April: Galaxy Pairs and Groups

April is the month for interacting galaxies, beginning with M81 (lower left) and M82 (upper right). These two galaxies, about 12 million light-years distant, passed close to one another about 20 million years before the light began its journey to my camera. Obviously, the galaxies are now farther apart than this image shows, but we would not see that for another 12 million years…confused yet? During this close encounter event, the larger and more massive M81 dramatically deformed M82 by gravitational interaction, stimulating a burst of star formation. The galaxies are still close together, with their centers separated by a linear distance of only about 150,000 light-years. Imaging. M81 and M82 can be imaged either together, as shown on the wide-field view, or individually, as shown on the next two pages. H-II regions have been emphasized in M82 by enriching the red channel with H-alpha data. Despite its large size, M81 shows little detail in its spiral arms unless long exposures are obtained away from light pollution. M82 is better framed with a smaller field of view. Both objects are bright enough for routine RGB methods or single-shot color cameras, but obtaining some data with H-alpha filters aids detection of the brilliant H-II glow around the core of M82.