Object of the Week January 06, 2013 - Sharpless 257 and the Three Snowballs

[reiner]

I actually wanted to write about Sharpless 261, Lower's Nebula, but when scanning Wikisky, I stumbled over this nice group of three Strömgren spheres just north of it, which had been a favorite of mine for a long time. So it will be Sharpless 257 and friends today, in the constellation of Orion, between Orion's head and the feet of Gemini.

IC 2162 (= Sh2-257)

Orion

RA 06 12 50
Dec +17 58 33

group of four emission nebulae

Sh257tricolor.jpg

Sharpless 257 (also IC 2162) forms together with Sharpless 254 and 255 a group of three nice Strömgren spheres, which a friend of mine and I dubbed the "Three Snowballs" due to their milky appearence at the eyepiece. The image above is a color composite of the DSS2 images.

Sharpless_257_DSS_labeled.jpg

This is a DSS red image with the different parts of the group labeled.

Sh257group_finder.jpg

And this is a finder chart from my Sharpless Observing Atlas.

My observing notes for this nice group during a night of very good transparency are as follows:

In the 24 mm eyepiece of my 22-Inch Dob, the two more compact nebula 255 and 257 were easily visible with H-Beta filters as milky, well defined areas, each surrounding a star in the middle. Sharpless 254 was somewhat more diffuse, but could be seen without retinal torture. The entire group fit into the field of view of the eyepiece and the contrast between the diffuse 254 and the more compact 255 and 257 was particularly beautiful.

With the UHC Filter, all three were still visible, yet much more difficult than with H-Beta. As to be expected for low-excitation HII regions, they were hardly visible with the OIII filter.

I tested as well the small nebula Sharpless 258, which is to the left on the image and might be some kind of reflection nebula. As there was nothing visible immediately, I did not try to push this, in particular as there are superimposed stars, which might be deceiving. Sh256, on the other hand, the small patch between 257 and 254, was well visible as a separate glow detached from the main mass of Sh257.


So this group is a nice target for the new year. A bit fainter than M42 but nevertheless a nice and interesting object. So...

"GIVE IT A GO AND LET US KNOW"

GOOD LUCK AND GREAT VIEWING!

[Clear Skies]

Sharpless 257 & friends, screenshot from my observing guides. Image is unmirrored.

Sharpless 257.jpg

[Steve Gottlieb]

Nice choice of challenge objects, Reiner. I'm going to have to reobserve these objects using an H-beta filter, based on your comments.

IC 2162 is generally assigned to Sh 2-257, but that identification is probably incorrect. Barnard's position precesses to 06 13 12 +17 58.4 (2000) and falls just east of Sh 2-255. In addition, Sh 2-255 is slightly more prominent visually (at least in my 18"), and probably more likely to have picked up by Barnard.

[reiner]

Sharpless 255 is definitely the most prominent part in the group, so the (common) assignment of 257 to IC2162 could well be wrong.

H beta was for me the filter of choice here. Dean Salman has narrowband images of this group, where there are distinct OIII areas around each of the central stars. Visually, the OIII filter was quite inefficient on this group.

[Marko]

Made this finder of wide area to include IC443 and others. Will take this to the field if it stops raining ... The chart is a bit busy but compact.

[Uwe Glahn]

These Snowballs are one of the best "Sharpless groups" in the sky Reiner. With 14,5" and Hß all four balls could be seen. I also tried Sh 258 which I could not detect.

14,5", 83x, Hß, NELM 6m5+
IC2162.jpg

[Paul Alsing]

Love the chart, Marko, right up my alley...

[Howard B]

I had a go at these last Saturday morning after moonset with my 28 and could barely detect 255 and 257. However, the sky was quite bright from auroral glow (20.7-ish SQM) and there were some thin high clouds here and there so conditions were well below average. The h-beta filter did the best job of showing two faint nebulosities and the OIII was just slightly inferior. Hopefully I can have another look under more favorable conditions next month.

[reiner]

Sharpless 254 can become very difficult under so-so conditions. Its surface brightness is considerably lower than that of the other two "snowballs". Even under good conditions, it is much fainter than the other two. You should give it another try, as this is really a nice group!

[Howard B]

These Snowballs are one of the best "Sharpless groups" in the sky Reiner. With 14,5" and Hß all four balls could be seen. I also tried Sh 258 which I could not detect.

14,5", 83x, Hß, NELM 6m5+
IC2162.jpg

Hi UWE,

I was finally able to have a decent look at this lovely bunch of Sharpless snowballs two Friday's ago (March 8) with my 28 f/4 under 21.35 SQM skies. Unlike my first attempt in January, this time the two brightest snowballs - 255 and 257 - stood out right away, with big 254 and little 256 only slightly more difficult. The h-beta filter had the best effect by far, with the NPB performing a distant second. All the snowballs were invisible without filters. 258 never showed a hint of itself at any power up to 408x, and my overall view was very much like your sketch - a subtle but wonderful treat. I haven't scanned my sketch yet but will try to remember to post it here when I do.

April 16th edit - I'm adding my sketch:

Sh255-258 crop.jpg