The Milky Way is our home galaxy, so you'd think we'd know it better than any other. But because we're looking at it from the inside, there are some basic facts — facts we know about other galaxies — that we're still figuring out.
Take size. Researchers tracked the motion of stars around the edges of Andromeda, the next spiral galaxy over, and showed it's more than 220,000 light-years across.
From the outside looking in, we're able to estimate Andromeda has 1 trillion stars.
With our clear pictures of Andromeda, we can count the number and types of arms in its spirals.