Cassiopeia A (Cas A; ) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies below 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 (9.25 in) with filters.

It is estimated that light from the supernova itself first reached Earth near the 1660s (±30 years) The optical component was first identified in 1950. or that a transient was recorded. Caroline Herschel noted that a star in the vicinity of τ Cas, HD 220562, fitted well with 3 Cas if a common error in sextant readings was made. Alternatively, the star AR Cassiopeiae could have been observed, again with the position recorded incorrectly. The position and timing mean that it possibly was 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, even with telescopes. The possible observation of the supernova resulting in the Cassiopeia A remnant would make the only exception. 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. 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. 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 (SNR G111.7-02.1) 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.

Supernova reflected echo

In 2005 an infrared echo of the Cassiopeia A explosion was observed on nearby gas clouds using Spitzer Space Telescope. 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. 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. In 2011 a study used spectra from different positions of the light echo to confirm that the Cassiopeia A supernova was asymmetric.

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.

See also

  • List of supernova remnants
  • Light echo

References