Dave Goulding’s Most Excellent Cosmic Quiz Question:
I’ve learned that the Andromeda galaxy is the only galaxy you can see without a telescope. Where do we look in the sky to see it?
The Andromeda Galaxy is the most distant object the unaided human eye can detect. The galaxy itself is a collection of about 300 billion stars located almost 3 million light-years from us. That means that when you’re looking at the Andromeda Galaxy the photons that are registering on your retinas are ending a speed-of-light journey through intergalactic space that began 3 million years ago.
Why is it called the Andromeda Galaxy? The galaxy carries that name because it is seen in the same general direction as the stars in the constellation of Andromeda, the chained-to-a-rock daughter of the boastful and vain queen Cassiopeia of ancient Roman mythology.
Late summer and autumn is an ideal time to find the Andromeda Galaxy.
On a clear night an hour or two after sunset, face the eastern sky and find the Great Square of Pegasus. It will look like a baseball diamond with “home plate” being lower-most above the horizon, then proceeding upwards and counterclockwise to “first base,” “second base,” and finally “third base.”
The star that marks the “third base” position in the Great Square of Pegasus is called Alpheratz. This star is also associated with the head of the princess Andromeda.
Now that you’ve located the head of the princess, look to your left (northward) for a long narrow funnel or cornucopia-shaped pattern of stars. This is the constellation Andromeda.
In the middle of this horn, about the same distance from Alpheratz as the space between the “bases” of the Square of Pegasus, is the star Mirach.
Look straight upwards a short distance and you’ll see a somewhat dimmer yellow star, and above that your eyes will just barely perceive a dim gray smudge that won’t quite come into focus.
A modest pair of binoculars will reveal an oblong gray smudge of light, bright in the center and faint on its edges. That’s the Andromeda Galaxy!
Now here’s the fun part. That little gray smudge is the light of hundreds of billions of stars that are located almost 3 million light years from your eyes. How far away is that?
Most the bright stars in the constellation Andromeda are about two hundred light years from us. That’s many trillions of miles.
Now imagine that you’re looking through a plate of glass that’s one foot in front of your face. On this plate of glass are microscopic bits of dust. Those are the stars of the constellation Andromeda. At this scale (200 light years = 1 foot) our Milky Way Galaxy, the galaxy within which our solar system is located, is about the size of a football field.
Looking through the plate of glass and far beyond the tiny specks of dust on the glass, nearly three miles away is the Andromeda Galaxy, about half-again the size of our own Milky Way Galaxy.
The stars you see with your unaided eyes as you perceive Andromeda on a crisp fall evening are, practically speaking, right in front of your nose, while the Andromeda Galaxy is far, far beyond them.
Enjoy the view – those photons from Andromeda have traveled a long way to get to you.