Cassiopeia A

Cassiopeia A is a young supernova remnant about 11,000 light-years away. Its expanding debris comes from a stellar explosion whose light likely reached Earth around 1680.

Supernova remnant: An expanding structure of shocked gas and stellar debris left by a supernova. Cassiopeia A is a nearby, young example of this class of object.
Cassiopeia A light echo: Scattered light from Cassiopeia A’s explosion, reflected by interstellar dust and observed centuries later. Its spectrum provides clues to the original explosion despite the lack of a clear historical sighting.
John Flamsteed: An English astronomer who compiled the first comprehensive star catalog using a telescope. A star Flamsteed recorded in 1680 near Cassiopeia A was later proposed as its visible supernova.
Nucleosynthesis: The formation of atomic nuclei through nuclear reactions in stars, explosions, and other astrophysical environments. Element-rich knots in Cassiopeia A expose products of nuclear burning and explosive synthesis.
Compact central object: A point-like X-ray source at a supernova remnant’s center, often a neutron star or black hole. Cassiopeia A’s central source lacks clear pulsations and has a distinctive thermal spectrum.
Supernova: A powerful stellar explosion that expels a star’s outer layers and can leave a compact remnant. The explosion created the debris and shock waves now observed as Cassiopeia A.
Chandra X-ray Observatory: A NASA space telescope designed to observe X-rays from high-energy astronomical sources. Its images resolve Cassiopeia A’s hot ejecta and expanding shock structure.
Supernova 1680: The presumed stellar explosion that produced Cassiopeia A, whose light may have reached Earth around 1680. Its date is inferred from the remnant’s expansion and a disputed historical star observation.
Cosmic rays: High-energy particles, mostly atomic nuclei, that travel through space at nearly the speed of light. Cassiopeia A’s shocks are studied as potential accelerators of cosmic-ray particles.
Neutron star cooling: The gradual loss of thermal energy from a neutron star through radiation and neutrino emission. Repeated observations of Cassiopeia A’s neutron star have been used to study its cooling rate.

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