... M31: The Andromeda Galaxy . ... Andromeda is the nearest major galaxy to our own Milky Way Galaxy. Our Galaxy is thought to look much like Andromeda. ... The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. ROSAT Explores The X-Ray Sky . ... Caught in the act earlier this summer by the Hubble Space Telescope, the volcanic moon Io (above and right of center) and its shadow (black dot) are seen here against Jupiter's clouds. ...