The Cloud’s OB associations are nothing less than dazzling in the telescope

The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968) and also showed that stellar associations are star-forming regions. OB associations are loose groupings of several thousand young stars, a small fraction of which are massive, luminous blue O- and B-type stars. They are distributed across large areas of the sky and share a common motion. Gravitationally unbound, OB associations are expanding systems, their stars moving away from individual common centres that may have core open clusters. Because the massive, luminous OB stars deplete their thermonuclear fuel relatively quickly, they have short lifetimes as a distinct object, generally only a few million years in age or less. Once the OB stars have gone supernova, and the outlying members have drifted apart, merging with the other field stars in the region, only the gravitationally bound open cluster is left as an observable object. (It certainly is peculiar to look at an OB association and consider that we are observing the transitional phase between the birth environment of stars and the field star population in the galaxy.)

Although they are extremely rare, comprising less than 1% of the total stellar population, massive stars play a crucial role in shaping their surroundings and contribute significantly to the evolution of galaxies and future generations of star formation by injecting energy into the interstellar medium through radiation, stellar winds, and supernova explosions. Individual massive stars can blow bubbles around themselves (diameters from fractions of 1 pc to a few x10 pc). An entire OB association’s massive members work together to create a superbubble (diameters of ~100 pc). And multiple generations of OB associations can form supergiant shells (diameters an astounding 1,000 pc and greater). And of course, as we all know, it’s thanks to the massive stars that we are here to think about massive stars!

The Large Magellanic Cloud is swarming with a magnificent array of glittering OB associations. They are distributed across the Cloud in great swathes of stars and they are very busy places to explore with a telescope! You can explore magnificent superbubbles with the next generation of OB associations around their peripheries busy birthing new stars. You can examine generations of OB associations in the nine gargantuan supergiant shells. Others OB associations are vast star clouds filled with stars and open clusters. Others are beautiful stellar nurseries. And yet others are beautiful congregations of stars of all shapes and sizes.

And thanks to the astronomers Peter Lucke and Paul Hodge we observers have a fabulous catalogue that provides us with a map of their distribution across the cloud as well as all the pertinent data we need to explore them. Their catalogue was published in 1970. It lists 122 objects with sizes from 15 to 300 pc and numbers of stars from a few to 225, down to a limit of V=14.7 mag. Most of the associations range in the ~15 to ~150 pc size, but sixteen – the ones with dimensions up to ~300 pc in diameter – were designated “star clouds” by Lucke and Hodge. Ninety-six percent of the associations are associated with nebulosity.

There are a number of ways to observe these fascinating objects:

A DEDICATED OBSERVING PROGRAMME

I found nothing gave me a better idea of the profound impact massive stars have on their local environments more than taking a leisurely tour from LH 1 all the way through to LH 122, focusing on each association and its local environment. It was as if they provided the skeletal makeup of the galaxy.

This is where you can find a chart with their locations, an image of each association to assist with identification, and a link to where you can find it:

SUPERBUBBLE OBSERVATIONS

When it comes to superbubbles, starting the observation with the OB association powering the enormous superbubble gives structure and context to the observation, quite apart from the fact that these splashy associations of stars in their superbubble vicinities are a wonderful sight! And then there are the younger OB associations in the bright H II regions along the peripheries of many of the superbubbles where the next generation of stars are being birthed.

SUPERGIANT SHELL OBSERVATIONS

Observing the OB associations in a supergiant shell not only offers a grand display from big, bright and sprawling to small, faint and contained, but also shows a fascinating correlation between their ages and locations in the supergiant shell.

STAR CLOUDS “THE OTHER WAY AROUND”

And finally, one gets a profoundly different perspective of the open clusters that reside in a LH star cloud when one observes them “the other way around” – in other words, by first observing the star cloud as the singular object it is (its size, shape, boundaries, pageant of glittering stars and the locations of the clusters within it), and then examining each of the clusters individually. Many of us zoom straight in on the clusters, merely noting that noting that cluster resides in LH such-and-such.