Cassiopeia A
 | |
| Event type | Supernova remnant, astronomical radio source  |
|---|---|
| Date | 1947 |
| Constellation | Cassiopeia |
| Right ascension | 23h 23m 26s |
| Declination | +58° 48′ |
| Epoch | J2000 |
| Galactic coordinates | G111.7-2.1 |
| Distance | 3.4 kpc (11,000 ly) |
| Remnant | Shell |
| Host | Milky Way |
| Notable features | Brightest radio source in the sky |
| Other designations | SN 1671, SN 1667, SN 1680, SNR G111.7-02.1, 1ES 2321+58.5, 3C 461, 3C 461.0, 4C 58.40, 8C 2321+585, 1RXS J232325.4+584838, 3FHL J2323.4+5848, 2U 2321+58, 3A 2321+585, 3CR 461, 3U 2321+58, 4U 2321+58, AJG 109, CTB 110, INTREF 1108, [DGW65] 148, PBC J2323.3+5849, 2FGL J2323.4+5849, 3FGL J2323.4+5849, 2FHL J2323.4+5848 |
 Related media on Commons | |
Cassiopeia A is a supernova remnant in the constellation Cassiopeia and the brightest extra solar radio source in the sky, with a flux of 2720 Janskys at 1GHz. It is believed to be approximately 300 years old but there are no historical records of any sightings of the progenitor supernova, probably due to interstellar dust absorbing optical wavelength radiation before it reached Earth. Possible explanations lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. These outer layers would cloak the star reabsorb much of the light released as the inner star collapsed.
3C461 is the name in the third Cambridge catalog of the radio source remnant associated with this supernova remnant. The name Cassiopeia A (or Cas A) is strictly used to refer to the optical component, but is by far its more common name. It is known that the expansion shell has a temperature of around 50 million degrees, and is travelling at more than ten million miles per hour.
3C461 is the strongest radio source in the sky beyond our solar system, and was among the first discrete sources to be found, in 1947. The optical component was first identified in 1950. In 1999, the Chandra X-Ray Observatory found a "hot point-like source" [1] close to the center of the nebula that is quite likely the neutron star or black hole predicted but not previously found.
Calculations working back from the currently observed expansion wave point to an explosion in 1667, although astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova in 1680. At any rate, no supernova has been visible to the naked eye from Earth since.