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Cassiopeia A (Cas A) is a supernova remnant in the constellation of Cassiopeia. The supernova that caused it is thought to have occurred around 300 years ago, though no definite recording of a supernova was made at that time. One possibility of it may have been made by John Flamsteed on August 16, 1680. Flamsteed observed a star with an apparent magnitude of +6 near the nebula but Flamsteed's position for the star does not correspond to any known star today. The nebula also houses a radio source, designated Cas A and first observed in 1947. For frequencies above 1 GHz, Cassiopeia A is the brightest radio source in the night sky.
Properties and Structure
At around 11,000 light years away, the nebula's apparent size of 5 arcminutes corresponds to a physical diameter of around 10 light years. The nebula's outer shell is expanding at roughly 4,000-6,000 km/s and is believed to have a temperature of 27.7 million Kelvin. The expansion rate suggests it cannot be more than a couple of hundred years old, though no record of a supernova exists at that time. This may be because the star ejected most of its outer layers or perhaps it was hidden behind a cloud of interstellar gas and dust.
At the centre of the nebula is a neutron star. It was the first neutron star to observed to possess an atmosphere containing carbon. The star is also cooling very quickly, decreasing by 4% over the ten years it has been observed by the Chandra Space Telescope. It is believed this indicates the core of the star is a superfluid. In 2013 the element phosphorus was also detected in the nebula as a whole in concentrations more than 100 times greater than the average for the Milky Way.
The nebula has been modelled in 3D and is the only supernova remnant for which this has been done. This model was generated using data from NASA's Chandra Space Telescope and its Spitzer Space Telescope. The image on the right is a false colour image generated from the observations of Spitzer (in infrared light) and Hubble (in visible light). From this it was discovered that there are perhaps 6 or so empty chambers within the nebula. These are large, two of them being 3 and 6 light years across.
- Cassiopeia A from constellation-guide.com
- Cassiopeia A from britannica.com
- Cassiopeia A: NASA'S Chandra Finds Superfluid in Neutron Star's Core from chandra.harvard.edu
- Exploring the Third Dimension of Cassiopeia A from nasa.gov