Messier 82 (NGC 3034) is a remarkable, peculiar galaxy in Ursa Major. M 82 was discovered along with its partner M 81 by Johann Bode, and both galaxies are sometimes known as Bode's Nebulae. Also called the Cigar Galaxy, M 82 has been spectacularly disturbed by a relatively recent encounter with M 81, and displays conspicuous dust lanes and heavy star formation. M 82 is the prototype irregular starburst disk galaxy.
Discovery and History
Messier 82 was discovered in 1774 by Johann Elert Bode, along with M 81; he described it as a "nebulous patch", which "is very pale and of elongated shape." Pierre Mechain independently rediscovered both galaxies in August 1779 and reported them to Charles Messier, who added them to his catalog in 1781.
M 82 is one of the few Messier objects to which William Herschel assigned a catalog number (H IV.79); Herschel usually carefully avoided giving his numbers to Messier's objects. William Parsons, the Third Earl of Rosse, was the first to remark on the dark dust lanes and patches visible in the central part of M 82.
Halton Arp included M 82 as No. 337 in his Catalogue of Peculiar Galaxies.
Amateur Observation
Messier 82 is relatively bright, at magnitude 8.4. It appears only 38' north of M 81, and can be seen in the same field of view with binoculars or a low power telescope. This magnificent cigar-shaped galaxy has a 9' x 4' halo, and contains a 5' core extended along its major axis. Because of its high surface brightness, the halo has a well-defined outline. The core is very irregular in profile, and severed into two almost equally long parts by a diagonal dark band.
The western part of the core is brighter and is studded with short dark streaks jutting out at different angles. The eastern half of the core has the largest unbroken area. Several bright knots are strung along the major axis; a couple appear nearly stellar at their sharpest points of concentration.
Supernova 2004am was discovered in M 82 on images taken at Lick Observatory in 2003. Supernova 2008iz was detected by radio observations in March 2008; and another possible radio supernova was reported in June 2009.
Properties, Starburst, and Structure
As a member of the M 81 group, M 82 is 12 million light years distant. It is separated from M 81 by at least 130,000 light years. Its core seems to have suffered dramatically from several close encounters with M 81, most recently about 100 million years ago. Tidal forces caused by gravity have deformed M 82, and caused star formation in its core to increase ten-fold compared to "normal" galaxies like the Milky Way.
In the core of M 82, the active starburst region spans a diameter of 500 parsecs. In 2005, the Hubble Space Telescope revealed 197 massive young clusters in the starburst core, with average masses around 200,000 Suns. A unique "superwind" flowing out of these clusters appears to be fueled by supernovae that occur about once every ten years. Tendrils of material extend away from the nucleus, suggesting a colossal explosion. Studies have revealed filaments expanding outward from M 82 at 600 miles per second. This turbulent explosive gas flow is also a strong source of radio noise, known as Ursa Major A or 3C 231.
The Chandra X-ray Observatory detected fluctuating X-ray emissions from an intermediate-mass black hole, of roughly 200 to 5000 solar masses, approximately 600 light-years away from the center of M 82. Like most galaxies, M 82 hosts a supermassive black hole at its center, with a mass of approximately 30 million solar masses.
M82 is much brighter at infrared wavelengths than it is in the visible part of the spectrum, and in fact M 82 is the brightest infrared galaxy in the sky.
Over 100 young globular clusters have been discovered in M 82 with the Hubble Space Telescope, their formation probably another side-effect of the close brush with M 81.
M 82 was previously believed to be an irregular galaxy. However, in 2005, two symmetric spiral arms were discovered in the near-infrared images of M 82. Due to M 82's high disk surface brightness, nearly edge-on orientation, and the complex network of dusty filaments present in optical images, the arms were not previously detected.