Tuesday, November 1, 2011

The Great Galaxy in Andromeda

Saturday night while I was up at the Vis I decided to take a break from imaging globular clusters to try my hand at what probably pretty much every astrophotagrapher ever has broken their teeth on: Messier 31, the Great Galaxy in Andromeda.

The Andromeda Galaxy is located in the direction of the constellation Andromeda, about 2.5 million light years away. It's not the closest galaxy to our own Milky Way, but it's the closest spiral galaxy and the one closest in size to the Milky Way in our Local Group of about 30 galaxies. (Intriguingly, although Andromeda is larger in size than the Milky Way, recent studies indicate that the Milky Way may have more mass. It also has a higher star formation rate, and a higher rate of supernovae.)

The Great Galaxy in Andromeda, Messier 31. Technically, only the innermost part of it. M32 (at top) and M110 (at lower left) are also visible.

While Andromeda may or may not be the most massive galaxy in our Local Group, there is no doubt that it's the brightest galaxy in the northern hemisphere. It's also the largest on the sky. In fact, if we could see it all, it would appear as wide as six full moons on the sky! Its huge size, however, is paradoxically what makes it so hard to see. All that light is spread over a large area, making it exceedingly faint. What you're seeing in this picture is no more than the central third of the galaxy; it stretches out on both sides almost as far again as seen in this picture (mostly because I didn't have a lot of time to image it, so I took the longest exposures I did in the time available).

Being as it is the largest on the sky and brightest in the northern hemisphere, Andromeda was the first galaxy to have its spectra taken (by Vesto Slipher in 1912) and the first galaxy to be confirmed as such (i.e. a system of star not materially associated with the Milky Way), by Edwin Hubble in 1922-23. This discovery increased the size of the known universe by several times, and was instrumental in the development of our current picture of the universe.

The Andromeda Galaxy is a fascinating star system, and I could write pages more on its various interesting features, but it's getting late here and I ought to get some sleep. After seeing how this picture came out, I'm tempted to try again and get a longer exposure to show fainter details, so perhaps I'll write more about it if I do.

Edit (11/5/11): Due to a suggestion from a master imager, I went back and re-reduced the data for this picture, stretching the light curve in a different manner to get better results. DeepSkyStacker, the program I use to do the data reduction, lets you apply different light curves to your photo after it does all the work to reduce the noise in the image. Basically, different light curves are better for bringing out different features. The one I chose (called "cube root") helped show the faint nebulosity better without simultaneously blowing out the the bright, dense core. So, here's the same picture, version 2:

The Andromeda Galaxy, Messier 31. Now much prettier!
If you compare the two pictures, you can see that the second one shows a lot more detail. I'm glad I learned how to apply different light curves, as my typical globular cluster shots didn't usually change much when I played around with them before so I wasn't quite sure what it did. But as you can see, it works wonders with nebulous subjects! (And since there aren't as many globular clusters to image during the winter, I'll probably be taking a little break from them to image some of the other cool objects in the night sky...)


  1. The Andromeda galaxy has always been one of my favorites to image. I think I've imaged it at least five times, and each time I figure out a new way to reveal the spectacular details in it. I like your image quite a bit; you should do a color series with a logarithmic stretch and post it.

  2. Definitely looks better! There is another trick called hybrid layering which I encourage you to try sometime. It allows you to combine a very long exposure, a medium exposure, and a short exposure such that you get all the faint stuff without blowing out the bright stuff. It is in effect a way of artificially increasing the bit depth of an image. Check it out sometime.


Think I said something interesting or insightful? Let me know what you thought! Or even just drop in and say "hi" once in a while - I always enjoy reading comments.