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Cassiopeia A
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Cassiopeia A supernova remnant, in a false-colour composite image synthesized from observations gathered in different spectral regions by three space-based observatories. Red areas, representing infrared data from the Spitzer Space Telescope, highlight warm dust in the remnant's outer shell. Yellow areas, representing visible-light data collected by the Hubble Space Telescope, show delicate filamentary structures made of warmer gases. Green and blue areas are X-ray data from the Chandra X-ray Observatory and reveal hot gas that was created when ejected material from the supernova collided at very high velocity with the surrounding gas and dust.
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Cassiopeia A supernova remnant, in a false-colour composite image synthesized from observations gathered in different spectral regions by three space-based observatories. Red areas, representing infrared data from the Spitzer Space Telescope, highlight warm dust in the remnant's outer shell. Yellow areas, representing visible-light data collected by the Hubble Space Telescope, show delicate filamentary structures made of warmer gases. Green and blue areas are X-ray data from the Chandra X-ray Observatory and reveal hot gas that was created when ejected material from the supernova collided at very high velocity with the surrounding gas and dust.
Cassiopeia A supernova remnant, in a false-colour composite image synthesized from observations gathered in different spectral regions by three space-based observatories. Red areas, representing infrared data from the Spitzer Space Telescope, highlight warm dust in the remnant's outer shell. Yellow areas, representing visible-light data collected by the Hubble Space Telescope, show delicate filamentary structures made of warmer gases. Green and blue areas are X-ray data from the Chandra X-ray Observatory and reveal hot gas that was created when ejected material from the supernova collided at very high velocity with the surrounding gas and dust.

Cassiopeia A

astronomy
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Written by
John Donald Fernie
Professor Emeritus, Department of Astronomy and Astrophysics, University of Toronto, Ontario. Author of Setting Sail for the Universe: Astronomers and their Discoveries.
John Donald Fernie
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Cassiopeia A, strongest source of radio emission in the sky beyond the solar system, located in the direction of the constellation Cassiopeia about 11,000 light-years from Earth. Cassiopeia A, abbreviated Cas A, is the remnant of a supernova explosion caused by the collapse of a massive star. The light from the event is estimated to have reached Earth between 1662 and 1700. Although the explosion must have been very powerful, no contemporary record—aside from a possible sighting by English astronomer John Flamsteed in 1680—exists of its having been observed, so the explosion may have happened behind an interstellar dust cloud. Today the remnant is also weakly observable at visible, infrared, and X-ray wavelengths, and it appears as an expanding ring of material approximately five arc minutes in diameter. The expansion rate of the remnant has been used to estimate how long ago the explosion occurred. At the centre of the remnant is a neutron star, which was the first detected to have a carbon atmosphere.

John Donald Fernie