M81, or the 81st object on Charles Messier's list, is the brightest member of one of the nearest galaxy clusters to our own. Actually this cluster's much larger than ours and contains no less than 34 members, although the larger part of those are dwarf galaxies that are beyond the reach of most amateur telescopes. A few others, however, are a real treat such as strange M82 which lies almost as close to M81 as measures the diameter of our Milky Way (I'll tell you more about M82 in my next post). On the other side of the cluster lies NGC4236, of which I told you here recently, at a distance of 12° in our sky or almost 4 million lightyears from M81. All of these galaxies are physically interacting with each other and especially M81 and M82 (and less known NGC3077) have a very strong bond. A few hundred million years ago a close encounter took place between M81 and M82 during which the latter was dramatically deformed. M81 on the other hand survived this encounter almost intact but still the enormous tidal forces generated a lot of star forming activity in it. It is difficult to see the many star forming regions with amateur telescopes but you might notice the brighter knots in the spiral arm just below the two little stars below the nucleus. The interaction with M82 has also sucked out a lot of matter from both galaxies which led to the formation of filamentary structures in between them. Some of these gas glouds have fallen back into M81, leading to even more starburst activity.
For those of you with a keen eye, you might notice a faint patch towards the left-hand border of my sketch. This is an irregular dwarf galaxy denominated UGC5336 (or Holmberg IX) which is a close companion to M81. It's very young, with an estimated age of merely 200 million years and therefore scientists believe that it may have formed out of the debris left from the M82 fly-by.
The M81 galaxy cluster's estimated to be 12 million lightyears away from us.