November 03, 2007

Ursa Minor

Today, I'd like to continue going through the constellations. With any luck, I'll hit all of the major ones visible in the sky prior to the upcoming star party I'm attending so that I can then use my notes if necessary! Next up is Ursa Minor, the constellation that includes the Little Dipper and the pole star, Polaris.

The main object of attraction here is Polaris, a Sirius-like star of 2nd magnitude located about 400 light years from Earth. Many people have the mistaken impression that Polaris is the brightest star in the night sky, but actually, it is only the brightest star in its little region of the sky. Overall, it ranks 48th. Sirius is the brightest, one we'll discuss another time when we're talking about Canis Major.

Polaris is not right at the pole. It is about 1.5 full moon diameters from the true North position. Over the course of the next 100 years, thanks to the twirling of Earth's axis of rotation, Polaris will get closer and closer to true North, reaching its closest angular distance right around the year 2100. For now, if you want to find true North, follow a line from Polaris toward the last star in the handle of the Big Dipper. About 3/4 of a degree from Polaris is true north. You can see Polaris move around in a tiny circle around the real pole in most circumpolar trail photos.

Historically, going back to observations by Herschel in 1780, it has been recognized that Polaris has a companion that is visible through small telescopes, known as Polaris B. About 30 years ago, however, it was discovered that Polaris has a bit of a wobble to it, indicating that it is being pulled back and forth by a much closer companion star. This companion was imaged by the Hubble Space Telescope, and the resulting discovery photo published in 2006.

Polaris is a Cepheid variable, about which I will say much more another time. It has a very mild variation (less than 2% of its brightness over the course of about four days) compared to typical Cepheids, and even that variation seems to be lessening with time as the star undergoes some sort of overall evolution. If you look at Polaris through a small telescope, you can see a loose ring of six stars in a circle just beneath Polaris. This is often referred to as the engagement ring with Polaris as the crowning jewel.

As the Earth's axis continues to precess over time, the indicator of true North will move away from Polaris in a big circle. At one time (about 2000 BC), Thuban was the closest bright star to the pole. Thuban is in the constellation Draco, which curls most of the way around Ursa Minor. In fact, Ursa Minor was once a wing of the dragon until the Greeks (specifically, Thales) changed it to a single small constellation for simplicity since so many mariners used the stars within it as a guide. Vega (in the constellation Lyra) was also a pole star thousands of years ago (and will be again in another 12,000 years or so).

The other two easily-visible stars in Ursa Minor (from within a city, anyway) are the two stars at the end of the Little Dipper, known as Kocab and Pherkad. These are often referred to in mythology as the Guardians of the pole or the two calves that guard the pole. Because the stars move in such a tiny little circle in the sky, they are often seen as a timid little herd of cattle hovering near their shepherd, Polaris.

The only deep sky object in this part of the sky, somewhat removed from the plane of the Milky Way galaxy, is the faint spiral galaxy NGC 6217, which really isn't even a good object for an 8" telescope. It is just too faint, which makes it virtually impossible to find without computerized guidance or some luck.

Posted by Observer at November 3, 2007 11:21 PM

Comments on entries can only be made in pop-up windows while those entries are still on the main index page. Sorry for the inconvenience this causes, but this blocks about 99.99% of the spam the blog receives.

"star party"?!? Unless you're in Hollywood, I don't expect to see those two words together...

Posted by: Humbaba on November 4, 2007 10:05 AM

Look up info on local amateur astronomy clubs and see when their next star party is scheduled. You don't have to take anything. Lots of amateurs bring their own scopes and just host an open house for everyone. Your kids would love it, but wait for Spring when Saturn is a good target in the early evening.

Posted by: Observer on November 4, 2007 01:57 PM

Ah, I see. By "party" you mean "nerdfest". :)

Posted by: Humbaba on November 4, 2007 05:30 PM