Cassiopeia A
Cassiopeia A is a supernova remnant in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately away within the Milky Way; 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 across from Earth's perspective. It has been seen in wavelengths of visible light with amateur telescopes down to 234 mm with filters.
It is estimated that light from the supernova itself first reached Earth near the 1660s, 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 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 by Martin Ryle and Francis Graham-Smith, astronomers at Cambridge, based on observations with the Long Michelson Interferometer. The optical component was first identified in 1950.
Possible observations
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, 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 in 2 months after that. Possible alternative explanations include an error in the position, or that a transient was recorded. Caroline Herschel noted that a star in the vicinity of τ Cas, HD 220562, fit well with 3 Cas if a common error in sextant readings was made. Alternatively, the star AR 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. 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.However, it is more probable that the "noon day star" was the planet Venus 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 the Milky Way has been visible to the naked eye from Earth since Kepler'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 remnant G1.9+0.3 reached Earth more recently than the first light from Cassiopeia A, but the associated supernova was not observed.
Expansion
The expansion shell has a temperature of around 30 million K, and is expanding at 4,000−6,000 km/s.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. 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.
Radio source
Cas A had a flux density of at 1 GHz in 1980. Because the supernova remnant is cooling, its flux density is decreasing. At 1 GHz, its flux density is decreasing at a rate of per year. 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, 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. 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. Cas XR-1 was discovered by an Aerobee rocket flight on 25 April 1965, at RA Dec. Cas X-1 is Cas A, a Type II SNR at RA Dec.The designations Cassiopeia X-1, Cas XR-1, Cas X-1 are no longer used, but the X-ray source is Cas A at 2U 2321+58.
In 1999, the Chandra X-Ray Observatory found CXOU J232327.8+584842, a central compact object that is the neutron star remnant left by the explosion.