Sharpless 308

Canis Major
RA 06 54 13
DEC -23 55 42

Another Sharpless, but not a true HII region, but a misclassification. Sharpless 308 is a windblown bubble around EZ CMa (WR 6), a Wolf-Rayet star, and hence a Wolf-Rayet ring nebula. It is a shock front formed by the fast and intense Wolf-Rayet wind of WR 6 that hits into the slower wind ejected by the star during its previous asymptotic giant branch (AGB) phase. It is very strong in OIII and much fainter in H alpha. Very recent results showed that this is the only WR bubble besides the Crescent Nebula that shows as well X ray emission.

At my latitude (48°N), Sharpless 308 is always very low and hence an object only suitable for nights with excellent transparency. Those at lower latitude will generally have much better observing conditions with this object.

Sh308.jpg
DSS color composite

Sh308inv.jpg
DSS blue inverted

There are excellent images by Don Goldman and Marco Lorenzi.

My observing notes:
Starting point for the star hop to Sharpless 308 is o1 CMa. This star is embedded in the open cluster Collinder 121, which is unspectacular even in binoculars and does not really stand out as a cluster. Shifting o1 CMa into the middle of the field of view of the finder eyepiece, you will be surprised by the view when you switch to OIII filtering (most spectacular, of course, with a filter slide or wheel). The OIII filter shows you a ghostly crescent of glowing gas, that extends from o1 CMa first towards NW and then towards N as part of a huge bubble of 40' diameter. This bubble is not related to o1 CMa itself, but is the blown of outer shell of EZ CMA, the Wolf Rayet star WR 6, about 20' N of o1 CMa, in the middle of the bubble.

With my 14" Dob, the western crescent is the brightest part and can be easily followed with OIII filter. Towards NW, the crescent becomes fainter, which corresponds to a sort of break-out structure in the bubble.

With my 22" Dob, I can follow the shell well beyond this area. In the NE part, there is another condensation where the shell becomes brighter before it slowly fades out in the eastern part.

Under excellent conditions with perfect transparency down to the horizon, I could see the entire shell, including the eastern part, that closes the circle towards o1 CMa. With 7mm exit pupil, the shell appeared filled with an extremely faint OIII glow, which disappeared at smaller exit pupils.


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GOOD LUCK AND GREAT VIEWING!