Castor and Pollux are so interesting because they are so typical and so modest as bright stars go.
Not that there is anything modest or common about bright stars in themselves. According to Planet Quest
by Ken Croswell, page 79, 80% of all stars in the Milky Way are small red main sequence stars (also known as red dwarfs), 9% are larger but still small K-type main sequence stars, 5% are white dwarfs, 4% are G-type main sequence stars like the Sun, and 2% are all other stars.
Very many of those rare "all other stars" must be A-type main sequence stars and K-type giants. According to Sky Catalogue 2000.0, Volume 1, Stars to Magnitude 8.0, 2nd Edition, by Alan Hirschfeld, Roger W. Sinnott and François Ochsenbein, which lists ~45,000 stars down to magnitude 8.0, the most common type of star listed in the catalog are K-type stars, most of which are certainly evolved giants. The second most common type listed there are A-type stars, most of them certainly main sequence stars. Or to put it differently, A-type main sequence stars
and K-type giants
are rare in absolute terms, but they are bright enough and common enough to dominate the sky
of the Summer Triangle
of the Big Dipper
, Kaus Australis
of the Teapot in Sagittarius
, and many others, are A-type main sequence stars. Arcturus
of Bootes, Dubhe
of the Big Dipper, Aldebaran
apparently in the Hyades
, and Kaus Borealis
, Kaus Meridionalis
, and Nash
of the Teapot of Sagittarius, and many others, are K-type evolved giants.
What is most interesting about Pollux
, apart from its planet, is how small and pale this star actually is. Its Johnson B-V is 0.991 ± 0.005, which is paler and more non-red than the color of any of the evolved giants that I have listed above, apart from Nash. Pollux, on the other hand, is the faintest K-type giants of all of those I have listed, only 30.66 ± 0.17 times solar in visual light.
From its distance of 34 light years, we calculate a total luminosity (incuding infrared radiation) for Pollux 46 times that of the Sun, and coupled with its temperature, a diameter some 10 times solar, making it smaller than most of its cool giant brethren and only a quarter the dimension of Aldebaran. Direct measures of angular size, however, yield a somewhat smaller diameter 9 times that of the Sun.
The relatively small size of Pollux means that it definitely wouldn't "eat" the Earth if it was placed at the center of the solar system! If I remember correctly, the distance between the Earth and the Sun is ~100 times the diameter of the Sun.
Interestingly, the distance to Pollux is very close to 10 parsecs. One parsec is 3.26163344 light-years, and the distance to Pollux is 33.785 ± 0.094 light-years, or 10.365 ± 0.029 parsecs.
An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if were viewed from a distance of exactly 10 parsecs (32.6 light years), with no extinction (or dimming) of its light due to absorption by interstellar dust particles.
The fact that the distance to Pollux is very close to 10 parsecs, and the visual luminosity of Pollux is 30.66 ± 0.17 solar, means that its absolute visual magnitude is very close to +1, or +1.0835 ± 0.0061. So Pollux comes close to being a "standard star", even though it appears to be faint for its class. But perhaps it isn't? Perhaps most K-type giants are smaller than we think? The really big bright ones stand out more than the others in the sky, and Pollux stands out just because it it nearby. Pollux is the most nearby red giant in the sky.
And Castor? Oh, at 47 times solar it's a modest A-type star - make that that it's a modest multiple star made up of six
components! A-type Castor A is a spectroscopic double, made up of one bright A-type component and a probably M-type second component, Castor B is another spectroscopic double, made up of one bright A-type component and a probably M-type second component, and Castor C is made up of two small possibly M-type components. The brightest A-type star of Castor is probably just a little bit brighter than Sirius, which is 23 times as bright as the Sun, and Castor A is definitely fainter than Vega.
The Castor system is indeed rather complicated, but in fact many stars more massive than the Sun belong to multiple star systems.
So both Pollux and Castor are modest stars with quirks, but in many ways they are typical and representative of the bright stars of the sky. For one thing, like most bright stars in the sky, Pollux, Castor A and Castor B are all more massive than the Sun, but they are not that
much more massive. According to Jim Kaler, Pollux is about 1.8 times as massive as the Sun (or 1.8 M☉
). Castor A, again according to Jim Kaler, is 2.4 M☉
, and Castor B is 1.9 M☉
. So yes, these stars are massive, but they are a far cry from the real, incredibly rare true behemoths and superheavyweights of the heavens.