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 unaided eye is two-and-a-half million light-years away. M31 is a giant galaxy in the constellation Andromeda. It’s high in the west 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 stars that contribute to the galaxy’s glow have expired since then — some with titanic explosions that briefly shine bri
