Jimmy Westlake: Andromeda Galaxy
October 5, 2008
Topping the alphabetical list of the 88 constellations is Andromeda, the beautiful daughter of Queen Cassiopeia and King Cepheus. This celestial royal family figured heavily in Greek mythology. Cassiopeia once boasted that her daughter was lovelier than the daughters of Poseidon. Angered by this, Poseidon sent a horrible sea monster to ravage the villages along the shoreline. The only way to stop the creature was to sacrifice Andromeda to the monster. Cassiopeia and Cepheus reluctantly chained their daughter to the rocks along the coast and left her to her fate. When the sea monster lumbered out of the depths of the sea and was about to eat poor Andromeda, the Greek hero Perseus jumped out from behind a rock and dangled the severed head of Medusa before the startled beast. One look into the eyes of Medusa, and the giant sea monster instantly turned into a mountain of stone. With this act of bravery, Perseus won the hand of Andromeda. The two were married and lived happily ever after, or so the story goes.
The stars of Andromeda twinkle brightly in our October sky, high in the east after darkness falls. One corner of the Great Square of Pegasus marks the princess’s head, a star named Alpheratz. From there, two diverging streams of stars trace out her body and legs. The bright stars Mirach and Almach lie along the lower stream of stars. Almach is a beautiful telescopic binary star.
Andromeda holds a secret. Far beyond her stars is a faint, fuzzy smudge of light visible to the unaided eye on a dark, moonless night. This is the Great Andromeda Galaxy, also known as M31, the closest major galaxy to our own Milky Way. At the staggering distance of 2 million light years, Andromeda’s galaxy is the most distant object visible to the unaided human eye. The stars that make up the familiar outline of our constellation Andromeda are merely foreground objects in our own galaxy. Alpheratz, for example, is only 97 light years away from us, so we see Alpheratz as it was 97 years ago. The light that we see tonight from the Andromeda Galaxy began its journey toward Earth two million years ago.
Binoculars or a small telescope provide a spectacular view of our neighboring galaxy. Like the Milky Way, the Andromeda Galaxy is a flattened, spiraling mass of hundreds of billions of individual stars. We see the galaxy nearly edgewise to our line of sight, with its bulging nucleus enfolded in its spiral arms.
Astronomers predict that in about 3 billion years, the Milky Way and Andromeda will suffer a galactic collision, but, because the galaxies are made mostly of empty space, they likely will pass through each other without so much as a single collision between individual stars. The two galaxies might even merge into a single super-galaxy, one that would dominate our little corner of the universe.
Professor 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 on the Web sites of CNN.com, NASA’s “Astronomy Picture of the Day” Web site, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines. His “Celestial News” article appears weekly in the Steamboat Pilot & Today. His “Cosmic Moment” radio spots can be heard on local radio station KFMU. Also, check out Jimmy’s Web site at http://www.jwestlake.com.