Everything about supergiant shells exists in a state of sheer enormity: their colossal size, the vast number and diversity of objects they contain, the great and enduring impact they have on observers

While the furious stellar winds of a single massive star blow a bubble, and the interacting stellar winds and supernova explosions of an entire OB association work together to create a superbubble, it takes the stellar winds and supernova explosions of multiple generations of OB associations to create a supergiant shell (SGS). They are the largest interstellar structures in a galaxy, having diameters an astounding 1,000 pc (3,260 light-years) and greater. Their diameters often exceed the scale height of the galactic gas disk, allowing them to puncture the gas disk and vent their hot interior gas into the galactic halo. SGSs have OB associations along the periphery or in the centre, with younger OB associations more often found along the periphery. After roughly 12 Myr, if no new OB associations have been formed, a SGS will cease to be identifiable at visible wavelengths.

SGSs in the Large Magellanic Cloud were first identified by Goudis & Meaburn (1978) based on visual inspection of Hα images of the ionized ISM. They tabulated four SGSs and this number was later expanded to nine by Meaburn (1980). They are known as LMC 1-9. Only two of these shells have been studied in detail, LMC 2 and LMC 4… LMC 2 because it is the most spectacular and LMC 4 because it is the largest.

I am not sure that observing is an adequate word… visiting the supergiant shells is more like setting sail on epic excursions that take you to some of the most captivating destinations in the universe.

LMC 1 offers us a modest exploration – simply the vast and gorgeous star cloud LH 15 with a few small clusters lying among its glittering stars. By contrast, LMC 2, located to the southeast of the gigantic Tarantula Nebula, is congested with such a multitude of complex and intricate superbubbles, star clouds, star-forming complexes, bubbles and blobs that it staggers the imagination.

LMC 3, located just to the northwest of the Tarantula Nebula, is massive and imposing yet it conveys a graceful sense of grandeur.

The largest of the SGSs, LMC 4, defines the word gargantuan at ~6,000 light-years in diameter. Its colossal periphery is jam-packed with superbubbles, supernova remnants, star-forming complexes, OB associations, clusters, and nebulae, and within its gigantic cavity the most graceful and enigmatic quaver-shaped nebula floats in solitary splendour.

LMC 5 is the only SGS that contains no OB association, but it does contain the superbly bright supernova remnant N49, the first extragalactic SNR ever discovered.

LMC 6’s expanse is curiously empty, but it makes up for it by hosting what is surely one of the prettiest complexes in the Cloud, N91, which itself hosts a rare Wolf-Rayet bubble. The SGS also contains  ghostly supernova remnant whose nickname honours a magnificent nose!

LMC 7 contains a fabulous star cloud, along with a fascinating star-forming complex, the “new” N79, that has been called “a future rival to 30 Doradus”.

LMC 8 is utterly charming with its two dominant star clouds resplendent with stars and small, delicate clusters, a boomerang-shaped OB association and its light dusting of nebulosity.

And last but by no means least, LMC 9 is a very old shell and its collection of OB associations and their related H II regions, including an elegant superbubble, are situated in a vast semicircle that is both intriguing and fascinating.

As if nine supergiant shells to explore isn’t enough, one can view the uniquely extraordinary site where two of these titanic monsters are colliding. Colossal LMC 4 is colliding with its smaller neighbour to its northwest and the impact zone has produced the young associations LH 52 and 53 in the dense starfield of NGC 1948 within the past few million years.

An exploration of the Cloud’s supergiant shells… astronomy doesn’t get better than this.