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M81 is similar in size and shape to our own Milky Way Galaxy, but approximately twice as massive.

The galaxy’s angular length covers nearly one-half of a degree in our sky- as wide as the apparent size of the moon. M81 has an apparent magnitude of 6.9, making it one of the brightest galaxies that can be seen from the Earth. Just at the limit of naked eye visibility, a handful of amateur and professional astronomers have boasted of being able to see it without the aide of a telescope or binoculars under exceptionally favorable conditions.

M81 is the brightest as well as the dominant galaxy of a nearby group called the M81 group. The second-most prominent galaxy of the group, M82, lies at a distance of only about 150,000 light-years from M81. Other members of the M81 galaxy group NGC 3077 and several other smaller galaxies.

It is hypothesized that M81 and M82 may have interacted in the recent past. Being the most massive of the pair, M81 received very little disturbance from the interaction. In optical and infrared light M81 only shows a delicate dust lane that runs perpendicular to its spiral arms. In contrast, visible remnants of the interaction between the pair are easily seen in a highly deformed and active M82. On a larger scale, radio observations show a common gaseous envelope surrounding M81, M82 and NGC 3077. The well-mixed gas and dust provide a reservoir of raw materials for future star formation in all three galaxies.

The above image of M81 combines data from the Hubble Space Telescope, the Spitzer Space Telescope and the Galaxy Evolution Explorer (GALEX) missions. The GALEX ultraviolet data were from the far-UV portion of the spectrum (135 to 175 nanometers). The Spitzer infrared data were taken with the IRAC 4 detector (8 microns). The Hubble data were taken at the blue portion of the spectrum.

Images courtesy:
Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/Harvard-Smithsonian Center for Astrophysics.

Because of its proximity and physical size, M81 is a great place to study stellar populations, spatial distributions, and globular and open cluster systems. It is also a prime candidate for studying stellar black holes and neutron stars that are orbiting regular stars. When gas from these stars falls on the black holes or the neutron stars, it is heated to temperatures of several million degrees and emits X-rays. Therefore we can identify those extreme objects by observing them with X-ray telescopes like Chandra. By comparing the HST and the Chandra images we can find where these X-ray sources are located (spiral arms, bulge, regions between arms), and from their optical properties (or the properties of the stars in their neighborhood) we can derive important information regarding their nature.

This information in turn, will allow us to understand how stars evolve and die to form black holes and neutron stars. Such studies are very difficult to do in our own Galaxy because a large fraction of it is obscured from our view. Because M81 is similar to our own Galaxy these results will also help us to understand how these extreme objects form in our own "Galactic neighborhood".