Astronomy Picture of the Day . ... Abell 2142: Clash of the Galaxy Clusters . ... Above is an X-ray image of Abell 2142, the result of the collision of two huge clusters of galaxies, and one of the most massive objects known in the universe. The Pipe Dark Nebula . ... The dark clouds, suggestively shaped like smoke rising from a pipe, are caused by absorption of background starlight by dust. ... NGC 1999: Reflection Nebula In Orion . ... Even as leap days go, today is a remarkable one. ...