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Hunting for the stars of December

The Sky Guy

Posted: Sunday, December 04, 2005

 

 

In December, the dominating constellation is Orion, the Hunter. It is many people’s favorite because of its brilliance.

Four stars arranged in a rectangle — red Betelgeuse, blue Rigel, Bellatrix and Saiph — outline the body, while three stars make up the belt with the Orion nebula as the sword beneath it.

Above Orion you find Taurus, part of it in the shape of an arrowhead including red Aldebaran. Above that is the Pleiades or Seven Sisters, a must-see with binoculars. To Taurus’ left is the pentagon Auriga with yellow Capella.

Beneath Capella appear Castor and Pollux, the twins in Gemini. Then we close this circle toward the horizon with Procyon and Sirius, the brightest star in the night sky. Jupiter resides further east, and also look for Cancer and Leo.

Some of the above constellations don’t show in the diagram. I described the late-evening sky. I trust you’ll be able to find Orion, so I opted to show the early-evening sky in the diagram instead to help you locate harder-to-spot celestial items.

There is more in the sky: The huge summer triangle, so called because it’s the dominant feature in the summer sky but still is visible until January in the west, is comprised of Vega, Deneb and Altair, another three of the brightest stars. Since these three stars are from different constellations (Lyra, Cygnus and Aquila), the summer triangle is not a constellation.

Early in the evening you also can see Arcturus in the constellation Bootes, easy to find by using the Big Dipper’s curvature and extending it toward the western horizon.

If you happen to be in a dark area, like a turnout on the Sterling or Seward highways, you’ll be able to see the Milky Way stretching from southwest to northeast.

How do you find Polaris, the North Star? Take the last two stars of the Big Dipper’s pan and mark off a distance five times their separation toward the north. You’ll end up at Polaris, which is the last star of the smaller and fainter Little Dipper.

Since the Little Dipper’s stars are fainter, you might need to be in a darker area without light pollution to see them. You also can imagine the two dippers as if you were flipping pancakes back and forth between the two.

Polaris is only 49th in brightness and thus appears fainter then people may expect. Its claim to fame is its fixed position of 60 degrees (marking latitude) above the northern horizon, not its brightness.

Venus is by now in the constellation Capricornus and is high enough the southwest horizon that it can be seen as a bright beacon during dusk.

Look in that direction, then turn around and see Mars high in the eastern sky, some distance from the Pleiades.

Saturn rises earlier in the evening and still is following Castor and Pollux. Jupiter appears brightly in the southeast during dawn.

This month I’d like to describe the constellation Auriga in a little more depth. It looks like a pentagon and is high in the eastern sky during evening, placed between Taurus and Gemini. Its brightest star is Capella. Its diameter is 13 times larger than our sun, 12 million miles compared to 900,000 miles. Its spectrum is very similar to the sun, hence its temperature of 11,000 F and its yellow color, which you may be able to see with binoculars, also are similar.

Putting these numbers together yields an energy output about 160 times as much as our sun’s, but at a distance of 45 light years from Earth, only some of its light reaches us, making it the sixth-brightest star in appearance outside our solar system.

Auriga also lies inside the Milky Way, so you see more fainter stars when looking at it than you would looking at the Big Dipper.

Not coincidentally, Auriga also bears some star clusters. These conglomerates of stars were formed within our galactic disk. These have the prosaic names M37, M36 and M38.

All are seen in the lower left corner of the pentagon (the opposite side from Capella).

While each are on the brink of visibility with the naked eye at magnitudes 5.5 to 6.5, all three are fine objects for binoculars. M37 contains about 150 stars within a diameter of 25 light years at a distance of 4,600 light years from Earth. M36 has only 60 stars within 14 light years and is 4,100 light years from us.

M38 has about 100 stars within 25 light years at 4,200 light years away. Since all three are roughly the same distance from us, they all appear almost equally bright.

Last, but not least, my community schools astronomy class has its last meeting the evening of Dec. 14 at Kenai Peninsula College.

Andy Veh is the physics and astronomy instructor at Kenai Peninsula College. This column appears on the first Sunday of each month. He can be reached at aveh@uaa.alaska.edu.



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