Recently IIS regular Renato1 asked why Magellanic massive clusters are shuttled between the current definitions of globular versus young massive versus open cluster. To us at the focuser knob, they are round, dense toward the core, and have no outstandingly brighter stars across their surfaces. What’s not to like?
Professionals, however, beg to differ. They can get a bit wrapped around the axle over this subject. Type LMC, Large Magellanic Cloud, globular cluster into an
ADS search box and watch the show. Don't do this until the weather folks predict a couple of cloudy nights. Brew a pot of coffee and set out a couple of toothpicks to prop open your eyelids. It’s amazing how much they have to say about so seemingly simple a definition.
Things move so quickly in the world of professional studies that it’s hard to pin down exactly what a universally accepted definition of ‘open’ versus ‘globular’ might be today. Some studies do their defining based on data in a few specific wavelengths available via the imaging equipment of the observatories that accept their RFTs (request for time). Others depend on mathematically derived values such as the ∏ (cap pi) value, which is the ratio between the average time it takes a star to cross the cluster and the average number of stars at a uniform distance from the core (called surface density) of the stars doing the crossing. I dunno about you, but I’m sure not waiting to set up the scope till all the professionals agree on this stuff.
More important is the question of binaries in globular clusters. Binarism is a key issue because a binary emits only around 75% of the light of the two stars if separate, but the local gravitational influence of a binary falls off more quickly with distance than the local influence of two separate stars. That has an effect on dynamic equilibrium, which in turn relates to how well a cluster can hold onto its outer stars. If a 40% binary rate typical of globulars under 1 billion years old can rise to 70% binaries by three billion years (also typical), the cluster will move towards sphericity. Cluster elongation (nonsphericity) negatively effects star loss rates because the stars can more easily slip free of the cluster’s gravitational well via the short axis and evaporate into the galaxy. The last thing a globular needs is easier paths of escape for its stars. Hence astronomers put a lot of weight into sphericity when they define what exactly is a globular and what is a massive open cluster.
We eyepiece-huggers down on the ground can see an out-of-round globular being made before our very eyes. The merging
massive cluster pair R136 in the heart of the Tarantula Nebula will almost certainly become a globular in, say, another billion years. Right now, at 200x or more in a 150mm or larger scope, we see a marvelous ball of glitter with a dark void around it, beyond which is the great whomping emission region of the Tarantula. That glitterball is two 10,000+ solar-mass clusters beginning to merge.
The result will be a way out-of-round globular for billions of years. Eventually it will ‘globularize’. For me, I don’t care what they call it. I want to look at it and wonder if it is looking back at me. Happens every time I put R136 in the eyepiece field.
=Dana
