In 2013 ESO released this beautiful image of N57 in the Large Magellanic Cloud with the title “The Odd Couple”. And an intriguingly odd couple they certainly are as the one is a bubble blown by a single star, the other a superbubble blown by an entire OB association! The smaller shell – N57C – is a rare Wolf-Rayet Bubble, 95 x 95 light years in size, that was blown by the Wolf-Rayet star, Brey 48. The larger shell is superbubble N57A (although the image slices off quite a bit of it), 650 x 325 light-years in size, that was blown by the OB association LH 76.
Unbelievably, N57 is not the only “superbubble and rare Wolf-Rayet bubble” odd couple in the Cloud! N206 also has a Wolf-Rayet bubble sitting just beyond the periphery of its superbubble. However, only a small portion of N206’s Wolf-Rayet bubble is visible and it is very faint at that, whereas N 57C is, in my opinion, about as perfect an example one can get of a Wolf-Rayet bubble in the eyepiece!
N57 lies on the southern periphery of LMC-4, the gargantuan supergiant shell in the north-eastern region of the Cloud. LMC-4’s diameter is a mind-boggling 6,000 light years and its periphery is dotted with prominent H II regions, superbubbles, bubbles, and supernova remnants. At low magnification the view is simply stupendous… it is among my most favourite places to hang out in the Cloud, each of the treasures as stunning as the next! This particular treasure is formally known as LHA 120-N57; it got its name in 1956 when Karl Henize catalogued it in his Catalogue of H-alpha emission stars and nebulae in the Magellanic Clouds. He subdivided the nebulae into five nebular components, from A to E (D is actually visible on the image – it is that tiny little pink glow between them.)
Here is an annotated DSS image of the odd couple:
I used my 16″ Dobs under a lovely Kalahari sky to explore this superbubble and its stunning companion.
As I always do with a superbubble, I began my observation of it with a look at the OB association responsible for blowing it – LH 76. Without a filter at 228x it is a gorgeous star cloud of at least three dozen resolved stars of mixed magnitude set against the beautiful glow of unresolved starlight. The cloud of stars is very intriguing in that one can discern the boundaries of the cloud of stars except to its north where the stars appear to have fizzled out, but without disturbing the roundness of the little cloud. There is a gorgeous string of stars that runs roughly N-S with a beautifully bright mag 10 star dangling off the southern end of the string – it is HD 269723, a yellow supergiant! On the western side of the cluster is another of the Cloud’s wonderful “stars” that are not stars at all, but very compact clusters – although this mag 12.7 HD 269714 doesn’t actually have that slightly bloated look others do (although maybe with averted vision it does… hard to tell when one’s imagination knows what very subtle detail to look for. The star cloud is nestled in against a beautiful pearly haze of nebulosity on the southwestern side.
Now something that I have long pondered, and long tried to see – the superbubble’s obvious “honeycomb” appearance so obvious in the DSS image. When I pop on the UHC filter, the bright nebulosity that envelops the southwestern side of LH 76 is very uneven and careful observation aided by averted vision allows one to see that the uneveness is actually a couple of curlicues of brighter nebulosity glowing against the overall nebulosity. I can make out one small glowing curlicue to the southwest of the star cloud; it is very wispy and filamentary-looking, an open to the NE. To its NE there is another… but actually it is less curlicue and more a softly glowing N-S streak-shape. There are other very small wraith-like curlicues in the nebulosity; very small feathery glows revealed by averted vision. As for the spindly honeycomb filaments so clear in the DSS image – as expected, they are not visible in my telescope, although clearly visible in my mind’s eye.
The ponder? In the early 1990s, astronomers were using ESO’s New Technology Telescope to image the remnant of the type-II supernova SN 1987, and serendipitously found a nearby nebula they named the Honeycomb Nebula (also not visible in my telescope, but not for lack of trying!)
I may be completely off-course but the honeycomb effect bears a remarkable similarity in the two images. In 2010, University of Manchester astronomer John Meaburn and colleagues studied the Honeycomb Nebula and came to the conclusion that its unique appearance is likely due to the combined effect of two supernovae. According to the team, a more recent explosion has pierced the expanding shell of material created by an older explosion. (They also added that the striking appearance of the Honeycomb Nebula is suspected to be due to a fortuitous viewing angle.)
Such fodder for pondering while observing N57!!
I gaze at LH 76, pondering its massive stars going supernovae and wondering at what I can’t see but know is there – who can’t love astronomy for its myriad pleasures!!
And now I nudged the telescope over to look at the superbubble’s companion… N 57C. It is absolutely dumbfounding at every magnification, with and without a filter. At 228x, and without the filter, it is a roundish (slightly elongated SW-NE) and faintish but distinctly glowing bubble, with a brighter inner filamentary ring surrounding a relatively large dark centre… with its mag 13 Wolf-Rayet star, Brey 48, lying in the dark centre – it really is the quintessential Wolf-Rayet bubble! It responds well to the UHC filter, and shows some superb detail; the inner filamentary ring is very prominent, not only brighter but also sharply defined both against the dark centre and the fainter outer nebulosity. The outer nebulosity is beautifully uneven; full of very subtle and very delicate gradations of softly glowing light. The outer edges of the bubble gradually grow both fainter and more diffuse until disappearing into the dark sky. Truly gorgeous!!
Brey 48 appears to lie in the centre in the entire bubble, although it lies north of the dark inner ring’s centre. A mag 12 star that lies on the southern edge of the bubble adds to the beauty of the scene. What a treat it is to see Brey 48’s stunning little glint of rare stellar light in my eyepiece! (Frankly, I will never ever get over the fact that the stars I see so clearly are in another galaxy!) And it is remarkable to gaze at that tiny star, shining so serenely in the eyepiece, and think about what’s really going on with and around it. And even more remarkable, what’s going to happen in the star’s not too distant future!
N57’s other Henize nebulae
N57B: In the DSS image, N57B appears to be lying on the outskirts of the superbubble’s honeycomb. In the eyepiece it appears far removed from the visible nebulosity and I would easily have missed it had I not known to look for it. It appears as the smallest, faintest little smudge of nebulosity that I picked up with averted vision; no edges, simply a ghostly little splot of very faintly glowing light. It has a lovely slightly orangey star in its neighbourhood – HD 269724, a red giant branch star.
N 57D: As I expected, not visible in my telescope.
N 57E: This is a very cool thing to see! Had I not known what it was, it would never have registered as anything more than a random faint star among a number of other random faint stars. But it is a young stellar object, a tiny, round, mag 13.7 droplet of faintly glowing nebulosity floating to the southwest of the cloud of stars… wow!! And a great way to end my evening’s exploration of this lovely odd couple.
Copyright © Susan Young 2019