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.
SEE MORE: Gorgeous New Milky Way Image Maps Our Galaxy's Dust
And the Milky Way? Until recently, we thought it was about 100,000 light-years across. But new studies show we may have been off by as much as 50 percent.
From the outside looking in, we're able to estimate Andromeda has 1 trillion stars.
Trying to count the Milky Way's stars from the inside means lots of them wash each other out. Our estimates are anywhere from 100 billion to 400 billion stars.
With our clear pictures of Andromeda, we can count the number and types of arms in its spirals.
But scientists have yet to agree on exactly how many arms the Milky Way has, or how they're arranged. Clouds of galactic dust get in the way of figuring it out.