Free Astronomy Magazine July-August 2015

SPACE CHRONICLES about 6 times the diameter of the full Moon. It is consid- ered a near-twin to the Milky Way gal- axy. Because the gas in Andromeda's ha- lo is dark, the team looked at bright background objects through the gas and observed how the light changed. This is a bit like looking at a glowing light at the bottom of a pool at night. The ideal back- ground "lights" for such a study are qua- sars, which are very distant bright cores of active galaxies powered by black holes. The team used 18 quasars residing far behind Androm- eda to probe how material is distribut- ed well beyond the visible disk of the galaxy. Earlier research from Hubble's Cosmic Or- igins Spectrograph (COS)-Halos pro- gram studied 44 distant galaxies and found halos like Andromeda's, but Hubble finds giant halo around the Andromeda Galaxy by NASA S cientists using NASA's Hubble Space Telescope have discovered that the immense halo of gas enveloping the Andromeda galaxy, our nearest massive galactic neigh- bor, is about six times larger and 1,000 times more massive than pre- viously measured. The dark, nearly invisible halo stretches about a mil- lion light-years from its host gal- axy, halfway to our own Milky Way galaxy. This finding promises to tell astronomers more about the evolu- tion and structure of majestic giant spirals, one of the most common types of galaxies in the universe. "Halos are the gaseous atmospheres of galaxies. The properties of these gaseous halos control the rate at which stars form in galaxies accord- ing to models of galaxy formation," explained the lead investigator, Nico- las Lehner of the University of Notre Dame, Indiana. The gargantuan halo is estimated to contain half the mass of the stars in the Andromeda galaxy itself, in the form of a hot, diffuse gas. If it could be viewed with the naked eye, the halo would be 100 times the diameter of the full Moon in the sky. This is equivalent to the patch of sky covered by two basket- balls held at arm's length. The Andromeda galaxy, also known as M31, lies 2.5 million light-years away and looks like a faint spindle,