A prototypical irregular type galaxy M82 forms a well known pair with M81. M82 is also the closest starburst galaxy to the Milky Way. As a starburst galaxy M82 shows a rate of star formation 10 times greater than our galaxy. Conditions for the starburst activity were believed triggered by a past close encounter with M81 between 300 and 600 million years ago. The two galaxies are imbedded in an immense gas cloud which may in part represent material pulled from those galaxies during the last encounter. Tidal tails and filaments comprise the extended cloud and seem to converge on the two galaxies. One compelling scenerio is that M82 may have been a more massive spiral galaxy at one time. The last encounter with M81 may have left M82 stripped of stars and gas with its remaining bulge and nuclear disk considerably truncated as it appears today. The tidal tails and filaments seen in the M81-M82 system presently may represent the remains of stripped gas that once belonged to the two galaxies.
The bursts of star formation have produced dramatic effects on M82. A collective outgassing of stellar winds from thousands of new stars and supernova driven shock fronts has caused the ejection of ultrahot gases (mostly hydrogen and nitrogen at temperatures of several million degrees) which extend out from the galactic core some several thousand light years. The superheated gases which arise from the combined particle winds of new stars and supernovae are collectively known as a galactic "super wind".
M82 is the prototypical starburst galaxy. Such a galaxy is experiencing an intense period of star formation that may last for ten million years or more. During the starburst period stars can form at rates tens to hundreds of times greater than the rates observed in normal galaxies. The accumulated brightness of new massive stars makes these galaxies some of the most luminous. The triggering of starbursts within galaxies is most often due to gravitational encounters with other galaxies. The resulting forces and shock waves compress existing molecular clouds located in the central regions of the galaxy inducing the formation of new stars. The new stars then produce fierce stellar winds which together with shock fronts from supernovae compress the interstellar medium triggering a chain reaction of further star formation. The wave of star formation can sweep through the entire central region of a galaxy where most of the gas exists. When the gas clouds are depleted star formation eventually ceases. Astronomers believe that most galaxies experienced periods of intense starburst activity in the remote past and their collective galactic winds have replenished the intergalactic medium with heavier elements such as carbon, oxygen and iron.