​                                                                 M81 - Bode's Galaxy

 

    The constellation of Ursa Major is the site of a beautiful spiral galaxy known as M81. This is one of the easiest and most rewarding galaxies for the amateur astronomer. It is a bright object, at magnitude 6.8, and can be easily located with any optical instrument. Some say it can be spotted with the naked eye under dark skies and ideal observing conditions. M81 is the brightest member of a group of galaxies called the M81 group. This galaxy is believed to have interacted with its close neighbor, M82, at some point in the past. It was also the site of a supernova explosion in 1993. M81 is located approximately 12 million light-years from Earth.

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    There are two subgroups in the M81 group: one group is associated with M81 and another is associated with the spiral galaxy NGC 2403. These two subgroups are moving toward each other. The total mass of the M81 group has been determined from the motion of galaxies within it to be 1 1012 solar masses. M81 has a mass of 6.7 1011 solar masses.

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    The M81 group also has a few galaxies with classifications similar to those of galaxies in the Local Group, and it was noticed by some astronomers that the linear sizes of the largest H II regions (which are illuminated by many OB stars) in these galaxies had about the same intrinsic sizes as their counterparts in the Local Group. This led American astronomer Allan Sandage and the German chemist and physicist Gustav Tammann to the (controversial) technique of using the sizes of H II regions as a distance indicator, because a measurement of their angular sizes, coupled with knowledge of their linear sizes, allows an inference of distance.