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Big Neighbor

October 23, 2016

Astronomical distances can be mind boggling. Our closest neighbor, the Moon, is a quarter of a million miles away — equal to about 10 trips around Earth’s equator. The closest planet, Venus, is always at least a hundred times farther. And the closest neighboring star system, Alpha Centauri, is about a million times farther still.

Yet even that enormous gulf is nothing more than a hop away compared to the most distant object that’s easily visible to the unaided eye. The Andromeda galaxy is more than half a million times farther than Alpha Centauri — two-and-a-half million light-years.

Right now, Andromeda stands about half-way up the eastern sky as night falls. To the eye alone, it looks like a faint patch of light about the width of the Moon. Binoculars bring it into better view.

Like our home galaxy, the Milky Way, Andromeda is a wide, flat disk. The visible part of the disk spans more than a hundred thousand light-years. But powerful telescopes show that stars taper off into space well beyond that, making the disk twice as wide as the Milky Way’s.

We see the galaxy almost edge-on, which raises another mind-boggling detail about astronomical distances. It takes the light from the far edge of the disk longer to reach our eyes than the light from the near edge of the disk. So there’s no way to see the galaxy at a single point in time. Instead, we see different parts of it as they looked over a span of almost a hundred thousand years.


Script by Damond Benningfield

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