M82 is a superb irregular galaxy located in the constellation of Ursa Major. It has an apparent magnitude of +8.4 and is visible with binoculars. Separated by only 38 arc minutes from M82 is an even brighter and equally stunning galaxy, M81. However, they are very different objects. M81 (mag. +6.9) appears almost face-on, is of a grand spiral deign and therefore exhibits prominent near perfect and well defined spiral arms. On the other hand, M82 (mag. +8.4) is edge on, appears long and thin and is known as the Cigar galaxy. It's classified as the prototype starburst galaxy in which stars are forming at exceptionally high rates.
These two objects are the largest members of the Ursa Major or M81 group of galaxies, which at a distance of 11.7 million light-years is one of the closest groups of galaxies beyond the Local Group. Spatially, M81 and M82 are close and are physically separated by only about 150,000 light-years. When seen through binoculars and telescopes at low powers they form a striking pair.
Both M81 and M82 were discovered by Johann Elert Bode on December 31, 1774. Pierre Mechain then independently rediscovered them in August 1779. He reported his observations to Charles Messier, who added both galaxies to his catalogue on February 9, 1781. Finding M82 is not particularly difficult as the Plough asterism of Ursa Major can be used as a starting point. First focus on Dubhe (α UMa - mag. +1.8) the northwest corner star of the bowl. The M81 / M82 pair is located 10 degrees northwest of this star, with M82 positioned directly north of M81.
The galaxies are best seen from the Northern Hemisphere during the months of March, April and May. From locations north of +21 degrees they are circumpolar and always visible.
M82 appears as a faint thin rod of light in binoculars, while M81 looks large and diffuse. The difference in size and shape of these two galaxies is apparent. Small 80mm (3.1-inch) telescopes at low powers show M82 as a slim grey needle of uniform light. A 150mm (6-inch) or 200mm (8-inch) instrument at high magnifications reveals dusty patches that cross the sharp surface of M82. The centre region is brighter that the edges.
In total, M82 spans 11.2 x 4.3 arc minutes of apparent sky. At a distance of 11.5 million light-years this corresponds to an actual diameter of 38,000 light-years, although it's less than half the diameter of M81 (90,000 light-years). Together, the pair form a popular visual and imaging target for amateur astronomers.
M82 is believed to contain at least 30 billion stars. On January 21, 2014, a bright type 1a supernova (SN 2014J) was discovered by Stephen J. Fossey and his students at the University College London (UCL) observatory in London.
M82 Data Table
|Object Type||Starburst galaxy|
|Distance (light-years)||11.5 Million|
|RA (J2000)||09h 55m 51s|
|DEC (J2000)||+69d 40m 43s|
|Apparent Size (arc mins)||11.2 x 4.3|
|Number of Stars||>30 Billion|
|Notable Features||Prototype starburst galaxy. Member of the M81 Group of galaxies|