If you’re thinking of a galaxy, the image in your head is probably the Andromeda Galaxy! Studies of this massive neighboring galaxy, also called M31, have played an incredibly important role in shaping modern astronomy. As a bonus for stargazers, it is also beautiful.

Have you heard that all the stars you see at night are part of our Milky Way galaxy? While that is mostly true, one star-like object located between the constellations of Andromeda and Cassiopeia appears fuzzy to unaided eyes. That’s because it’s not a star, but the Andromeda Galaxy, its trillion stars appearing to our eyes as a 3.4 magnitude patch of haze. Why so dim? Distance! It’s outside our galaxy, around 2.5 million light years distant—so far away that the light you see left M31’s stars when our earliest ancestors figured out stone tools. Binoculars show more detail: M31’s bright core stands out, along with its wispy, saucer-shaped disc. Telescopes bring out greater detail but often can’t view the entire galaxy at once. You may be able to make out globular clusters and at least two of its orbiting dwarf galaxies: M110 and M32. Surprisingly, persistent stargazers can still spot M31’s core in moderate light pollution if skies are otherwise clear.

Modern astronomy was greatly shaped by studies of the Andromeda Galaxy. A hundred years ago, the idea that there were other galaxies beside our own was not widely accepted, and so M31 was called the “Andromeda Nebula.” Increasingly detailed observations of M31 caused astronomers to question its place in our universe – was M31 its own “island universe,” and not part of our Milky Way? Harlow Shapley and Heber Curtis engaged in the “Great Debate” of 1920 over its nature. A few years later, Edwin Hubble estimated M31’s distance as far outside our galaxy’s boundaries. And so, the Andromeda Nebula became known as the Andromeda Galaxy. These discoveries inspire astronomers to this day, who continue to observe M31 and many other galaxies for hints about the nature of our universe.