The star Castor, at magnitude 1.57, is the 23rd brightest star in the sky. Although it has the Bayer designation α Gem, it is actually fainter than β Gem (Pollux). Hence, it has been suggested that one of these stars may have changed in luminosity during the past four centuries since Bayer named them. At the northeast corner of Gemini, Castor and Pollux make a most attractive sight; Pollux is an orange giant star, and Castor a contrasting white.
Name and Mythology
Castor and Pollux are the two "heavenly twin" stars that give the constellation Gemini (which means "twins" in Latin) its name. The name Castor refers to one of the twin sons of Zeus and Leda. The star also has the Arabic name Al-Ras al-Taum al-Muqadim, which literally means "The Head of the Foremost Twin". The Chinese recognized Castor as Yin, one of the two fundamental principles upon which all things depend.
Castor A and B
Castor has no physical relation with Pollux, and at a distance of 50 light years it is half again as far away as its mythological companion. In fact, Castor is a complex system of six stars in three binary groupings, all mutually orbiting each other.
Castor was first discovered to be a visual binary in 1678 by Giovanni Cassini - the first gravitationally bound objects observed beyond the Solar System. It is a fine double star for medium amateur telescopes. α Gem's close A-B components are separated by up to 6.8", and appear brilliant white. Castor A and B, of magnitudes 2.0 and 2.9, orbit one another over a period of 440 years. The stars' mean separation is approximately 107 AU - about the diameter of Pluto's orbit - and they move in an eccentric orbit (e=0.343) that takes them as close as 68 AU and as far apart as 133 AU. They are currently about 4.5" apart, and are separating, making them easier to observe.
Each of the components of Castor is itself a spectroscopic binary, making Castor a quadruple star system. These stars exhibit a periodic Doppler shift in their spectral lines, indicating a pair of stars (Castor Aa and Ab) in orbit around one another. Castor Aa is 2.15 solar mass main-sequence star of spectral class A1 V, with a temperature around 9500 K, and a luminosity of over 34 suns. Its companion (Castor Ab) is an M5 V red dwarf containing 0.4 - 0.6 solar masses. They circle each other at a mean distance of about 0.12 AU - a tenth of Mercury's distance from the Sun - in an eccentric orbit (e=0.499) that takes only 9.21 days to complete.
Castor B's components (Castor Ba and Bb) are very similar. Castor Ba is a 1.7 solar mass main-sequence star of spectral type A2 V, with a temperature around 8300 K and a luminosity of about 14 suns. Castor Bb is a M2 V red dwarf containing 0.4 - 0.6 solar masses. They are separated on average by only 0.03 AU, with a nearly circular orbit (e=0.01), and a period of 2.93 days.
Castor C
A magnitude 8.8 red dwarf lies 73" south of the bright pair. It has the same parallax and proper motion, and seems to be a true physical member of the system. At a distance of about 1,100 AU from the inner pairs, its orbital period must be at least 10,000 years long. It is designated Castor C. This companion is also a spectroscopic binary, with a period of 19.54 hours; its components (Castor Ca and Cb) are separated on average by only 0.018 AU, in a near-circular orbit.
Castor C is also an eclipsing binary, with a magnitude range of 9.1 to 9.6. It is one of only a few known eclipsing binary systems where both components are red dwarf stars. It too consists of nearly identical stars, much cooler low-mass class M1 Ve dwarfs with temperatures of only about 3280 K, and (from their orbit) masses of 0.60 and 0.62 suns. They have a combined visual luminosity only 5% of the Sun's. Due in part to their close proximity, tidally enforced rapid rotation is expected to induce high chromospheric activity in both stars. Both are flare stars, and have been given the variable-star designation YY Geminorum.
Motion and Evolution
Star systems like Castor are thought form when their contracting birth clouds divide, then divide again as a result of rapid rotation. Castor is the title member of a group of widely dispersed stars known as the Castor Moving Group. The group also includes Vega, Fomalhaut, Zubenelgenubi, and Alderamin. The properties of the whole set suggest an age for its member stars of about 200 million years. Yet there are still questions. The luminosities and temperatures of Castor A and B show them to be 370 million years old, while those of Castor C imply and age of 30 to 85 million years. Something is wrong, either with observation or theory; or Castor C was not originally part of the system, and was somehow added later.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]