Messier 31, the Andromeda Galaxy, is the largest and most-distant object that’s easily visible to the unaided eye. Under dark skies, it looks like a skinny cloud about as wide as the Moon. Right now, it’s about a third of the way up in the northeast at nightfall.

M31 is two-and-a-half million light-years away. In other words, the light you see from the galaxy tonight began its journey across the cosmos two-and-a-half million years ago. The galaxy is roughly 150,000 light-years across – bigger than the Milky Way – and may contain a trillion stars.

It’s also the hub of its own galactic empire – it’s orbited by more than three dozen smaller galaxies. And a recent study revealed many new details about the satellites. Astronomers spent years looking at them with Hubble Space Telescope. And they supplemented the new observations by going through older ones.

They found that most of the stars in the smaller galaxies had been born by about 12 billion years ago – when the universe was about one-tenth of its present age. And star formation had all but stopped by about eight billion years ago. Galaxies that are bigger and farther from M31 gave birth to stars a little longer than those that are small and close.

One of the bigger satellites might have rammed through M31 a few million years ago. That stirred things up throughout the empire surrounding big, beautiful M31.

More about M31 tomorrow.

Script by Damond Benningfield