M 81, M 82 and Galactic Cirrus

M81 and M82 were discovered by Johann Elert Bode in 1774. A few observers have reported that M81 is visible to the naked eye under perfect conditions. M81 is the main galaxy of a galaxy group neighboring the Local Group of galaxies.

The M81 group of galaxies is quite similar to our Local Group. The central galaxy is the large barred spiral M81.

M82 is the brightest infrared galaxy in the sky, just 125,000 light-years away from M81. A close encounter of both galaxies took place about 600 million years ago. As a result, star formation is still ongoing today. Several supernova explosions have created strong winds that carry hot gas out of the center of the galaxy, forming Hα filaments. The hot matter cools down, and part of it falls back to the center again. On the other hand, M81 shows no signs of the encounter.

East of M81, the galaxy UGC 5336 (Holmberg IX) appears as a weak patch of light. NGC 3077 can be seen near the edge of the image. Both galaxies, as well as PGC 28731, belong to the M81 group. Most of the other diffuse spots are remote galaxies in the background. The whole image is covered with a very faint nebula sometimes called IFN or Integrated Flux Nebula. Nevertheless, “Galactic Cirrus” may be a better name for these faint structures that can be seen in many locations in the sky. They were discovered visually and photographically by Johann Georg Hagen in the 1930s. It is believed that the galactic cirrus consists of gas and dust reflecting the light of our galaxy. Some of the faint clouds visible in the image are intergalactic rims between M81 and M82.

Very faint objects like the IFN can normally not be photographed from light-polluted areas, especially when the radiated light is more or less uniformly distributed over the visible spectrum. Only with a combination of fast telescopes and very long exposure times can such objects become visible.