Object of the Week June 16, 2013--- NGC 6337

NGC 6337

Scorpius

RA
17h 22m 15.65s

DEC
-38º 29' 01.4"

MAG 12.3, central star mag 14.9

Type: Planetary nebula, size 49” x 45”

NGC 6337, the Cheerio Nebula is our focus this week. A lovely annular planetary nebula very much like M57, it’s located in southern Scorpius a few degrees west and south of the Stinger stars. At magnitude 12.3 and 49” x 45” arc seconds in size it’s reasonably bright and large and will take magnification well. Its central star’s magnitude is 14.9.

Attachment 719

6337 is special because of all its central stars – check out the DSS image and you’ll see a straight line of five stars bisecting the very round ring of NGC 6337, and they’re the visual challenge here. Of course only one is the physical central star of 6337 – it’s the one closest to the bright star that’s just inside the southern edge of the ring.

I've found the non-filtered view to be the most satisfying mostly because it’s the only way to see the three faint central stars. The two brighter stars on the end of the five star line are seen fairly easily, but the three in the middle are about as difficult to see as the single central star in M57 during poor seeing. The nebula stands out with higher contrast using either UHC (good) or OIII (better) filters but they do dim the stars and erase any hope of seeing the faint triple. My sketch is a composite – the nebula was drawn using an OIII filter and the stars added without a filter, all using 408x. You’ll note that I've seen only two of the three faint central stars so far. Has anyone seen all three?

Attachment 720 Attachment 721

I've observed 6337 from the Golden State Star Party at about +40 degrees latitude with my 28 inch scope, and had similar views through my 13 inch from the Big Island of Hawaii at around +19 degrees latitude, so elevation above the horizon really matters. My best view was with the 28 inch – see the sketches above - but it was only slightly better than with the 13 inch.

In the paper “The Outflows and Three-Dimensional Structure of NGC 6337: A Planetary Nebula with a Close Binary Nucleus”, Ma. T. García-Díaz, D. M. Clark, J. A. López, W. Steffen and M. G. Richer (http://iopscience.iop.org/0004-637X/...1633.text.html) discuss the three dimensional structure of 6337 as being greatly influenced by the true central star being a tight double star. The other four stars just happen to lay in our line of sight.
The discussion of how the authors constructed a 3D map of 6337 is worth the effort to read their paper. Having some idea of 6337’s internal dynamics brings it to life, making knowledge of its physical nature as powerful an observing tool as a nebula filter or averted vision.


“Give it a go and let us know!”

Good luck and great viewing!

Excellent target, Howard. It's surprising how good this planetary looks, even to mid-northerners. The only other perfectly annular planetary that far south that resolves as well (from +38 north) is IC 5148, though I may be forgetting one or two others.

John Herschel, who discovered it on 28 Jun 1834, made three observations as well as a sketch and was clearly impressed by this object. He caught the two outer stars but missed the difficult tight trio.

"A beautiful delicate ring, of a faint ghost-like appearance, about 40" diameter; in a field of about 150 stars, 11 and 12 mag and under. In it is one star 12 mag very conspicuous, and one 15 mag much less so. Near it are two stars 14 and 15 mag, and south of it at a distance 60" is another."

Howard, you open the PN season with one of my favorites.

When I first saw a picture of that PN with its CS line I becomes one of the "most wanted" object in the sky. I first tried it with my 16" from 47° latitude N. The PN could be seen as a faint round ring, no detail, no CS line...only 5° above the horizon.

When I first traveled to Namibia (23°S) I first tried it with 12", 375x: "bright ring even without filter, ~40" diameter, 13mag star N within the ring, very faint 15mag star S, always something within the ring but no star itself can be resolved, perhaps a faint line N-S elongated, the ring itself is better defined at its NW and NE edge"

A few nights later I tried it with 24" and 400x and the results amazed me deeply: "bright, absolutely round, distinct ring structure with some details (knots) in it, stellar spot at the W edge, the NE quadrant also brighter and better defined, within the ring shifted from the middle to SW faint line composed of 3 faint stars, line easy to see but difficult to separate, middle star a little bit brighter"

24", 400x, NELM 7m+
Attachment 728

That's the finest sketch of 6337 I've seen Uwe, and you saw all three of the central stars! Fantastic, and yet another reason to go to the southern hemisphere one day. I imagine that if 6337 traded places with M57 it would be one of the most famous deep sky objects and immensely popular. Like real estate, location makes a huge difference.

Thanks Howard,

you definitely have to go to the south. It is incredible when the central milky way walks over your head and the the Magellanic Clouds chase each other.

I don't know what hip NGC 6337 has for the southern observers. Aren't some Australian Observers here in the forum. What I think - the south is the hemisphere of PN. There are lots of other fantastic PN like NGC 2899, 3210, 5189, 5844, or 6302 which steal NGC 6337 the show.

Greetings from the hemisphere of galaxies.

I have but one observation of this in a view from a 33" that often is not fully optimized for mirror alignment. This was from very dark skies but as it was a shared view it was rushed so I was unable to use my own eyepieces to tweek and squeeze what was possible. I'll have to try this in my 18" with my own selection of eyepieces and filters but I doubt I will see the line of dim stars or the center one. Wonderful selection Howard.

NGC 6337 Type: Plan Position: 17 22 15.6 -38 29 02 Con: Sco
Date: 06/12/10 Time: 23:49 Site: WSprDSR
Scope: 33ObF5.0 Eyepiece: 18mm Power: 220
NELM: 6.7 SQM: 21.70 Seeing: 4 Transparency: 4
Description: ' 1/20fov Unfiltered show donut shape. 1/4 ratio of ring
thickness to dia. Dim FS in middle of ring on W edge and
brighter FS on inside edge of ring on NE side where ring may be
fatter on NE side. No Central star'

Although with a much smaller aperture, here is my log entry from 8/7/04: Using Celestron 11" SCT with 20mm Nagler Type II (140X): Located SW of Gamma Scorpii and SE of the Bug Nebula. Using an O-III filter, it appears round, and about 40" diameter. With averted vision, an approximately 13th mag. star is seen superimposed upon the E edge, which is slightly brighter than the rest of the annulus. The nebula is very slightly darker toward the middle. At the NW edge of the field is a double star with an 8.8 mag primary and a 13th mag. companion at PA 340. A 12th mag. star lies 1/2 way to the SSE edge of the field.

Stargazr
Land of Oz Observatory
http://www.astrolandofoz.com

A bit late but I will add my observation from 2012, Namibia (I'm quite new to DSF).

16", f/4.5, 225-300x, 2012.06.12/13. Hakos, Namibia:
Attachment 2045Attachment 2046

I very much like this planetary as well (just like other rings). This was the first planetary I drew from the southern hemisphere and I really wanted to see all three stars inside it, but saw only two. And they were very hard.
As I came upon this thread I was pretty surprised to see Howard's drawing showing 2 stars just like mine. Very good observations above!