M81 and M82
Description (Adapted from Steve Mandel, Robert Gendler, APOD, and SEDS):
This is a shot of the famous M81/82 pairing in Ursa Major. This image is optimized to present the "Integrated Flux Nebula", the material which looks like fog surrounding these galaxies. The term, "Integrated Flux Nebula", or "IFN", was initially coined by Steve Mandel, as part of his "Unexplored Nebulae Project". These clouds are unusual in that they do not reflect light from any individual star or star clusters, instead these "high-latitude" nebulae reflect the total combined light (integrated flux) of all the stars in the Milky Way.
M81 and M82 were discovered by Johann Elert Bode in 1774. M81 is one of the easiest galaxies to observe for amateur astronomers. With its total visual magnitude of about 6.8, it can be found with most amateur small telescopes. Bode described M81 as a "nebulous patch", about 3/4 of a degrees away from M82, and it "appears mostly round and has a dense nucleus in the middle". Pierre Méchain independently rediscovered both galaxies as nebulous patches in August 1779 and reported them to Charles Messier, who added them to his catalog after his position measurement on February 9, 1781. M81 is the brightest member of the nearby galactic group, the "M81 group". The true distance of M81 is 12 million light years. Investigations performed in 1994 have indicated that M81 has probably only little dark matter, hence is estimated to have less than that of average galaxies.
M82 is a prototype member of the class of galaxies called, "starburst galaxies". As a starburst galaxy, M82 shows a rate of star formation 10 times greater than our galaxy. A member of the M81 group, M82 is also about 12 million light years distant. In infrared light, M82 is the brightest galaxy in the sky; it exhibits a, "infrared excess" (it is much brighter at infrared wavelengths than in the visible part of the spectrum). The accumulated brightness of new massive stars makes these galaxies some of the most luminous.
A clear bridge of gas exists between M81 and M82, showing velocity characteristics resulting from a tidal encounter between the two galaxies 300-600 million years ago. In both galaxies, colliding gas has created a recent abundance of bright new stars and also triggered the "starburst" status of M82. The gravity from each galaxy affects the other during each subsequent orbital pass, once 4.6 billion years or so. Each orbit brings them as close as 10,000 light years, and their current distance from each other is about 120,000 light years. Eventually, only one galaxy will remain.
The triggering of starbursts within M82 is the result of the gravitational encounter with M81. The resulting forces and shock waves compress existing molecular clouds and induce the formation of new stars. The new stars then produce stellar winds which trigger a chain reaction of further star formation. When the gas clouds are depleted star formation eventually ceases.
The Dwarf irregular galaxy, Holmberg IX, (seen below M81 in the image) most likely condensed from tidal debris left over from this event. (Note the hydrogen-rich star-forming nebular regions clearly visible in this dwarf galaxy.)
