Messier 81 (also known as NGC 3031 or Bode’s Galaxy) is a spiral galaxy about 12 million light-years away, in the constellation Ursa Major. Due to its proximity to Earth, large size, and active galactic nucleus (which harbors a 70 million M supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy’s large size and relatively high brightness also make it a popular target for amateur astronomers.

An infrared image of Messier 81 taken by the Spitzer Space Telescope. The blue colors represent stellar emission observed at 3.6 μm. The green colors represent 8 μm emission originating primarily from polycyclic aromatic hydrocarbons in the interstellar medium. The red colors represent 24 μm emission originating from heated dust in the interstellar medium.

Most of the emission at infrared wavelengths originates from interstellar dust. This interstellar dust is found primarily within the galaxy’s spiral arms, and it has been shown to be associated with star formationregions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are very effective at heating the dust and thus enhancing the infrared dust emission from these regions.