The Cygnus Loop
The Cygnus Loop (also known as the Veil Nebula) is a supernova remnant, the detritus of the explosive death of a massive star about ten to twenty thousand years ago. Detailed modeling of its spectacular filamentary shape suggests that the explosion occurred inside an interstellar cavity created by the progenitor star. As is common in astronomy, many of the precise physical properties of the object are rendered uncertain by the uncertainty of its distance. For decades scientists used a value of about 2500 light-years based on analyses of its gas motions by Hubble in 1937 and Minkowski in 1958. Many recent distance estimates have varied over a wide range generally consistent with this one, but the most cited value is a 2005 measurement of between 1500 and 2100 light-years.