Astronomy Picture of the Day

Explanation: This giant spiral galaxy, Messier 101 (M101), was photographed by the Ultraviolet Imaging Telescope (UIT). UIT flew into orbit as part of the Astro 2 mission on-board the Space Shuttle Endeavour in March 1995. The image has been processed so that the colours (purple to white) represent an increasing intensity of ultraviolet light. Pictures of galaxies like this one show mainly clouds of gas containing newly formed stars many times more massive than the sun, which glow strongly in ultraviolet light. In contrast, visible light pictures of galaxies tend to be dominated by the yellow and red light of older stars. Ultraviolet light, invisible to the human eye, is blocked by ozone in the atmosphere so ultraviolet pictures of celestial objects must be taken from space.