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Trifid nebula. Optical image of the Trifid nebula (M20, NGC 6514). North is at top. This is an emission nebula, a large cloud of gas and dust that glows as the hydrogen gas it contains is ionised by the radiation from hot young stars within it. It lies in the constellation Sagittarius, and is between 2000 and 9000 light years away. The stars seen at centre were born 100,000 years ago. Photographed by the Gemini North Telescope, Mauna Kea, Hawaii, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/NATIONAL OPTICAL ASTRONOMY OBSERVATORIES/ SCIENCE PHOTO LIBRARY)