Cygnus Loop

The Cygnus Loop is a loop-shaped radio source (emission nebula) interpreted as a supernova remnant, specifically as material ejected from a past supernova (SN) which has since traveled considerable distance. It spans considerable angular size (an angular diameter of around 3 degrees), and portions of it are visible, collectively known as the Veil Nebula, which includes visible nebulae NGC 6960, NGC 6992, NGC 6995, and IC 1340. Determination of the date of the presumed SN depends on the determination of the distance to the loop, which is challenging and determinations have varied: a recent Gaia determination is 725 pc which suggests the SN occurred 20,000 years ago, but some determinations have been a fraction of that. There have been searches for an associated neutron star (such as was found in the Crab Nebula) but none has been identified.