Astronomy Picture of the Day

Explanation: What shines in the gamma-ray sky? This simulated image models the intensities of gamma rays with over 40 million times the energy of visible light, and represents how the sky might appear to the Gamma-ray Large Area Space Telescope (GLAST) after its first year in orbit. Familiar steady stars are absent from the dramatic 80x80 degree field which looks directly away from the centre of the Galaxy. Instead, the Geminga and Crab pulsars - bizarre, spinning stellar corpses known to be neutron stars - are the two brightest gamma-ray sources. These and other gamma-ray bright objects in the field, monstrous active galaxies and still unknown sources, have been detected by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the orbiting Compton Gamma-Ray Observatory. However, most of the simulated sources are new - extrapolating current ideas and anticipating discoveries resulting from GLAST's improved gamma-ray vision. The central broad band of faint gamma-ray emission is due to high-energy cosmic rays colliding with interstellar gas in the outer spiral arms of the Milky Way, while below is a diffuse energetic glow from prominent molecular clouds in Monoceros, Orion, Auriga, and Taurus. Intended to explore extreme environments in the distant cosmos and planned for launch in 2005, GLAST is under development by NASA, U.S., and international partners.