Monday, October 26, 2009

Going deep

A few comments on Saturday's APOD, a photo of the so-called Deer Lick Group of galaxies in the constellation Pegasus. The large galaxy is NGC 7331, thought to be pretty much a twin of our own Milky Way Galaxy. The photo gives a nice sense of three dimensions.

NGC 7331 is about 40 million light-years away, and the four fuzzy spots above it in the photograph are other galaxies about ten times further away.

We live in a galaxy very much like NGC 7331 -- a hundred billion stars or so in a spiral disk about 100,000 light-years wide. Our Sun is a rather nondescript yellow star about two-thirds of the way out from the center. It makes one great spin with the galaxy every 200 million years.

The sketch here, from my 365 Starry Nights, shows our part of the galaxy edge-on, about 2000 light-years thick, with a view out from our star toward the "Great Square" of Pegasus. I should say "through" the Great Square of Pegasus, because all of the stars that define the constellation are in our own galaxy. Alpheratz, Algenib, Markab and Scheat are the stars we see as the corners of the Square. (Enif is the Flying Horse's "nose".) The Square is what I call "the Window" in the book. The Deer Lick Group lies just outside of the window frame.

The hundreds of stars you see in the APOD photograph are in the Milky Way Galaxy, between us and the edge as we look out. In fact, most of them are rather close neighbors. The galaxies in the photograph are far beyond.

Imagine the Milky Way Galaxy as a frisbee. The nearest spiral galaxy, Andromeda, would be another frisbee across the room, and NGC 7331 would be a frisbee in the house across the street. The other four galaxies in the Deer Lick Group group would be frisbees in the next block. All of this is just our own little corner of the universe. The most distant galaxies we photograph would be frisbees in the next town. And don't forget the hundreds of billions of other frisbees scattered all over the place.

By the way, the galaxies in the photograph are their actual apparent size. The foreground Milky Way stars, however, are vastly smaller than their images in the photograph. The brighter stars (apparent brightness) make bigger "splotchs" of light as it "soaks in" the film.

Now ponder all of that as you sit there sipping your morning coffee. I will leave it to you to find out how the group of galaxies got their unusual name.