This object is a huge faint
supernova remnant (SNR) located 3000 light years away in the constellations of Auriga and Taurus. The remnant has a span of 150 light years and an apparent diameter of almost 3 degrees. This image is a 4 panel mosaic.
The image, in the HOO palette, captures HII filaments in red and OIII filaments in cyan, with hydrogen the more dominant gas. Many of the other SNRs I have imaged have a more equal balance of hydrogen and oxygen. The IC 443 nebula (Jellyfish Nebula) is one SNR which has a similar HII dominance to SH2-240. Another similarity to the smaller IC 443 nebula is that the spherical nature of the SNR is still mostly intact in both nebula.
Numerous irregular dark clouds somewhat obscure the nebula, particularly at the lower center and in the upper right quadrant. Some extended lobes are seen, which could be due to various factors such as non-uniform density of the interstellar medium, or non-symmetric expulsions from the explosion. The most significant lobes are polar-opposite left and right. It is interesting that, in contrast to the rest of the nebula, the outermost extent of these left and right lobes appear to be dominated more by oxygen than by hydrogen.
A pulsar near the center of this nebula, not visible in this image, is the progenitor star. The bright red HII region to the lower left is Sh2-242, unrelated to the Sh2-240 SNR.
This object is also known as Simeis 147 (S147). This identifier was established in a paper by Gaze and Shajn in a Crimean journal in the 1950's. The name Simeis originates with an older astronomy facility operated by the Crimean Astrophysical Observatory in the city of Simeiz. The only other popular Simeis object is Simeis 57 (The Propeller, sometimes referred to as DWB 111).
My
previous image of this object, using my Tak 85, was one of my longest integration times at 20 hours, compared to this one at 7 hours.
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