The Great Galaxy in Andromeda

Date: October 16-31
Time: 6:45 pm, one hour after sunset
Place: the northeastern sky

This is a followup to my previous post, but goes a bit deeper. People who attend our star gazing sessions get to see very deep into space. One of their most frequently asked questions is “How far can you see with that telescope?”

The answer to that question depends on many factors, not the least of which is the clarity, darkness, and steadiness of the night sky, something astronomers call seeing. It also depends on how big your telescope is. So perhaps a more interesting question is “How far can you see with your eyes?” If your sky has good seeing, the answer to that question is 2.3 million light years, or around 140 quintillion miles — that’s 140 with 18 zeros after it!

This farthest visible object is the Andromeda Galaxy, a spiral of some 100 billion stars very similar to our own Milky Way Galaxy. Also known as M31, from the Messier Catalog, the Andromeda Galaxy is the closest neighboring galaxy to our home galaxy. If you can see it at all, what you’ll see is an elongated smudge of light located slightly below the point one third of the way from Alpheratz to Schedar.

Alpheratz, in Pegasus, and Schedar, in Cassiopeia (the “W”) are both fairly bright stars. You’ll have no trouble finding them. M31 is a bit tougher. To maximize your chances of seeing this object, wait for a night without a Moon. Then turn off all outside lights, go outside, and give your eyes at least 20 minutes to adjust to the dark. Now look for that faint smudge. When you spot it, you’ll be able to claim you saw the farthest object visible to the human eye. And if you do, you should probably spend some time memorizing those numbers.

How to Find the Nearest Galaxy
Q&A: Desert Dust