|Observation data (Epoch J2000)|
|Supernova type||IIb |
|Host galaxy||Milky Way|
|Right ascension||23h 23m 26s|
|Peak magnitude (V)||6?|
|Distance||11 kly (3.4 kpc)|
|Notable features||Strongest radio source beyond our solar system|
Cassiopeia A (Cas A) is a supernova remnant in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. It had a flux density of 2720±50 Jy at 1 GHz in 1980; its flux density at 1 GHz is decreasing at a rate of 0.97±0.04 percent per year. This decrease means that at frequencies below 1 GHz Cas A is now less intense than Cyg A. The supernova occurred approximately 11,000 light-years (3.4 kpc) away in the Milky Way. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective (within the Milky Way Galaxy). Despite its radio brilliance, however, it is extremely faint optically, and is only visible on long-exposure photographs.
It is believed that first light from the stellar explosion reached Earth approximately 300 years ago 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 (although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by John Flamsteed on August 16, 1680). 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 have cloaked the star and reabsorbed much of the light released as the inner star collapsed.
X-ray source 
In 1979, Shklovsky predicted that Cas A had a black hole. In 1999, the Chandra X-Ray Observatory found a "hot point-like source" close to the center of the nebula that is quite likely the neutron star or black hole predicted but not previously found.
Although Cas X-1 (or Cas XR-1), the apparent first X-ray source in the constellation Cassiopeia was not detected during the June 16, 1964, Aerobee sounding rocket flight, it was considered as a possible source. Cas A was scanned during another Aerobee rocket flight of October 1, 1964, but no significant X-ray flux above background was associated with the position. Cas XR-1 was discovered by an Aerobee rocket flight on April 25, 1965, at RA 23h 21m Dec +58° 30′. Cas X-1 is Cas A, a Type II SNR at RA 23h 18m Dec +58° 30′.
The designations Cassiopeia X-1, Cas XR-1, Cas X-1 are no longer used, but the X-ray source is Cas A (SNR G111.7-02.1) at 2U 2321+58.
Calculations working back from the currently observed expansion point to an explosion that would have become visible on Earth around 1667. Astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova on August 16, 1680, when he catalogued a star near its position. Another suggestion from recent cross-disciplinary research is that the supernova was the mythical "noon day star", observed in 1630, that heralded the birth of Charles II, the future monarch of Great Britain. At any rate, no supernova in the Milky Way has been visible to the naked eye from Earth since. Observations of the exploded star through the Hubble telescope have shown that, despite the original belief that the remnants were expanding in a uniform manner, there are high velocity outlying eject knots moving with transverse velocities of 5,500−14,500 km/s with the highest speeds occurring in two nearly opposing jets. When the view of the expanding star uses colors to differentiate materials of different chemical compositions, it shows that similar materials often remain gathered together in the remnants of the explosion.
Observation of supernova reflected echo 
Recently, an infrared echo of the Cassiopeia A explosion was observed on nearby gas clouds using Spitzer Space Telescope. The recorded spectrum proved the supernova was of Type IIb, meaning it resulted from the internal collapse and violent explosion of a massive star, most probably a red supergiant with a helium core which had lost almost all of its hydrogen envelope. This was the first observation of the infrared echo of a supernova which explosion had not been directly observed which opens up the possibility of studying and reconstructing past astronomical events.
See also 
|Wikimedia Commons has media related to: Cassiopeia A|
- Krause, Oliver; et al. (2008). "The Cassiopeia A Supernova was of Type IIb". Science 320 (5880): 1195–1197. arXiv:0805.4557. Bibcode:2008Sci...320.1195K. doi:10.1126/science.1155788. PMID 18511684.
- Fesen, Robert A.; Hammell, Molly C.; Morse, Jon; Chevalier, Roger A.; Borkowski, Kazimierz J.; Dopita, Michael A.; Gerardy, Christopher L.; Lawrence, Stephen S. et al. (July 2006). "The Expansion Asymmetry and Age of the Cassiopeia A Supernova Remnant". The Astrophysical Journal 645 (1): 283–292. arXiv:astro-ph/0603371. Bibcode:2006ApJ...645..283F. doi:10.1086/504254
- Baars, J. W. M., Genzel, R., Pauliny-Toth, I. I. K., Witzel, A. (1977). "The Absolute Spectrum of Cas A; An Accurate Flux Density Scale and a Set of Secondary Calibrators". Astronomy and Astrophysics 61: 99. Bibcode:1977A&A....61...99B.
- Stover, Dawn (2006). "Life In A Bubble". Popular Science 269 (6): 16
- Hughes DW (1980). "Did Flamsteed see the Cassiopeia A supernova?". Nature 285 (5761): 132–133. Bibcode:1980Natur.285..132H. doi:10.1038/285132a0.
- Fabian, A. C. (2008). "Astronomy. A blast from the past". Science 320 (5880): 1167–1168. doi:10.1126/science.1158538. PMID 18511676.
- Shklovsky IS (1979). "Is Cassiopeia A a black hole?". Nature 279 (5715): 703. Bibcode:1979Natur.279..703S. doi:10.1038/279703a0. PMID 16067999.
- Pavlov GG, Zavlin VE, Aschenbach B, Trumper J, Sanwal D (2000). "The Compact Central Object in Cassiopeia A: A Neutron Star with Hot Polar Caps or a Black Hole?". Astrophysical Journal 531 (1): L53–L56. arXiv:astro-ph/9912024. Bibcode:2000ApJ...531L..53P. doi:10.1086/312521. PMID 10673413.
- Celebrating 5 Years with Chandra: Cassiopeia A
- Bowyer S, Byram ET, Chubb TA, Friedman H (1965). "Observational results of X-ray astronomy". In Steinberg JL. Astronomical Observations from Space Vehicles, Proceedings from Symposium no. 23 held in Liege, Belgium, 17 to 20 August 1964. Symposium (International Astronomical Union). no. 23: 227–39. Bibcode:1965IAUS...23..227B.
- Fisher PC, Johnson HM, Jordan WC, Meyerott AJ, Acton LW (1966). "Observations of Cosmic X-rays". Ap J. 143: 203–17. Bibcode:1966ApJ...143..203F. doi:10.1086/148491.
- Byram ET, Chubb TA, Friedman H (Apr 1966). "Cosmic X-ray Sources, Galactic and Extragalactic". Sci. 152 (3718): 66–71. Bibcode:1966Sci...152...66B. doi:10.1126/science.152.3718.66. PMID 17830233.
- Friedman H, Byram ET, Chubb TA (April 1967). "Distribution and Variability of Cosmic X-Ray Sources". Sci. 156 (3773): 374–8. Bibcode:1967Sci...156..374F. doi:10.1126/science.156.3773.374. PMID 17812381.
- Webber WR (December 1968). "X-ray astronomy-1968 vintage". Proc Astron Soc Australia.: 160–4. Bibcode:1968PASAu...1..160W.
- Oullette, Jennifer. "Did Supernova Herald the Birth of a King?". Discovery.com. Retrieved 18 April 2011.
- Fabian, Andrew C. (2008). "A Blast from the Past". Science 320 (5880): 1167–1168. doi:10.1126/science.1158538. PMID 18511676.