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3C 461 3C461 3C461 is the name in the third Cambridge catalog of the radio source remnant to the most recent supernova in this galaxy's history. It is associated with the optical counterpart Cassiopeia A (aka G111.7-2.1), a faint nebula ten light years across and 3.4 kiloparsecs from Earth. It is known that the expansion shell has a temperature of 50 million degrees, and is travelling at more than ten million miles per hour. 3C461 is the strongest radio source in the sky beyond our solar system, and was among the first discrete sources to be found, in 1947. The optical component was first identified in 1950. In 1999, the Chandra X-Ray Observatory found a "hot point-like source" http://chandra.harvard.edu/fifth/casa/ close to the center of the nebula that is quite likely the neutron star or black hole predicted but not previously found. It is known that 3C461 is in fact the remnant of a powerful supernova, although it must have been a comparatively dim one considering this was a nearby event occurring within the Milky Way. Explanations for the apparent discrepancy currently lean toward the idea that the source star was unusually massive and had previously ejected much of its outer layers. Calculations working back from the currently observed expansion wave point to an explosion in 1667, although astronomer William Ashworth and others have suggested that the Astronomer Royal John Flamsteed may have inadvertently observed the supernova in 1680. At any rate, no supernova has been visible to the naked eye from Earth since.
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