Jimmy Westlake: Andromeda vs. The Milky Way
October 6, 2017
The stars of the constellation 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 to the north trace out her body and legs. Almach, at Andromeda's left foot, is a beautiful telescopic binary star, one golden and one blue.
Andromeda was the beautiful daughter of Queen Cassiopeia and King Cepheus, a celestial royal family that figured heavily in Greek mythology. Cassiopeia once boasted that her daughter, Andromeda, was more beautiful than the daughters of Poseidon. Angered by this boast, Poseidon sent a horrible sea monster to ravage the kingdom. 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 sea and was about to chomp down on poor Andromeda, the hero of the story, Perseus, swoops in on his flying steed, Pegasus, and dangled the severed head of the Medusa before the startled beast.
One look into the eyes of Medusa and the giant sea monster turned into a mountain of stone. With this act of bravery, Perseus won the hand of Andromeda, the two were married, and, of course, they lived happily ever after.
Andromeda, the constellation, holds a secret. Far beyond her stars lies a faint, fuzzy smudge of light, visible to the unaided eye on a dark, moonless night. This is the Great Andromeda Galaxy, or M31, the closest big galaxy to our own Milky Way.
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Located at the staggering distance of 2.5 million light years, Andromeda's galaxy is the most distant object easily visible to the unaided human eye. The light that we see tonight from the Andromeda Galaxy began its journey toward Earth 2.5 million years ago. For comparison, we see the star Alpheratz as it was only 97 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 collection of hundreds of billions of individual stars. We see the galaxy nearly edgewise to our line of sight, with its bulging nucleus wrapped up in its spiral arms.
Astronomers have determined that the Milky Way and Andromeda Galaxy are moving toward each other, trapped in a gravitational free fall. In about three billion years, the Milky Way and Andromeda will suffer a galactic collision, but, since the galaxies are made mostly of empty space, they will likely pass through each other without so much as a single collision between stars.
The two galaxies eventually will merge into a single super-galaxy, one that would dominate our little corner of the universe. Let me be the first to propose the name "Milkdromeda" for our new home. Welcome to Milkdromeda.
Jimmy Westlake recently retired from full-time teaching at Colorado Mountain College's Steamboat campus after 19 years as their professor of physical sciences and is looking forward to spending a lot more time under the starry sky. His “Celestial News” column appears in the Steamboat Today newspaper monthly. Check out Westlake's astrophotography website at http://www.jwestlake.com.