Astronomers using a vast survey of the sky around the Andromeda Galaxy have found evidence that it has had two huge meals in the distant past, two separate events where smaller galaxies were physically consumed by the far larger one. Both events were long ago, and one was much earlier than the other, but — like a gourmand with both ketchup and mustard stains on their shirt — the evidence remains.
At a distance of 2.5 million light years, Andromeda is our closest large galactic neighbor. It's a spiral galaxy, like the Milky Way, and has very roughly the same mass (about 1–2 trillion times that of the Sun). It's bigger than us at about 200,000 light years wide, and has roughly twice as many stars. It's so close and big that it can be seen with the naked eye from mildly dark skies.
Like our galaxy, Andromeda has two major components: a flat disk (with spiral arms) made up of old and younger stars, and a huge halo of old stars surrounding it. Both outside and inside this halo are several smaller galaxies, including M32 and NGC 205, both of which are visible using a small telescope.
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