 A false color image composed of data from three sources: Red is infrared data from theSpitzer Space Telescope, gold is visible data from theHubble Space Telescope, and blue and green are data from theChandra X-ray Observatory. The small, bright, baby-blue dot just off-center is the remnant of the star's core. | |
| Event type | Supernova |
|---|---|
| IIb[1] | |
| Date | 1947 byMartin Ryle andFrancis Graham-Smith) |
| Constellation | Cassiopeia |
| Right ascension | 23h 23m 26.0s[2] |
| Declination | +58° 48′ 41″[2] |
| Epoch | J2000 |
| Galactic coordinates | 111.734745°, −02.129570° |
| Distance | c. 11,000ly[3] |
| Remnant | Shell |
| Host | Milky Way |
| Notable features | Strongest radio source beyond theSolar System |
| Peakapparent magnitude | c. 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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Cassiopeia A (Cas A;listenⓘ) is asupernova remnant (SNR) in the constellationCassiopeia and the brightest extrasolarradio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000light-years (3.4 kpc) away within theMilky Way;[3][4] given the width of theOrion Arm, it lies in the next-nearest arm outwards, thePerseus Arm, about 30 degrees from theGalactic anticenter. The expanding cloud of material left over from thesupernova 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.[5]
It is estimated that light from the supernova itself first reachedEarth near the 1660s (±30 years)[3], although there are no definitively corresponding records from then. Cas A iscircumpolar at and above mid-Northern latitudes which had extensive records and basic telescopes. Its likely omission in records is probably due tointerstellar dust absorbing optical wavelength radiation before it reached Earth, although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae byJohn Flamsteed in 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 absorbed much of the visible-light emission as the inner star collapsed.
Cas A was among the first discrete astronomical radio sources found. Its discovery was reported in 1948 byMartin Ryle andFrancis Graham-Smith, astronomers atCambridge, based on observations with theLong Michelson Interferometer.[6] The optical component was first identified in 1950.[7]
Calculations working back from the currently observed expansion point to an explosion that would have become visible on Earth around 1667. AstronomerWilliam Ashworth and others have suggested that theAstronomer RoyalJohn Flamsteed may have inadvertently observed the supernova on 16 August [O.S. 6 August] 1680, when he catalogued a sixth-magnitude star 3 Cassiopeiae, but there is no corresponding star at the recorded position. It is estimated that the supernova should have reached a magnitude of 3.2 at its maximum and decayed to the 6th magnitude (as observed by Flamsteed) in 2 months after that.[1] Possible alternative explanations include an error in the position,[8] or that a transient was recorded.Caroline Herschel noted that a star in the vicinity ofτ Cas, HD 220562, fit well with 3 Cas if a common error in sextant readings was made.[9] Alternatively, the starAR Cassiopeiae may have been observed, again with the position recorded incorrectly. The position and timing mean that it may have been an observation of the Cassiopeia A progenitor supernova.[10] 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 ofCharles II, the future monarch of Great Britain.[11]
However, it is more probable that the "noon day star" was the planetVenus that reached its maximum morning brightness two days earlier, allowing day time visibility in a clear sky. A bright supernova in Cassiopeia would have been visible for months and there would be more observation records as Cassiopeia is visible above the horizon any night in Europe.
No supernova occurring within theMilky Way has been visible to the naked eye from Earth sinceKepler's Supernova of 1604. Apart from the possible observation of the supernova resulting in the Cassiopeia A remnant, no supernova has been observed in our Galaxy since 1604, even with telescopes. First light from the supernova remnantG1.9+0.3 reached Earth more recently than the first light from Cassiopeia A, but the associated supernova was not observed.
The expansion shell has a temperature of around 30 millionK, and is expanding at 4,000−6,000 km/s.[3]
Observations of the exploded star through theHubble 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.[3] 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.[4]
Cas A had a flux density of2720 ± 50Jy at 1 GHz in 1980.[12] Because the supernova remnant is cooling, its flux density is decreasing. At 1 GHz, its flux density is decreasing at a rate of0.97 ± 0.04 percent per year.[12] This decrease means that, at frequencies below 1 GHz, Cas A is now less intense thanCygnus A. Cas A is still the brightest extrasolarradio source in the sky at frequencies above 1 GHz.
Although Cas X-1 (or Cas XR-1), the apparent first X-ray source in theconstellationCassiopeia was not detected during the 16 June 1964,Aerobeesounding rocket flight, it was considered as a possible source.[13] 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.[14] Cas XR-1 was discovered by an Aerobee rocket flight on 25 April 1965,[15] atRA23h 21mDec +58° 30′.[16] Cas X-1 is Cas A, a Type II SNR atRA23h 18mDec +58° 30′.[17]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, theChandra X-Ray Observatory foundCXOU J232327.8+584842,[18] acentral compact object that is theneutron star remnant left by the explosion.[19]
In 2005 an infrared echo of the Cassiopeia A explosion was observed on nearby gas clouds usingSpitzer Space Telescope.[20] The infrared echo was also seen byIRAS and studied with theInfrared Spectrograph. Previously it was suspected that a flare in 1950 from a centralpulsar 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.[21] The infrared echo is accompanied by a scatteredlight echo. The recorded spectrum of the optical light echo proved the supernova was ofType IIb, meaning it resulted from the internal collapse and violent explosion of a massivestar, most probably ared 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 wasasymmetric.[22]
In 2013, astronomers detectedphosphorus in Cassiopeia A, which confirmed that this element is produced in supernovae throughsupernova 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.[23]
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