Two Wolf-Rayet shells in Canis Major

Jamie Dillon

Wednesday night the 26th, I got up to the Peak and had the park to myself, far as I could tell. Set up on the pads in front of the Observatory and got 2 and a half hours of decent skies. What I most wanted to see was Sh2-308 in Canis Major, a Wolf-Rayet shell that I'd only heard about from Steve Gottlieb. It's not in Uranometria. Steve describes finding this really interesting object from Grey Pine at Lake Sonoma: /reports/2003.02.01.12.html Titled Blowing Bubbles in the Wind, one of the great alltime OR's, including a quasar whose light started here before our Sun formed. Steve lists some other Wolf-Rayet shells, two in the southern parts of our skies, along with Thor's Helmet also in CMa, ngc 2359, and the Crescent in Cygnus, ngc 6888.

Add two to that list. The Bubble Nebula in Cassiopeia, ngc 7635. Not far from there, over the border in Cepheus, is Sh2-157. Now I can't find where I got the idea that the Bubble Nebula is a WR shell, but I have this lingering impression that it is. Sh2-157 is in the astro literature as probable. So here's a fun and compact observing project, checking out Wolf-Rayet shells, of which we can see 5 from here. They're all striking in the eyepiece, and easy in medium aperture.

What's also cool about these is that they're the result of really hot energetic stars blowing off huge amounts of matter in their stellar winds, making big bubbles in the interstellar medium. A Wolf-Rayet star is a star in the late stages of its evolution that's strange for several reasons. Wolf-Rayets would be classed as O-type stars if their spectra weren't so weird, on the far end of the OBAFGKM sequence, the hottest brightest baddest most massive stars. They live according to the classic honkytonk line, "live fast, love hard and leave a beautiful memory." They can't be classed as O stars partly because they've blown off all of their hydrogen into these prodigious stellar winds, and are fusing helium for their main fuel. They don't live long as stars go because they run out of fuel quickly, so we don't see many from here. They certainly end up as supernovae.

Sh2-308 was the 5th on my list. Easy hop, just north of 16CMa, a naked eye star from a dark place. What fascinated me most of all was that the shell clearly looked like a shockfront around its central star, which itself is just north of 16. There was even some structure on the western edge, come faint broad curdling along the arm moving past that bright 16 star.

Of course I checked out Thor's Helmet not far from there. Much brighter and denser than 308, with all kinds of interesting structure. An article I read put the age of 308 at 70,000 years. It's over 40' wide in the eyepiece, clearly having had time to spread out. Fun to compare the two.

Also, that night was first light for a new Orion OIII filter I'd gotten. Have had my Lumicon OIII since I got my scope, 9 years now. Get James Turley to gnash his teeth over those older Lumicon filters - optically fine, but they never cared that the frames didn't thread into any eyepiece. I'd used it in blinking mode all these years, holding it over the eye lens. Having used an Ultrablock as well for years, I was aware of the moral advantages of being able to screw a filter onto the back of the eyepiece and leave it there. The views of these two shells with the new filter were gratifying, as was a look at M42.

This was all with Felix, a Celestron 11" f/4.5 Dobs with optics made by Discovery Telescopes. Was using a 22 Pan, 16mm UO Koenig, 10mm and 6mm Radians and that new Orion OIII.

Saturn looked good, with tan bands on the disk, a clear Cassini division, and Titan, Rhea, Tethys and Dione all close in. When I was setting up around 8:30 the limiting magnitude was around 5.7. A high haze came in by 9:30 and it dropped with around 5.2. Seeing was 3/5, moderate. Saturn was decent as noted. It took work at 210x to get the F star around the Trapezium. By 11 pm, temps dropped, clouds started to move in and the dew got serious. Good cues to go home. I was extremely satisfied to get my first new object logged since New Year's Day, after several skunkings, and to get that Sh2-308 onto the old retinae. Also of course tissue-restoring to be under a rural sky with a bunch of stars.

We winter observers are the hard core, say it loud, we're nuts and we're proud.

DDK