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Thread: Object of the Week January 20, 2013 - A Wolf-Rayet ring nebula - Sharpless 308

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  1. #1
    Member reiner's Avatar
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    Object of the Week January 20, 2013 - A Wolf-Rayet ring nebula - Sharpless 308

    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.


    "GIVE IT A GO AND LET US KNOW"

    GOOD LUCK AND GREAT VIEWING!
    Reiner

    22" and 14" Dobs on EQ platforms and Deep Sky Observing
    www.reinervogel.net

  2. #2
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    Cool Ring Reiner,

    I used the souther latitude of La Palma (29°N) on the top of the Roque de los Muchachos (2420m/7940ft) to observe and sketch this WR-ring.

    With my 14,5" and magnification between 54x and 83x (AP 6,9mm-4,5mm) I could see the heavily structured W side with twisted edges. The nebula of the ring could easily track the whole circle with a brighter part in the N. Within the ring I could not observe nebula.

    Sh2-308.jpg
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  3. #3
    Member Steve Gottlieb's Avatar
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    Great choice, Reiner -- this is one of my favorite, relatively obscure nebulae and on the short list of Wolf-Rayet shells. Here is my last observation of this object --

    18" (2/4/08): remarkable view of this giant Wolf-Rayet shell at 73x and OIII filter (excellent filter response). Virtually the complete outline could be traced for at least 35' diameter. The shell is striking along the 25' western border as it gently arcs from north to south with a sharp contrast against the sky immediately to the west. This region also has some weak filamentary structure and brightness variations. At the south end of this long arc, the rim curves more sharply east, crossing directly through a triangle of mag 7.5/8/9 stars and dimming as it passes just south of 3.9-magnitude 16 CMa. To the east of 16 CMa, the SE portion of the shell is the only difficult part to follow but can be picked up again on the eastern edge of the rim. From this point the rim continues directly north where material appears to pool up on the north end in a 10' brighter, circular patch. Superimposed on this obvious glow are a number of mag 10-12 stars. The rim dims again for a short stretch at the NW end before we reach the bright western rim again. At the geometric center of the shell is the massive Wolf-Rayet star EZ CMa (mag 6.7-6.9), midway between 16 CMa at the south end (15' separation) and the brighter patch at the north end. The interior is filled with very low surface brightness, patchy nebulosity.
    Steve
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  4. #4
    Administrator/Co-Founder Dragan's Avatar
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    Check out todays APOD. It's a wonderful image of Reiners OOTW from last January

    http://apod.nasa.gov/apod/ap131224.html
    Clear Dark Skies,
    Dragan Nikin
    25" f/5 Obsession #610 "Toto"
    30" f/4.5 OMI EVO #1 "Tycho"
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