N206 – An Elegant Superbubble

7th Oct 2018

NGC 206 DSS colourI can think of few things I enjoy more than peering into the Large Magellanic Cloud’s incredible collection of superbubbles. And I find that subsequent visits are even better as a superbubble appears even more beautiful and fascinating than the memories of it burned into one’s mind. Indeed, once you have been captivated by their mystery and their beauty and the sheer magnitude of what they are, these glorious places demand return trips. And there is always something more to see, even if it is but a strand of faint nebulosity one missed on a previous visit.

And thus it was with the showpiece N206 located in the southeast outskirts of the Large Magellanic Cloud… In fact, it is impossible to resist return visits to a complex that hosts a beautiful and richly-wispy superbubble that has expanded to a diameter of ∼365 light years, which astronomers tell us suggests an age of ∼2 million years; a huge ghostly supernova remnant; a superb star-forming cluster; two OB associations, LH 66 and LH 69, which harbour 12 and 29 OB stars, respectively; two binary Wolf-Rayet stars, one of which lies in the superbubble (it’s a member of LH 69), the other lies just beyond the periphery of the superbubble and has blown a rare Wolf-Rayet bubble; a tiny tight cluster masquerading as a star; an entire complex that bears a remarkable resemblance to M8 (like all tourists, I enjoy seeing something that reminds me of home); and plenty of other attractive swirls, streaks and patches of nebulosity…

Yes indeed, once the wonders of a superbubble have been experienced, there is an overwhelming imperative to do it again!

Both in images and through the eyepiece N206 is, to my eye, the most elegant of all the LMC’s superbubbles… with all the allure the word suggests – it’s lovely, graceful, has beautifully understated superbubble features, and is absolutely captivating. (I am surprised that such a lovely object is strangely ignored by astrophotographers.)

Formally known as LHA 120-N 206, this lovely complex got its name in 1956 when Karl Henize catalogued it in his Catalogue of H-alpha emission stars and nebulae in the Magellanic Clouds, and subdivided it into four (A-D) nebular components, although C and D are not included on the image below as they are both not only very isolated and don’t appear on the image, but they are also very small and faint – C is a miniscule 16.8 mag young stellar object and D is an exceedingly tiny and beyond-faint emission object. (I am surprised that Karl Henize didn’t break this object up into multiple N-designations as there are several distinguishable and clearly related patches of nebulosity within the complex.)

 

N206

N206 showcasing its captivating elegance in this annotated DSS image

 

I used my 16″ Dobs under pristine Kalahari skies to explore this lovely superbubble.

 

LH 69 and LH 66

RA: 05h31m24s    Dec: -71°04′24″  

RA: 05h26m45s    Dec: -71°05′18″  

Without a filter the complex’s nebulosity is subtle and interesting. N206A (NGC 2018) is very noticeable – a smallish, bright, circular patch from which faint and smudgy nebulosity appears to fan out to the north, the west and the southwest in a beautifully hazy patchwork of uneven nebulosity that fades into the background like early morning mist being burned off by the rising sun, but not before giving one a hint of its superbubble shape. Three noticeable patches of foggy nebulosity are strung out in a NW-SE direction along the superbubble’s SW periphery, and to the northwest lies an isolated, elongated patch, detached from the superbubble. (Looking at the unfiltered nebulosity, I wonder why John Herschel only described NGC 2018 and not the complex as a whole?)

The stars of LH 69 also appear to stream out from N206A’s lovely bright patch – scattering in a NW direction in three distinct groupings. Just west of N206A lies a nice trio of stars that form a slender triangle, the apex pointing north. The easternmost “star” of the base is mag 11.5 HD 269676 – and it has a slightly bloated look, and for good reason – it is not a single star but a massive, compact cluster containing several O-type stars! The second grouping lies northwest of this grouping and it forms an unusual little asterism, looking like an off-kilter Delphinus, its ε Del equivalent (HD 269660) a lovely mag 11 sparkler. The third grouping lies northwest of the little faux dolphin and it holds a special treat – Brey 44 – a Wolf-Rayet binary with spectral type WN4:b+O!! It – or more accurately, they – appear as a lovely little mag 12.9 white star in the eyepiece, and also form a nice “pair” with mag 12.6 HD 269656, an OB-type star that lies just NNW of it. Frankly, whenever I look at a Wolf-Rayet star, my mind boggles knowing that I am looking at an example of the hottest stars known to man (and presumably any other star-gazing civilizations out there; after all, the sky may be congested with intellects) and that it is not long for this universe and will end its brilliant life in spectacular style!

