Messier 82 is gravitationally bound to its much larger companion, Messier 81 (not seen in these images). It is therefore approximately the same distance from us and is located in the constellation Ursa Major. M82 is classified as an irregular galaxy, but its unusual appearance has led many to describe it as a prototypical example of a peculiar galaxy. It is also one of the brightest infrared and radio sources in the celestial sky, a result of galaxy-wide bursts of star formation. Such galaxies are also known as starburst galaxies.
Visible: DSS (left) and Visible Color: Robert Gendler (right)
The DSS visible-light image (left) suggests that M82 might be nothing more than an edge-on galaxy. But it appears to be surrounded by a fuzzy halo, and wisps of optical emission are evident along a line that is roughly perpendicular to the major axis of the galaxy. The color photo (right) is of shorter exposure, and hence the central regions are not saturated (as in the DSS photo). This image provides us with a better opportunity to observe the dust lanes within the central regions of the galaxy. The color image confirms the presence of obscuring dust near the nucleus of Messier 82. Two features of the color image suggest that large amounts of dust are present in M82. First, note the reddish hue near the galaxy center. Dust grains are more effective at scattering short-wavelength blue light than long-wavelength red light. Hence, less of the blue light reaches our eye, leaving a reddened tint. [This same phenomena produces the colorful red sunsets in dusty or smoggy atmospheres.] Second, dark absorption lanes are seen to be slicing across the width of the galaxy. The most prominent dust lane appears to be bisect the galaxy. For a spectacular deep composite optical/near-IR image of M82 and the crossing dust lanes, visit http://www.seds.org/messier/more/m082_m1.html.
Near-Infrared: 2MASS, Mid-Infrared: IRAS and Far-Infrared: IRAS
Now examine the near-infrared image (above, left). It does not suggest that M82 might be peculiar or laden with dust. This is because near-IR radiation effectively pierces through dust to reveal the underlying distribution of old stars within the galaxy. On the other hand, mid- and far-infrared radiation (center and right) is an excellent tracer of dust in galaxies. Unfortunately, the IRAS images of M82 are of relatively poor spatial resolution, and all details noted above are lost.
The most notable feature of the IRAS images is the fact that the galaxy has apparently been rotated by about 90 degrees. In reality, the orthogonal stretch is a result of the peculiar rectangular shape of the infrared detectors used aboard IRAS. Messier 82 was scanned in a manner such that the detector's length dimension coincided with a line running roughly from the upper right to the lower left.
Radio: VLA + Merlin (left), Ultraviolet: ASTRO-1 UIT (right)
The radio image (left) is a composite picture produced by using two widely separated ground-based telescopes, the VLA in New Mexico ane MERLIN in the UK, which were electronically linked through a procedure known as interferometry. By linking the telescopes in this manner, astronomers are able to obtain an image that is capable of yielding the spatial resolution of a single telescope equivalent to the distance separating the two telescopes! By combining telescopes in this way, one creates an interferometer, or array of individual telescopes all pointing at the same astronomical source. [In reality, both the VLA and MERLIN are interferometers themselves, comprised of smaller arrays of telescopes.] The image from the radio interferometers reveals the chaotic structure of radiation emanating from Messier 82.
The ultraviolet photo (above, right) also attests to the peculiar nature of M82. The peculiar shape is partially a result of obscuring dust on the foreground side of the galaxy. The bright point-source to the south-west (lower right) is a foreground star in our Milky Way galaxy.
Finally, the x-ray image of M82 confirms the bizarre nature of this galaxy; it looks nothing like the other images in this gallery. Most of the x-ray emission is concentrated in the central regions of M82. The bright spots in the center are supernova remnants and X-ray binaries. The luminosity of the X-ray binaries suggests that most contain a black hole. The diffuse X-ray light in the image extends over several thousand light years, and is caused by multimillion degree gas flowing out of M82.