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The Andromeda galaxy is big, bright, and close. In fact, it’s our nearest large galactic neighbor. That means it’s the easiest big galaxy for astronomers to study. And because of that, they may have a better understanding of some of the galaxy’s details than they do of our own galaxy, the Milky Way.
The Milky Way is more difficult to study because we’re inside it, so astronomers can’t see the whole thing. It’s like trying to map a forest from deep inside it — you’ve got a great view of the trees around you, but you can’t see anything else.
Astronomers can see all of the Andromeda galaxy. And there’s a lot to see — it spans about six times the width of the Moon in our sky.
Many of the measurements of Andromeda depend in large part on its distance — about two-and-a-half million light-years.
At that distance, astronomers calculate that Andromeda’s disk spans more than 200,000 light-years — twice the diameter of the Milky Way. The galaxy contains perhaps a trillion stars — several times the number in the Milky Way. And when you include the galaxy’s invisible dark matter, it may be up to twice as massive as the Milky Way — a hefty close galactic neighbor.
And if you have a nice, dark sky, you can see the Andromeda galaxy as well. To the unaided eye, it looks like a faint smudge of light. It’s low in the northeast as night falls right now, to the left of the Great Square of Pegasus.
More about the Andromeda galaxy tomorrow.