Jimmy Westlake: See 3 galaxies this week
September 18, 2012
What's the farthest thing you can see without a telescope? Would you believe 15,000,000,000,000,000,000 miles on a clear night? That's 15 quintillion miles!
On a moonless early autumn evening, one can actually see an object that far away. In fact, the fall sky offers the opportunity to see the two most distant objects visible to the unaided human eye.
In the northeastern sky, about midway between the familiar W-shaped star pattern of Cassiopeia's Chair and the Great Square of Pegasus, you'll find a faint wisp of light, like a tiny, detached piece of the Milky Way. Charles Messier catalogued this fuzzy patch as the 31st object on his list of comet impersonators in 1764, so we now call it Messier 31, or M31 for short. It also was known for centuries as the Great Andromeda Nebula when it was thought to be a spinning vortex of hot gas in our Milky Way. But in the 1920s, American astronomer Edwin Hubble determined that Andromeda's great nebula was no nebula at all — it turned out to be a near perfect twin of our own Milky Way galaxy at the mind-boggling distance of 2.5 million light years (15 quintillion miles). From that distance, even a collection of 100 billion suns appears in our sky as a faint wisp on a clear, dark night.
The discovery that M31 was a whole other galaxy demonstrated that the universe was far larger than we had imagined, and soon astronomers confirmed thousands of other galaxies.
Andromeda's galaxy just happens to be the closest and brightest of the distant galaxies visible from the northern hemisphere, literally in our cosmic backyard. Astronomers now recognize that the Milky Way and Andromeda galaxies are the two dominant members of a small collection of about three dozen galaxies affectionately known as the Local Group.
There is another spiral galaxy in our Local Group and it, too, faintly is visible to the unaided eye on exceptionally dark, clear nights. It is Messier's object No. 33, or M33, also called the Pinwheel Galaxy. It is smaller, fainter and, at 3 million light years, a bit more distant than M31. It's therefore more difficult to see. As luck would have it, M33 is found only one hand-span below (east of) Andromeda Galaxy in our fall sky.
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Consider yourself among the most visually gifted people if you can spot the faint wisp of M33 with the naked eye. I have accomplished it on a few occasions, not by looking straight at it, but by using averted vision, glancing off to the side to focus its image on a more sensitive part of the retina. Of course, binoculars put M31 and M33 within easy visual grasp and show them as much more than just mists in the night.
What's the third galaxy visible to the unaided eye on moonless autumn evenings? Why, it's our own Milky Way Galaxy, arching high overhead like a colorless rainbow — the third and grandest galaxy of fall.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out Westlake's astrophotography website at http://www.jwestlake.com.
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