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If you gaze long enough into a dark, starry sky — a sky that’s not polluted by streetlights or the Moon — it seems like you can see forever. That’s not quite the case, but you can come pretty close: The most distant object that’s easily visible to the eye alone is two-and-a-half million light-years away.
M31 is a giant galaxy in the constellation Andromeda. It’s high in the east-northeast at nightfall, and looks like a skinny smudge of light that’s wider than the Moon.
That smudge is the combined glow of hundreds of billions of stars — a cosmic pinwheel that’s even bigger than our own galaxy, the Milky Way. We’re viewing it from just above the edge, so it looks a bit like a fat cigar.
We’re also seeing M31 as it looked far in the past. It takes the galaxy’s light two-and-a-half million years to reach us, so we see it as it looked two-and-a-half million years ago. Millions of the stars that contribute to the galaxy’s glow have expired since then — some with titanic explosions that outshine the entire rest of the galaxy combined. We won’t know which ones, though, until their light reaches Earth — perhaps millions of years in the future, or perhaps as early as tonight.
We do know that M31 and the Milky Way are moving closer together, so over the next few billion years M31 will grow larger and brighter. Eventually, it and the Milky Way may merge — and some other galaxy will take M31’s place on the edge of forever.
Script by Damond Benningfield, Copyright 2011