Cassiopeia A

Cassiopeia A (Cas A) (listenⓘ) is a <a href="/facts/Supernova_remnant/tYboe2n4">supernova remnant</a> (SNR) in the constellation <a href="/facts/Cassiopeia_(constellation)/1SvXVyPT">Cassiopeia</a> and the brightest extrasolar <a href="/facts/Astronomical_radio_source/FEJFMcI8">radio source</a> in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 <a href="/facts/Light-year/IGc5NdQb">light-years</a> (3.4 <a href="/facts/Parsec/SwSsk57J">kpc</a>) away within the <a href="/facts/Milky_Way/ulF6lccN">Milky Way</a>; given the width of the <a href="/facts/Orion_Arm/Vo4Clzk5">Orion Arm</a>, it lies in the next-nearest arm outwards, the <a href="/facts/Perseus_Arm/lm1TFb0z">Perseus Arm</a>, about 30 degrees from the <a href="/facts/Galactic_anticenter/KyLz7FUx">Galactic anticenter</a>. The expanding cloud of material left over from the <a href="/facts/Supernova/Pfo2J23l">supernova</a> 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.
It is estimated that light from the supernova itself first reached <a href="/facts/Earth/HLLmDfj0">Earth</a> near the 1660s, although there are no definitively corresponding records from then. Cas A is <a href="/facts/Circumpolar_constellation/6WNxzu8u">circumpolar</a> at and above mid-Northern latitudes which had extensive records and basic telescopes. Its likely omission in records is probably due to <a href="/facts/Interstellar_dust/Uy6hlL29">interstellar dust</a> absorbing optical wavelength radiation before it reached Earth, although it is possible that it was recorded as a sixth magnitude star 3 Cassiopeiae by <a href="/facts/John_Flamsteed/hptRt6IK">John Flamsteed</a>. 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 <a href="/facts/Martin_Ryle/bjgMtM4k">Martin Ryle</a> and <a href="/facts/Francis_Graham-Smith/FofyFC49">Francis Graham-Smith</a>, astronomers at <a href="/facts/University_of_Cambridge/0la7aX9o">Cambridge</a>, based on observations with the <a href="/facts/Long_Michelson_Interferometer/WI3XSACM">Long Michelson Interferometer</a>. The optical component was first identified in 1950.

Cassiopeia A open-in-new

Cassiopeia A