Cassiopeia A (Cas A) (listen) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way;[2][3] given the width of the Orion Arm, it lies in the next-nearest arm outwards, the Perseus Arm, about 30 degrees from the Galactic anticenter. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective. It has been seen in wavelengths of visible light with amateur telescopes down to 234 mm (9.25 in) with filters.[4]

Cassiopeia A

A false color image composed of data from three sources: Red is infrared data from the Spitzer Space Telescope, gold is visible data from the Hubble Space Telescope, and blue and green are data from the Chandra X-ray Observatory. The small, bright, baby-blue dot just off-center is the remnant of the star's core.
Event type	Supernova remnant, astronomical radio source
IIb[1]
Date	1947
Constellation	Cassiopeia
Right ascension	23h 23m 24s
Declination	+58° 48.9′
Epoch	J2000
Galactic coordinates	111.734745°, −02.129570°
Distance	11,000 ly (3.4 kpc)[2]
Remnant	Shell
Host	Milky Way
Progenitor	unknown
Progenitor type	unknown
Colour (B-V)	unknown
Notable features	Strongest radio source beyond our solar system
Peak apparent magnitude	6?
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
Preceded by	SN 1604
Followed by	G1.9+0.3 (unobserved, c. 1868), SN 1885A (next observed)
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It is estimated that light from the supernova itself first reached Earth near the 1690s, although there are no definitively corresponding records from then. Cas A is circumpolar at and above mid-Northern latitudes which had extensive records and basic telescopes. Its likely omission in records is 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 16 August 1680[5]). 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 re-absorbed much of the light released as the inner star collapsed.

Cas A was among the first discrete astronomical radio sources found. Its discovery was reported in 1948 by Martin Ryle and Francis Graham-Smith, astronomers at Cambridge, based on observations with the Long Michelson Interferometer.[6] The optical component was first identified in 1950.[7]

Earlier discovery

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 16 August 1680, when he catalogued a star near its position. Another suggestion from recent cross-disciplinary research is that the supernova was the "noon day star", observed in 1630, that was thought to have heralded the birth of Charles II, the future monarch of Great Britain.[8] At any rate, no supernova occurring within the Milky Way has been visible to the naked eye from Earth since.

Expansion

The expansion shell has a temperature of around 30 million K, and is expanding at 4000−6000 km/s.[2]

Observations of the exploded star through the Hubble Space 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.[2] 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.[3]

Radio source

Cas A had a flux density of 2720 ± 50 Jy at 1 GHz in 1980.[9] Because the supernova remnant is cooling, its flux density is decreasing. At 1 GHz, its flux density is decreasing at a rate of 0.97 ± 0.04 percent per year.[9] This decrease means that, at frequencies below 1 GHz, Cas A is now less intense than Cygnus A. Cas A is still the brightest extrasolar radio source in the sky at frequencies above 1 GHz.

X-ray source

Although Cas X-1 (or Cas XR-1), the apparent first X-ray source in the constellation Cassiopeia was not detected during the 16 June 1964, Aerobee sounding rocket flight, it was considered as a possible source.[10] Cas A was scanned during another Aerobee rocket flight of 1 October 1964, but no significant X-ray flux above background was associated with the position.[11] Cas XR-1 was discovered by an Aerobee rocket flight on 25 April 1965,[12] at RA 23^h 21^m Dec +58° 30′.[13] Cas X-1 is Cas A, a Type II SNR at RA 23^h 18^m Dec +58° 30′.[14] 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.

In 1999, the Chandra X-Ray Observatory found CXOU J232327.8+584842,[15] a "hot point-like source" close to the center of the nebula that is the neutron star remnant left by the explosion.[16]

Supernova reflected echo

In 2005 an infrared echo of the Cassiopeia A explosion was observed on nearby gas clouds using Spitzer Space Telescope.[17] The infrared echo was also seen by IRAS and studied with the Infrared Spectrograph. Previously it was suspected that a flare in 1950 from a central pulsar could be responsible for the infrared echo. With the new data it was concluded that this is unlikely the case and that the infrared echo was caused by thermal emission by dust, which was heated by the radiative output of the supernova during the shock breakout.[18] The infrared echo is accompanied by a scattered light echo. The recorded spectrum of the optical light echo 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 light echo of a supernova whose explosion had not been directly observed which opens up the possibility of studying and reconstructing past astronomical events.[1][7] In 2011 a study used spectra from different positions of the light echo to confirm that the Cassiopeia A supernova was asymmetric.[19]

Phosphorus detection

In 2013, astronomers detected phosphorus in Cassiopeia A, which confirmed that this element is produced in supernovae through supernova nucleosynthesis. The phosphorus-to-iron ratio in material from the supernova remnant could be up to 100 times higher than in the Milky Way in general.[20]

Gallery

• 
  Cassiopeia A seen by the 24-inch Ritchey-Chrétien reflector at the Mount Lemmon Observatory
• 
  Cassiopeia A observed by the Hubble Space Telescope
• 
  x-ray image of Cassiopeia A taken with the Einstein Observatory
• 
  Cassiopeia A was the first light image of the Chandra X-ray Observatory
• 
  Cassiopeia A seen by Spitzer and showing different chemical elements and dust within the supernova remnant

See also

• List of supernova remnants
• Light echo

References

External links

Wikimedia Commons has media related to Cassiopeia A.

• Arcand, Kimberly K.; Jiang, Elaine; Price, Sara; Watzke, Megan; Sgouros, Tom; Edmonds, Peter (15 December 2018). "Walking through an exploded star: Rendering supernova remnant Cassiopeia A into virtual reality". Communicating Astronomy with the Public Journal. 24: 17. arXiv:1812.06237. Bibcode:2018CAPJ...24...17A.
• "3D visualisation of Cassiopeia A". BBC News.
• "Virtual reality and augmented reality model viewer to explore a 3D hydrodynamic model of supernova remnant Cassiopeia A". Sketchfab; INAF-Osservatorio Astronomico di Palermo (S. Orlando).

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