Andromeda Galaxy

StarDate: October 16, 2009

You are missing some Flash content that should appear here! Perhaps your browser cannot display it, or maybe it did not initialize correctly.

audio/mpeg icon

We can't travel into the past, but we can get a glimpse of it. Every time we look at the Moon, for example, we see it as it was a little more than a second ago. That's because sunlight reflected from the Moon's surface takes a little more than a second to reach Earth. We see the Sun as it looked about eight minutes ago, and the other stars as they were a few years to a few centuries ago.

And then there's M31, the Andromeda galaxy -- the most distant object that's readily visible to human eyes. This great amalgamation of stars stands almost directly overhead late this evening. When viewed from a dark skywatching location, far from city lights, it looks like a faint, fuzzy blob. But that blob is the combined glow of hundreds of billions of stars -- seen as it looked more than two million years ago.

Andromeda is similar to our own Milky Way galaxy. It's a flat disk that spans more than a quarter-million light-years. Its brightest stars form spiral arms that make the galaxy look like a pinwheel. Yet the galaxy is so far away that its structure is visible only through telescopes.

The light from M31 has to travel about two and a half million light-years to reach us. That's about 15 billion billion miles -- the number 15 followed by 18 zeroes. Yet even across such an enormous gulf, the galaxy is so bright that we can see it -- faintly -- with our own eyes, crossing high overhead late tonight.

Script by Damond Benningfield, Copyright 2006, 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

The one constant in the Universe: StarDate magazine


©2014 The University of Texas McDonald Observatory