Galaxy, any of the systems of stars and interstellar matter that make up the universe. Many such assemblages are so enormous that they contain hundreds of billions of stars. Virtually all galaxies appear to have been formed soon after the universe began, and they pervade all space that is viewable by modern telescopes.
One of greatest discoveries of the 20th Century was that there are galaxies beyond our own, and astronomer Edwin Hubble was a key figure in this revelation.
Gamma rays are high-energy photons produced by some of the most violent events in the universe. We explore this powerful radiation in more detail here.
Meet 20 great astronomers who made the history books astronomy.com - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from astronomy.com Daily Mail and Mail on Sunday newspapers.