N158 – a Celestial Geode

8th Oct 2018

ngc2081-ccI am a keen rock hound, and a geode is among the most beautiful objects one can find. Geodes are the mysterious treasure-boxes of the geological world. Undistinguished plain rock from the outside, they reveal a cavity in the middle filled with beautiful crystals when they are cut or broken open. The rough exterior of a geode gives no indication of the treasures held within its core – its composition is only discovered when it is cracked open. The crystals are most often clear quartz, although they are sometimes amethyst or calcite or other minerals. The size and formation of the crystals and the different shades of colour make each geode special – from those literally bursting at the seams with the most gorgeous crystals to those that contain the most delicate outcropping of crystals, no two geodes are alike.

A walk in the Lebombo Mountain area of northern Zululand can, on the rare occasion, garner one a stunning amethyst geode. The area consists of ancient Karoo basalts. The outflow of these basalts covered most of South Africa, and heralded the break-up of Gondwanaland approximately 180 million years ago. The rapidly cooling basalts trapped gas bubbles, and silica-rich fluids that contained trace amounts of iron percolated drip by drip through the cavities. Over millions of years, this liquid formed crystals – six-sided pyramids of amethyst – with colours that range from the very palest lilac to the most gloriously deep violet.

You may be forgiven for wondering what amethyst geodes have to do with a blog on one of the Large Magellanic Cloud’s rich collection of superbubbles! But peering into the heart of a superbubble is like peering into the heart of a celestial geode… instead of a bubble of rock with a crystalline treasure at its heart, a superbubble’s cavity contains at its heart the most gorgeous starry treasure. And like terrestrial geodes, no two are alike!

N158 is just such a celestial geode… elongated in a NNE-SSW direction, it consists of a glorious superbubble in the NNE, blown by the OB association LH 104 and filled with its gorgeous star cloud that contains some superb stellar treasures, and in the SSW lies a brighter and more compact star cloud, LH 101, floating in a beautiful swathe of silky nebulosity.

 

An amethyst geode from Zululand and the N158 superbubble geode from the Large Magellanic Cloud

The amethyst geode from Zululand and the superbubble N158, a celestial geode in the Large Magellanic Cloud. There is an extraordinary similarity in exploring the crystalline heart of the amethyst geode with my 40x magnifying glass and the stellar heart of N158 with my 16 inch telescope.

 

Formally known as LHA 120-N158, this glorious 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. It lies within the spectacular supergiant shell LMC 2, home to the stupendous Tarantula Nebula, five superbubbles, a swarm of other Henize nebulae, star clouds, clusters, supernova SN1987A… one could spend an entire summer exploring the multitude of objects it has to offer! Here is an annotated DSS image of N158:

 

The utterly gorgeous N158

The utterly gorgeous N158

 

I used my 16″ Dobs under a pristine Kalahari sky and I began with the heart of the celestial geode…

 

LH 104 (NGC 2081)

LH 104 is dominated by B-class supergiants and with no filter at 228x it is a superb star cloud of around twenty or so resolved stars (many of which are ~mag 13-14), set against the beautiful glow of unresolved starlight and surrounded by the softest and most raggedly uneven wash of the bright HII glow that is so obvious in the DSS image. Three Wolf-Rayet stars are among the cloud’s treasures – mag 13 Brey 94, binary mag 13.1 Brey 95 (WN4b+O8) and mag 12.2 Brey 95a. And a very special treat lies on the northeast side – mag 11.9 star HD 38489 is an extreme luminous blue variable, whose spectrum is similar to that of our very own eta Carinae!! (I can’t help but wonder, on a flight of fantasy, if it hosts its own Homunculus-like nebula!)

