Cygnus Loop Nebula

Until 1999, the most often-quoted distance to the supernova remnant was a 1958 estimate made by R. Minkowski, combining his radial velocity measurements with E. Hubble’s proper motion study of the remnant’s optical filaments to calculate a distance of 770 parsecs or 2500 light-years. However, in 1999, William Blair, assuming that the shock wave should be expanding at the same rate in all directions, compared the angular expansion along the sides of the bubble (visible in Hubble Space Telescope images) with direct line-of-sight measurements of the radial expansion towards the Earth and concluded that the actual size of the bubble was about 40% smaller than the conventional value, leading to a distance of about 1470 ly.

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