Astronomy Picture of the Day

Explanation: The shockwave from a 20,000 year-old supernova in the constellation of Cygnus supernova explosion is still expanding into interstellar space. The collision of this fast moving wall of gas with a stationary cloud has heated it causing it to glow in visible as well as high energy radiation, producing the nebula known as the Cygnus Loop (NGC 6960/95). The nebula is located about 2500 light-years away. The colours used here indicate emission from different kinds of atoms excited by the shock: oxygen-blue, sulphur-red, and hydrogen-green. This picture was taken with the Wide Field and Planetary Camera 2 on board the Hubble Space Telescope.