foremost twin”, although a later catalogue compiled by Egyptian astronomer Al Achsasi al Mouakket called the Calendarium listed Castor as ”Aoul al Dzira”, or “the first in the paw”.

To the Romans, Castor and Pollux were associated with the legendary twin brother Romulus and Remus who founded the city of Rome, while in India the two stars depicted the Ashwins, known as the twin horsemen of the dawn. In China, Castor and Pollux form part of an asterism known as “North River”, in which Castor is referred to as “[the] Second Star of North River,” although they are also sometimes taken to symbolize the dual forces of nature referred to as Yin and Yang.

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