The brightest radio source in our heavens, other than the sun (which appears bright because if ¡s so close to us), is a supernova remnant in the constellation Cassiopeia, First efforts to identify an optical object at its location were rather disappointing, Nothing as blazingly obvious as the Crab Nebula was found. Instead, very faint filamentary material is seen on photographs, the only visible remains of an exploded star. The presence of dust between the sun and this remnant, which is located 10,000 light-years away, may be preventing us from seeing the object. Radio waves ignore such dust and have revealed Cas A to be a glorious display of radio emitting filaments. The radiograph of Cas A shown in Figure 4.2 is one of the most stunning radiographs ever made. The image appears ring-like, suggesting a shell of material ejected by an explosion. Observations of motion in the filaments indicate that the explosion must have occurred in 1680 AD, but no record exists of anyone having seen it back then.
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