January 15, 2008

Canis Minor

Sticking to the Winter Circle, let's move around to another constellation that joins Orion, Taurus and Auriga in this pattern: Canis Minor. Being a very small constellation that's close (but not too close) to the plane of the Milky Way, there just aren't any good, bright deep sky objects here. It's a little too far from the Milky Way for clusters and big regions of star formation and too close to be able to see through all the gas and dust to the galaxies that surely lie beyond our galaxy in this direction.

The highlight, of course, is Procyon, the 7th or 8th brightest star in the sky (tough to rank stars when so many vary in brightness by a little bit) and one of the closest, being about 11 light years away (only about three times further than the Alpha Centauri system). To the Romans, this was known as Antecanis. To the Greeks, Procyon. Both translate roughly to "before the dog" since this star rises just before Sirius and Canis Major. The usual shorthand story for this constellation is that it is the smaller of Orion's two hunting dogs, but Ridpath's Star Tales has a much longer variant involving different characters if you are interested.

Like the Sun, Procyon is a main sequence star, just slightly hotter. What makes it bright is its proximity to us, not so much its intrinsic luminosity. It is about 40% more massive than the Sun and is thought to be a bit younger at 3 billion years old (compared to the Sun's 5 billion year age). It also has a white dwarf companion orbiting at an average distance of 15 AU (which would put it somewhere between Saturn and Uranus in our solar system). This white dwarf is the degenerate core remnant of a more massive companion (perhaps twice Procyon's mass or more) that has already finished its main sequence Hydrogen burning and undergone a planetary nebula phase, losing all of its outer layers millions of years ago.

Being hotter and brighter than the Sun, if Procyon has orbiting planets, they would have to be roughly 1.2 to 2.5 AU from the star in order to be considered habitable. Though there is some interest in finding a potentially habitable planet in such a nearby system, it seems to me very unlikely. First of all, the orbit of such a planet would be somewhat unstable with two very massive objects already in the system. Secondly, Procyon puts out a lot more of its light at higher energies compared to our Sun, and all of these high energy photons would be potentially harmful for life on a planet without a thick ozone layer to protect it. Finally, the evolution of the nearby massive companion surely would have affected the environment of any planet in the vicinity in a negative way, so even if the planet might be habitable, the odds of its being inhabited are slim.

The other bright star in this constellation is Gomeisa, a name that Kaler indicates comes from the Arabic for "the weeping one". Gomeisa is actually much larger and brighter than Procyon (though it is also a main sequence star), but it is over 10 times further away. This star is rotating so quickly that, similar to Gamma Cassiopeia (which I discussed previously), it has thrown off a disk of material that emits its own light. The disk is four times larger than the star itself, and it generates bright emission lines in the spectrum of the system. This star also lights up the surrounding gas and dust much like the stars in the Pleiades.

Posted by Observer at January 15, 2008 04:04 PM

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