First-light image from the Swift X-ray Telescope, of the Cassiopeia A supernova remnant. This object is the remnant of a gigantic stellar explosion that occurred in about 1680. The explosion heated the surrounding gas and the remnants of the star to temperatures of several million degrees Celsius. The hot gas has been expanding and cooling for the past 325 years. This image is a true X-ray color image: the lowest X-ray energies are shown in red, the medium energies are in green, and the highest energies are in blue.

Photo from NASA's Swift Observatory

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