Jimmy Westlake: Fall offers glimpses of galaxies
October 25, 2011
One of my favorite times of year for sky-watching is early fall, when the nights are clear and cool but not yet frigid. On moonless autumn evenings, one almost can see to infinity. In fact, the fall sky offers the opportunity to see the two most distant objects visible to the unaided human eye.
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 M31. 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. From that distance, even a collection of 100 billion suns appears in our sky as a faint wisp seen on a clear night.
The discovery that M31 was a whole other galaxy demonstrated the universe was far larger than we imagined, and it didn't take long before dozens, then hundreds, then thousands of other galaxies were confirmed. 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 cluster of about three dozen galaxies called the Local Group. Most of the other members are little guys — so small and faint that seeing them requires large telescopes or long-exposure photography. Many of them orbit our galaxy as tiny satellites, and the same is true for Andromeda.
There is one other large spiral galaxy in the Local Group of galaxies, and it, too, is faintly visible to the unaided eye on exceptionally dark, clear nights. It is Messier's object number 33, or M33 for short, also called the Pinwheel Galaxy. It is smaller, fainter and, at 3 million light years, more distant than M31 and is proportionately more difficult to see. As luck would have it, M33 is found only one hand-span below the Andromeda Galaxy in our fall sky. 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 several occasions, not by looking straight at it but by using averted vision to focus its image on a more sensitive part of the retina. Of course, binoculars put both M31 and M33 within easy visual grasp and show them as much more than just mists in the night. Both are visible near the star clouds of our Milky Way galaxy arching overhead, the third and grandest galaxy of fall.
Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published across the world. Check out Jimmy's website at http://www.jwestlake.com.