Sometimes, amid all the attention lavished on space observatories like the Hubble Space Telescope, we forget that the big scopes here on terra firma are expanding astronomers' horizons too.Take, for example, this recently released image of Messier 81 (its northern half, anyway). Taken with Japan's Subaru telescope atop Mauna Kea, the image reveals a diffuse glow (appearing bluish here) skirting beyond the galaxy's beautiful spiral disk. It's part of a curious gas-rich ring known as Arp's Loop, which likely resulted from an interaction with neighboring M82 and NGC 3077 some 200 to 300 million years ago.
The Hubble Space Telescope recorded a nice view of the entire galaxy a few years ago. But with an aperture of 27 feet (8.2 m), Subaru and its prime-focus camera resolved many individual stars hovering near M81's disk. A team led by astronomer Michael K. Barker (University of Edinburgh) wanted to know whether they constitute part of a halo surrounding M81, much as a cocoon of stars and globular clusters surrounds our Milky Way.
The suspicion is that halos are evidence of a galaxy's appetite for devouring some of its neighbors — in the Milky Way's case, unfortunate victims that used to hover nearby in the Local Group. When two galaxies merge, the ensuing tug of war for gravitational control throws out many stars (or hydrogen gas that turns into stars) out into looping, inclined orbits. These far-flung suns serve as a smoking gun of the merged systems' violent encounter.
Barker's team analyzed 40,000 stars beyond the galaxy's visible disk and concludes that M81's halo is distinctly different from the Milky Way's. It's both brighter and more abundant in "metals" (astronomer-speak for any element heavier than helium). These differences must be telling us something about how M81 evolved — but the story line isn't complete yet. Perhaps this giant pinwheel feasted on more small galaxies than the Milky Way did, or maybe the starry envelope is evidence of tidal tugging from M82 and NGC 3077. The researchers aren't even sure if they're seeing a true halo or just an extension of the visible disk.
In any case, M81 is one of the easiest galaxies to spot in a backyard telescope. Located some 12 million light-years away, it has the total light of a 7th-magnitude star. These evenings you'll find it high overhead near the Big Dipper's bowl. Click here for a nice finder chart published several years ago in Night Sky magazine.