Cassiopeia A

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Cassiopeia A
Cassiopeia A Spitzer Crop.jpg
Observational Data
Designation Cas A
Right ascension 23h 23m 26s[1]
Declination +58° 48′[1]
Constellation Cassiopeia
Type of object Supernova remnant
Dimensions 3 arcminutes[1]
Distance from Earth 11,000 ly[1]

Cassiopeia A (Cas A) is a supernova remnant in the constellation of Cassiopeia.[1] 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.[1] The nebula also houses a radio source, designated Cas A and first observed in 1947.[1] 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.[1] 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.[1] 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.[2] The star is also cooling very quickly, decreasing by 4% over the ten years it has been observed by the Chandra Space Telescope.[3] 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.[1]

The nebula is a very bright radio source, the most luminous visible from Earth for frequencies above 1 GHz. Below this, the radio source Cygnus A is brighter.

The nebula has been modelled in 3D and is the only supernova remnant for which this has been done.[1][4] 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.[1]