The LMC is a terrible place to plan for a long, lovely holiday. A simple sweep at 30x using an 80mm refractor unveils a dazzling variety of every kind of object except empty space. There are over 1800 star clusters alone. Broaden your definition of ‘interesting’ and you’ll wish you hadn’t: the
Bica et al survey done in 1998 lists 7847 ‘extended objects’. Thankfully, that doesn’t include binaries.
Paddy, IIS’s resident guru about matters Magellanic, has done a magnificent job of making sense out of our Galaxy's closest neighbours. Quite by accident I came across a star chart that showed 10 globular and open clusters far to the south of Paddy’s limits. They turned out to be some of the more demanding objects I’ve ever tried, and therefore worth ever minute out there in the cold winds of the Karoo (not cold enough for the mozzies, alas) looming over the eyepiece like a vulture waiting for something to glimmer. Six hours over four nights and all I had to show for it was between three and a dozen tick marks under the various objects listed by number. I wonder if psychologists have considered studying we astronomers for latent masochistic tendencies coupled with a curious record of narcolepsy at dawn.
The 10 clusters I introduced yesterday are piffles in a puffball on the grand scale of things. Yet they have been so little studied that they may have the potential to add a new facet to globular cluster lore: the chemical history of delayed adolescence. What’s more, two of them—NGC 1777 and 2209—are open clusters, not globulars. Those two are also the toughest to get an unequivocal sighting. I’m still not entirely sure what I saw of them wasn’t wishful glimmer.
Yet it wasn’t for days after I observed them that I perused the literature using their NGC and IC numbers and discovered I had stumbled upon what we non-PhD lot regard as a sort of Ocular Holy Grail—something very few people have ever noticed, and even fewer have studied. The NED and Simbad data on them makes pretty skimpy reading. An ADS search for ‘LMC halo globular cluster’ brings up Kenneth Freeman’s name in items #3 and #4—in papers dated 1983 and 1987. As a category of study, the Magellanic halo clusters are a PhD or two awaiting suitable candidates.
Start with the difference between a globular and an open cluster. Do the Magellanic halo clusters follow different set of rules than the Milky Way’s? Let’s try age. When we think ‘globular cluster’ we automatically think of wizened decabillionarians glowing faintly into their dotage in serene loneliness. But the youngest fully accepted globular cluster is NGC 1818 in the upper LMC, at 40 million years. (This cluster has been featured 8 times on APOD since 1997.) Remoteness? Only a few globulars actually reside in the outer halo of our Galaxy, and most of them are the cores of former dwarf galaxies strewn like chicken bones on a plate after a feast by our Milky Way. (Why does the Milky Way make me think of Henry VIII?) The Milky Way bulge hosts over 20 contrarian GCs that were born before the bulge, watched their neighbourhood turn into a boxy tenement, and survived to tell the tale. Haute Province 1 (Ophiuchus) is presently the closest to the galactic core (1630 light years) and is roughly 13 billion years old. Core concentration? Nope there, too: Six Milky Way massive open clusters are far denser than Pal 3 in Sextans—we can see four of them in our scopes as NGC 3603, Westerlund 1 and 2, and R136 in the Tarantula Nebula. Some Class XII globulars such as NGC 7942 Aquarius are looser and less massive than a young hotshot like the Jewel Box. Poor NGC 7942 is falling headlong into its likely last passage through the Milky Way disc. Presently the loosest known globular, it is likely to end up a swarm of dissociated stars in roughly half a billion more years. (We could call this ‘Cluster’s Last Stand’ but that moniker
has already been assigned to Palomar 13.)
As you might expect, professional astronomers have a more complicated notion of the term ‘globular cluster’ than our airy eyepiece impressions of a beautiful speckled ball. They are more impressed with the shape and contents of the cluster’s horizontal branch in the Colour Magnitude Diagram—having a blue tail is is a prize asset in the horizontal branch’s celestial kennel. Astronomers also fancy the magnitude difference between a cluster’s red giant tip and the horizontal branch red clump; this is a measure of hydrogen envelope loss as the core fuses helium into carbon and oxygen. Arrival at the horizontal red clump signifies a star’s heaviest mass loss phase is over, a data point which helps determine the age and internal dynamics of the entire cluster. Unsurprisingly, astronomers have a name for this, the d(V-I) index, and aren’t you glad you don’t have to explain it all to your fiancée’s mom and dad when your qualifications as a candidate bridegroom arise over the dinner table. Astronomers also wring their hankies over whether a cluster exhibits an anti-correlation between its oxygen and sodium abundances, an innocuous-sounding phrase that carries a big stick when it comes to defining a globular cluster’s status. The [Na/O] reciprocal arises in connection with two less-known cyclical catalytic processes that affect an ageing star’s atmospheric chemistry, the Neon-Sodium and Magnesium-Aluminium-cycles. This is an estimable topic because the richness of earthly life forms we enjoy daily is the leftover dandruff of complex cyclic nuclear reactions in the horizontal branches and AGB phases of stars long since vanished into the cosmic graveyard. (If you can’t live another moment without knowing what the professionals talk about between midnight and 5 a.m., you can read all about the importance of the horizontal branch in globular cluster studies
here.)
I apologise for dumping all this into your lap at this ungodly hour, so I’ll put the horse back in front of the cart for now. All these factors and the uncountable pages of astrophysical jargon arising out of globular cluster studies have a very obvious shortcoming: what we know about the minute-by-minute pulsebeat of globular clusters comes almost entirely from Milky Way studies. MW globulars are very old and very chemically evolved compared with the youngsters in the Magellanic Clouds. Of the 10 Mensa clusters that we can see using a good scope on a good night, all are between 300 million and 3 billion years old. This age bracket is something of a gold standard for LMC halo globulars, but it is roughly 1/4 to 1/3 the age of most Milky Way globulars. Why? Surprisingly, the subject has not been studied all that well. The most detailed and yet digestible is
a 2007 study of 15 of the LMC’s middle-age globular clusters. It's an appetiser. There's lots more to come on the Mensa table.
Hence, when we visually observe the 10 Mensa globulars, we really are looking at objects about which no one really knows very much at all. See any or all of them and you can put away your scope for the night pleased that finally you have looked at something that hasn’t been picked over by legions of observers before you.