NGC 6960 is a part of the well known Cygnus Loop, the beautiful supernova remnant. In 2018 I started a project to look for effects of expansion in my first narrowband images. My OIII-data (53 x 120 s) compared with the POSS1 and POSS2 exposures using blue-sensitive photographic plates shows significant changes:
A closer look reveals that there are two different effects:
On the one hand, the gas systematically shifts to the outside, on the other hand the brightness distribution changes.
Even before the supernova explosion, the strong stellar wind of the supernova candidate created a cavity in the edge of which the nebula matter, e.g. oxygen, was concentrated. The shock wave of the supernova now stimulates the gas to glow by impact ionisation.
The filaments thus continue to spread and at the same time are traversed by the faster shock wave. This causes new filaments to light up on the outside - i.e. in the direction of propagation - (some places are marked by arrows as an example) and they fade again later on the reverse side.
Version A - images from 1953 / 1988 / 2018
Version B - images from 1953 / 1988 / 2018 with superimposed intermediate images in steps of 25%
image credit:
ESO and Digitized Sky Survey 1 (1953)
ESO and Digitized Sky Survey 2 (1988)
and own work (2018)
The complete bicolor-version 2018
is here 
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