The superbubble responds well to the UHC filter – the soft ragged wash is revealed as a beautifully bright (but still somewhat ragged-looking) nebulous glow that surrounds the star cloud. It has the most fabulous shape – resembling a hybrid between a circle and a triangle! (On a geological note, most geodes are also oddly “hybrid-shaped”.) The north-western side is quite faint – an uneven patch of very soft and translucent nebulosity, and with no edges; it simply dissolves into the starry background. The north-eastern and eastern sides are very attractive – there are beautifully brighter and smudgy strands and patches scattered along the length of soft and translucent nebulosity. The superbubble is brightest and most detailed along its southern rim. The nebulosity looks like a bright, somewhat warped and tattered ribbon of nebulosity against the faint overall background haze. Its brightest section is BSDL 2722, a beautifully bright knot that looks as if it is pushing the fainter surrounding nebulosity into the star cloud, for there is a lovely hazy headland of nebulosity that bulges northwards into the star cloud. A bright strand of nebulosity sweeps away from the bright knot towards the east and in fact, the whole southern portion has a number of streaks and wisps within it. Lovely!! A slightly bloated-looking mag 11.2 “star” – HD 269936 – lies off the south-western edge of the superbubble’s rim… but it isn’t a star at all… it is actually another of the extremely compact clusters of which there are a number in the Cloud!

Two clusters worth a visit lie just outside the rim of the superbubble. NGC 2091 is a small, fairly bright mag 12 oval, oriented E-W that lies to the east of the bright strand of nebulosity that sweeps eastwards along the southern rim; looking as if a chunk was broken off and flung aside. It has nicely defined edges and only one faint star is resolved almost dead centre. The other little cluster is a very small mag 12 cluster, H88 308, lying outside the rim’s north-eastern edge. The cluster shows as a very small oval of faint unresolved starlight, oriented SSE-NNW with well-defined edges.

 

LH 101 (NGC 2074)

LH 101 is gorgeous!! Without a filter and at 228x it is a large, beautifully bright semi-circular crown of stars, open to the SW and surrounded by the loveliest C-shaped mist of pearly nebulosity. The crown of stars is marked by lovely brighter stars at either end. On the north-western side of the crown lie two bright stars oriented SW-NE. The north-eastern of the two, HD 269927, consists of a pair of OB stars (mags 11.4 and 11.8) and a mag 12.5 Wolf-Rayet star (Brey 91). The lovely bright white mag 10.3 star to its SW is A0-class HD 269923. The bright star on the south-western end of the crown is mag 12.5 CPD-69 471, an O3-class supergiant. And between these bright stars lies between 15 and 20 fainter to very much fainter resolved stars, lying in a lovely pool of unresolved starlight. It really is a lovely crown of stars! The nebulosity responds well to the UHC filter; it is bright, large, beautifully uneven and has lovely edges that melt rapidly into the background.

The small dark streak that cuts into the nebulosity on its eastern side is known as the “Seahorse of the Large Magellanic Cloud”. It is a gigantic dust pillar structure roughly 20 light-years long; condemned to disappear over the next million years as more stars in the cluster form, their light and winds slowly blowing away the dust pillars. What an object to see!! But alas, it remained unseen by my eyes despite a long and diligent search. I wonder if larger telescopes will pick it up?

 

The Seahorse of the Large Magellanic Cloud

The Seahorse of the Large Magellanic Cloud. Image credit ESO

 

I ended off my evening in N158 by visiting three small visual treats. HD 269924 is a lovely orangey mag 10.4 star with a faint but distinctly glowing “tail” of nebulosity that appears as if it is streaming away from the star to the south before curving to the southwest. It looks like a strange and wonderful comet!

N158B is a tiny, faint droplet of dimly glowing nebulosity; and I ended with a look at N158D (also known by the prosaic name SUMSS J053917-693329). It appears as a small, slightly arc-shaped glow of pale nebulosity… and was a wonderful final object to view because it leads the eye in a south-westerly direction to yet another superbubble… N160… what a glorious place the Large Magellanic Cloud is!! I could spend my whole life here!!

 

Copyright © Susan Young 2019

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