|Right Ascension||23 : 23.4 (h:m)
|Declination||+58 : 50 (deg:m)
|Visual brightness|| 6? (mag)
|Apparent Dimension||5 (arc min)
This is the youngest known supernova remnant in our Milky Way Galaxy, and the strongest extrasolar radio source in the sky. Calculating its expansion back, astronomers have found that the supernova must have blown up around the year 1667. Strangely, it was not widely noticed by that epoch's astronomers. However, as astronomy historian William Ashworth found out in 1980, it was perhaps observed by John Flamsteed on August 16, 1680, who cataloged a star near its position as "3 Cassiopeiae". However, he did not recognize it as a supernova, or "New Star", or anything particular else, and simply cataloged it as ordinary star. As this star was not noticed elsewhere, it cannot have been much brighter than 6th magnitude.
Later astronomers didn't find any sufficiently bright star near Flamsteed's position, and classified his catalog entry as erroneous.
As the supernova was rather close, its observed maximum brightness was extremely faint for a supernova, only about 250,000 solar luminosities, fainter than the brighter "normal" stars. This indicates that it was heavily obscured by interstellar matter.
The supernova remnant was found among the earliest discrete radio sources, in 1947 by radio astronomers from Cambridge, England, and is the strongest radio source in the sky beyond the solar system. This radio source was first named Cassiopeia A and later cataloged 3C 461. Its optical counterpart coudn't be found until a more precise position was obtained, by radio interferometry in 1950. Consequently, David Dewhirst of Cambridge obtained first deep optical photos of this region in the sky and discovered a strange faint nebula, which was then investigated by Walter Baade and Rudolph Minkowski with the then-new Palomar 5-meter telescope (Baade & Minkowski, 1954). Spectroscopic observations soon confirmed its nature as the rapidly expanding shell of a supernova remnant, also cataloged as G111.7-2.1. Within two years, American astronomers were able to determine its angular expansion rate, and calculated back that the expansion must have started around the year AD 1667, as mentioned above. Nevertheless, until Ashworth' publication of 1980, it was thought that the supernova had not been observed because of heavy obstruction. Even now there are some doubts, as the position of "3 Cassipeiae" does not exactly co-incide with that of Cassiopeia A, and some historians think Flamsteed may simply have cataloged an erroneous position of another star.
Cassiopeia A was the first target to be photographed by Nasa's new Chandra X-ray Observatory (CXO), also known as AXAF; this is also our image here (see NASA Press Release 99-206). It was always one of the preferred targets of X-ray astronomers, in particular with the Chandra orbital observatory.