by Robert J. Vanderbei
April is galaxy month.
The arc of the Milky Way—our home galaxy—dominates the summer and winter nighttime skies. But in fall and spring, depending on the time of night, the Milky Way is usually seen making a ring around the horizon.
For example, at midnight on April 15 the Milky Way will be both setting in the west and rising in the east. Hence in April the bulk of our galaxy is out of our field of view, and we get great views of objects that lie beyond the Milky Way—such as other galaxies.
Shown here is a picture of a patch of sky in the constellation Canes Venatici, the Hunting Dogs. The image contains several galaxies, the biggest of which is Messier 106. Click the pic to see the full-size version.
—Picture by Robert J. Vanderbei
M106 is a barred spiral galaxy about 26 million light-years away from us. Based on analyses of x-ray images, it is thought that M106 contains a supermassive black hole at its center into which material from the galaxy is steadily falling.
The second biggest galaxy in the picture, seen at the lower right, is called NGC 4217. It is a spiral galaxy, but from our vantage point, it appears edge-on.
These two galaxies are about the same distance from us and hence are neighbors of each other. If you look carefully, you will spot at least four or five additional smaller, fainter galaxies.
M106, NGC 4217, and perhaps some of the other galaxies in the picture belong to a gaggle of galaxies called the Canes II Group.
Unfortunately, light pollution has made it difficult for most observers to see galaxies through a telescope in their backyards, as most of us live either in cities or the suburbs. To see these objects visually through a telescope, you have to go to a place where the night skies are very dark.
But there is an alternative. If you are willing to settle for a photographic record as opposed to a direct visual observation, you can take a picture of many very faint objects even from light-polluted locations.
I took the picture shown here from my driveway in light-polluted New Jersey. I attached a CCD camera to my four-inch refractor and collected several two-minute exposures that I then stacked on my computer to make the final picture. The total exposure time was 196 minutes.
There is another pretty grouping of three galaxies visible in April called the Leo Trio, so named because they can be found in the constellation Leo.
(Get more on how to find Leo in the spring skies.)
The Leo grouping consists of M65, M66, and NGC 3628. These three galaxies are all about 35 million light-years from us and therefore—just as with the Canes II group—they are neighbors of each other.
This picture is a stack of 31 three-minute exposures. Again, click to see it larger.
—Picture by Robert J. Vanderbei
Note that anyone can stack astroimages using Photoshop: File -> Scripts -> Load Files Into Stack.
However, it’s not the easiest way to do it—in fact, I think it would be quite tedious to stack a large number of images without a customized script.
I use a program called MaxIm DL. It’s pricey but full featured.
There are a few other popular choices, such as CCDstack and ImagesPlus, and there are freeware programs such as DeepSkyStacker and Registax II (which is designed for imaging planets but can be used for deep-sky objects, too).
I haven’t used it myself, but for those who prefer freeware, I’ve heard only good things about DeepSkyStacker.
Robert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.