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National Aeronautics and Space Administration

Andromeda (M31)

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The Andromeda Galaxy, M31, dominates an extent of M31’s disk, making it possible for exclusive group of objects outside our own galaxy astronomers to begin to understand the local that are visible to the unaided eye. Under dark skies, physical conditions that promote or inhibit M31 can be seen as a cloudy patch of light, formation. UV imaging is particularly well suited although even small telescopes reveal its substruc- for studying regions with infrequent (but ongoing) ture in the form of a central bulge and spiral disk. , such as the outermost part of the galactic disk. The Andromeda Galaxy and are the largest, most massive in the “Local A Different View of the Group.” Oddly, M31 seems to contain less than the Milky Way, even though its disk is more The Galaxy Evolution than double the diameter of the Milky Way’s. Explorer was Because it lies only 2.5 million light years away, launched on April 28, M31 is an extremely well-studied , 2003. Its mission is to giving us a detailed, external perspective of a study the shape, galaxy similar to our own. However, much remains brightness, size and to be learned. distance of galaxies across 10 billion Revealing How Form years of cosmic his- tory. The Explorer's GALEX Space Telescope images of the Andromeda 50-centimeter-diameter (19.7-inch) telescope Galaxy in the (UV) portion of the sweeps the skies in search of ultraviolet-light have provided new insight into the star sources. formation process ultimately driving galaxy evolution. UV imaging traces primarily emission Caltech leads the Galaxy Evolution Explorer from the atmospheres of hot stars, most of which mission and is responsible for science operations were formed in the past few hundred million years. and data analysis. NASA's Jet Propulsion The GALEX data vividly reveal the global pattern Laboratory, Pasadena, California, manages the of “recent” star formation within M31, enhancing mission and built the science instrument. The the contrast of these features by excluding the light mission was developed under NASA's Explorers originating from ordinary, cooler stars. Program managed by the Goddard Space Flight Center, Greenbelt, Maryland. South Korea and Though large-scale spiral arms dominate the France are the international partners in the mission. GALEX image of the Andromeda Galaxy, it is the fine resolution and sensitivity of the UV data that For more GALEX images and information, visit make it especially scientifically valuable. GALEX http://www.galex.caltech.edu. resolves small associations of young, massive stars (and even luminous individual stars) over the entire

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