Unlike LH 69, LH 66 doesn’t have any bright(ish) stars for us. Lying to the southwest of LH69, it is a smaller group of very faint scattered stars.

 

The Superbubble with the UHCfilter

One gets a beautiful response with the UHC filter! Pop on the filter and you can immediately see that you are looking at a superbubble! The nebulosity is round and very uneven and smudgy but with some brighter patches; it looks like a blackboard after it has wiped in the most cursory fashion. It has some very dark patches in it that look like dust lanes. Very unusual… but then again, everything about a superbubble is unusual! And in fact, charmingly unusual is that the complex bears an uncanny resemblance to M8!

N206A is lovely with the filter – a very prominent, smooth, glowing, circular patch of pearly light with beautifully hazy edges. Interestingly, this cluster encompasses 14 OB stars but I can see nothing of them, although (is it imagination?) with averted vision the smooth glows seems to have that slight graininess that hints of stars. What looks like a beautiful but fainter bow wave sweeps out in two lovely curves from it to the north and south; the southern one slightly brighter than the northern one and both fading into the general faint patchiness. Three faint but noticeable patches are strung out in a NW-SE direction along the superbubble’s SW periphery. The southernmost patch is N206B (BSDL 2120) and it appears as a faint, elongated E-W patch of foggy mist with a very faint star embedded in it just west of centre. Heading NW, the next patch is BSDL 2048, a faint roundish patch with a couple of very faint, averted vision members of LH 66 embedded in it. The third patch is BSDL 2005, and it appears as a faint round fog with a faint member of LH 66 embedded almost dead centre.

The northern edge of the bubble appears as a fractured and wispy arc of very faint nebulosity; it looks like an almost-transparent strand of faintly glowing light melting into the background. Averted vision helped, but even so, it was very faint, very splintered, but very definitely the rim.

The western side of the superbubble fades into a lovely starry nothingness… except for an absolute treasure that lies just beyond its periphery…

… A Wolf-Rayet bubble!

 

Brey 40a

RA: 05h29m33.2s    Dec: -70°59′35″  

Wind-blown Wolf-Rayet bubbles are among the most rare, intriguing and tantalising objects to observe. Our own galaxy offers us but a handful to observe… the LMC has an incredible six that we can observe, offering observations from the utterly gorgeous to a faint and wispy arc-shaped portion of the ring. Brey 40a is the latter. (NGC 2020, lying just outside the periphery of the superbubble N57 is the former and to my eye, is the quintessential W-R bubble!)

On the image one can see that the star, and the visible part of its bubble that lies about 40″ to its south (BSDL 1985), are embedded in the extended and incredibly faint and diffuse nebulosity of N206. In the eyepiece, Brey 40a, the mag 13.4 Wolf-Rayet star (a binary with types WN4+O8) appears as a small, bright little star. Because the extended nebulosity is invisible in the telescope, the visible fragment of its ring appears as an isolated, ethereal mistiness; it is beautiful and fragile-looking; and on the western side of the mistiness there is a short, dagger-thin little streak of slightly brighter brightness. The bubble’s mistiness has no defined edges… it dissolves gently into the surrounding sky. It has the very slightest curve – not much more than an inkling of a bubble-curve.

A cluster, KMHK 979, lies just NE of Brey 40a, but I could see nothing of it (it certainly puzzled me; such an odd location for an open cluster!)

 

SNR B0532-71.0

RA: 05h31m33.2s    Dec: -71°00′03″  

I tried for that large SNR (~95 x 95 light years in diameter) one can see in the image, but with little expectation of seeing anything at all. I used everything at my disposal – the UHC filter, the OIII filter, averted vision, time… but alas, as expected not even a hint of it. But it was actually a nice way to end my revisit to N206 – that to my eyes all that remained of the massive star that died in a titanic explosion had vanished into nothingness in the beautiful starry background surrounding this elegant superbubble.

 

Copyright © Susan Young 2019

 

 

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