(W78, Sharpless 103)
(supernova remnant in Cygnus)
The Cygnus Loop is a loop-shaped radio source (emission nebula)
interpreted as a supernova remnant, the loop interpreted as material
ejected from the supernova (SN) which has traveled considerable distance.
It spans considerable angular size, 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 SN depends on the determination
of the distance, which is challenging and has varied: a recent
Gaia determination is 735 pc which suggests the SN
occurred 21,000 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.
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