Castor and Pollux
Published on Feb 10, 2007 at 6:06 pm.
3 Comments.
Filed under stars.
This time of year, Orion is high in the sky in the evenings. I’ve already posted about the constellation Orion in the past, so today I want to talk about two stars northeast of Orion. Go out and look towards Orion. In the northern hemisphere, you will see two stars up and to the left of Orion about a couple of fists width away (in the southern hemisphere, they will be down and to the right). These are the stars Castor and Pollux. Castor is the more northern one, and Pollux is the brighter one (and if you look carefully, it has a slight orange hue to it). Castor and Pollux are the heads of the twins of the constellation Gemini.
From mythology, Castor and Pollux are the twin sons of Queen Leda of Spartus. Castor was the son of Spartan King Tyndareus, and he was an expert horseman and horse trainer. The star Castor was called Eques, the Horseman, by Ovid. According to mythology, Pollux, though Castor’s twin, was fathered by Zeus, the king of the gods, and thus immortal. Ovid called this star Pugil, the Boxer. The star Castor was also called Apollo, and Pollux was called Hercules. Those names were apparently occasionally used until about the 18th Century when the names Castor and Pollux again became dominant. These two stars, bright and obvious, near to one another in the sky, form a natural pair, so it is not surprising that most cultures associated them with one another in some way. To the Chinese, they were Yin and Yang.
Though Pollux is the brighter of the two stars, Castor was dubbed the alpha star by Johannes Bayer (The Bayer designations are Greek letters assigned to each star in a constellation, generally by order of brightness). Castor is nearly 52 lightyears distant. Castor is a young star system, perhaps only 200 million years old. Castor is also a binary star. I first learned of this many years ago when I was just starting off in astronomy, while still an amateur astronomer, and I was excited to try to observe it. Alas, no matter how hard I tried, my little telescope never showed Castor as anything but a bright dot. Well, now I know that was because the components were simply too close together for my instrument at the time. But, these stars have about a 350 year period, and they were at their closest in 1969. They have since been getting farther apart, and are now far enough apart for most amateur telescopes, other than the smallest, to show them as two very close stars. The primary stars have magnitudes of about 2 and 3, but there is also a much dimmer companion a ways away that is about magnitude 9.5 that seems to be associated with Castor, probably a very far flung companion in a loose orbit around the main pair. But, careful studies of these stars show that each of the three is also, itself, a pair of stars! Each of the primary stars is a spectral type A1 or A2 main sequence star (very similar to Sirius), and each seems to be orbited by a small red dwarf star. The much dimmer third companion is itself a pair of somewhat larger red dwarfs. That makes Castor a six star system!
Pollux, the brighter of the pair, is nearly 2/3 of the distance to Castor. With a surface temperature of 4865K, Pollux is a cooler star than the stars of Castor, whose surface temperatures are near 10,000K. However, Pollux shines with nearly the same total light as Castor because it is physically a much larger star, what we call a red giant (actually orange hot, but the term is still “red giantâ€). A red giant is a star that has used up the hydrogen in its core and has left the main sequence. It is dying. Pollux has slightly fewer metals than the Sun, and that suggests that it may be older. However, it also is more massive than the Sun, by at least 70 percent, and more massive stars don’t live as long. At any rate, it probably left the main sequence fairly recently. Pollux is the closest giant star, but not the closest star off the main sequence. Sirius has a white dwarf companion. A white dwarf is the end stage of most stars’ evolution. So, in the past, Sirius B was the closest red giant star. Now, that distinction goes to Pollux. But, it probably won’t hold that status long. Procyon, a little over 1/3 of the distance to Pollux, is just barely off the main sequence, as a subgiant. In the future, it will be the nearest giant star, and it may become so before Pollux finally dies. When that happens, it will be really impressive, as Procyon will likely have a visual magnitude of -3 or -4, making it by far the brightest star in the sky (other than the Sun!).
Pollux has another distinction. It has a planet, and it was the first naked eye star found to have a planet. This planet orbits is a nearly circular orbit about 1.6 AU from the star with a period of about 590 days. This planet is a gas giant, with a mass about 2.3 times that of Jupiter. Though this planet is at about the same distance that Mars is from the Sun, Pollux is some 30 times more luminous than our Sun. That means that this planet will be warm, at least 380 Celsius (over 700 Fahrenheit). Pollux will look over its skies as a bloated orange ball, nearly five times larger than the Moon or Sun appear in our sky. The Sun would appear as about magnitude 4.9 seen from Pollux’s planet. Sirius, the brightest star in our nighttime sky would be only a first magnitude star from Pollux. But, Castor, Pollux’s twin as seen from Earth, would be slightly brighter than magnitude zero, perhaps magnitude -0.25 from Pollux. This planet is a gas giant, but it is in a low eccentricity orbit. This leaves open the possibility of other planets in the Pollux system. But, in order for any planet to be anything like Earthlike, it would have to be a bit farther from Pollux than Jupiter is from our own Sun.
While we are thinking about how the sky would look from Pollux’s planet, I thought that I’d imagine the view from Castor. Below, I’ve created a view from Castor, looking back towards the Sun. I used Starry Night Pro to come up with this, and it insists on showing the Sun, but really the Sun would be a pitiful magnitude 4.9 star seen from Castor. But, you see three bright stars to the right of the Sun, they are (from top to bottom) Pollux, Procyon, and Sirius. Pollux would be magnitude about -0.2 from Castor, and Sirius would be a second magnitude star. Procyon would appear as a third magnitude star. Interestingly, Sirius and Procyon would appear as a pair of stars from Castor much as Caster and Pollux do from Earth, and part of a trio of stars as seen from Pollux that would be as distinctive as Orion’s belt is from Earth.
-Astroprof
(Star Fields created by Starry Night Pro)
Jerry Hodge on March 27, 2007 at 11:36 am: 1
I am new to astronomy, but doing quite well. Just identified Castor and Pollux last night. I am in New Jersey, about 25 miles from New York City. I was also able to see Procyon last night. Do you recommend any good books for people new to Astronomy. It is very exciting. You article is simple and very easy to understand. I now need to learn the differecn between stars.
Thank you,
Jerry Hodge
Astroprof on March 27, 2007 at 3:07 pm: 2
Thanks.
Several guides come to mind. One that I saw recently was The Concise Atlas of the Starsby Serge Brunier. It has photographs of constellations, and clear overlays that identify stars. It seems a good teaching tool. Another one, that seems great for a newcomer to astronomy is Touring the Universe by Ken Graun. It has star charts, tips for new astronomers, and basic information about binoculars and telescopes.
sd on November 27, 2009 at 12:44 am: 3
Since I cannot send emails.
A suggestion.
Trace a line to the most luminous star
closest to the Sun, i.e. Sirius. Then from Sirius trace a line to the most luminious star nearest it; and
continue this for a number of iterations. For example 28 iterations. Then do the same for the list of most luminious stars nearest the Sun. The compare the two sets of lines.
What do you see?
Maybe you’re the first to see it?