Globulars -vs open clusters

[Weltevreden SA]

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 (http://adsabs.harvard.edu/) 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 (http://www.wolaver.org/Space/30Doradus_R136.htm) 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

[Paddy]

Thanks for this wonderfully luminous and illuminating post Dana! I love the way you've put the science in and then come back to the experience at the eyepiece.

[Weltevreden SA]

Thanks, Paddy. I have a sort of unwritten rule for myself, 'If I can't see it, I don't write about it.' That may limit me to what an 8-inch scope can pull in under good skies, but it's also enough to keep me going out there night after night. I salve my bruises at never being able to get a bigger scope (this is S Africa) by wandering over to the IIS images forums. You fellas have some marvelous astrophotographers down there. These days I go to IIS before WikiSky when I want to know what a certain object looks like.

=Dana

[ChrisM]

Thanks Dana for a very interesting post. Is there an explanation as to why a binary typically emits only 75% of the light that would ordinarily be emitted by the two stars if separate?

Chris

[Weltevreden SA]

Thanks for the question, Chris. The 75% figure is related to bound stellar systems (aka clusters). It is a log-normal distribution function (http://en.wikipedia.org/wiki/Log-normal_distribution) (sorry about that) determined by he properties of star clusters as bound systems versus associations as non-bound. In a bound system the more massive stars clump toward the centre in part because they are more massive, but more important because they drive away the dense formation gas that made them in the first place. The mass of the gas ejected can be two to three times the mass of the remaining stars. This makes for a significant mass-energy imbalance. The technical term is virial disequilibrium (http://en.wikipedia.org/wiki/Virial_theorem). Several things happen as a result: (a) orbital densities become tight in the central region and many of the massive stars form close binaries whose orbits are only a few tens of their diameters, hence they rapidly rotate, (b) close binaries bulge into teardrop shapes facing each other called Roche lobes, which fill with envelope gas, (c) the quantity of this Roche gas varies depending on the stars' distances, but generally is 1/4 to 1/3 the total envelope surface mass; (d) the Roche gas is thinner and cooler so it no longer radiates as much light per unit surface area, and (whew) finally, (e) the log-normal distribution for the combined number of stars versus the total amout of emitted light comes out to the peak of a curve at 75%. It's a calculated rule of thumb that has been shown to have utility. Other calculated functions have found utility as well, e.g., the half-light and half-mass radii, the dynamic time (orbit time of a typical star) and crossing time (time it takes for a typical star to cross the cluster). The word 'typical' in this context is a little like faces in a crowd: each is different close up, but they mostly look the same at a distance.

[ChrisM]

Thanks Dana for the detailed explanation. I find it amazing how much is known about these far-away stellar systems, and it all helps to enrich the observing experience.
Chris

[Renato1]

Thanks for the interesting information, Dana.

Though I still feel we need a new class of DSO - maybe an OGC (Open Globular Cluster) to distinguish between the clusters that look almost the same as Globular Clusters (the ones that formed when the galaxies formed, and haven't formed stars since (except for Omega Centauri)) and regular open clusters which usually don't look much like globular clusters and have less stars.
Regards,
Renato.

[Weltevreden SA]

Your point is a good one, Renato. Surrounding the technical distinction between open and globular (permanently bound or not) is a maze of contributing factors that varies considerably around just two entities: system particle density and system energy density. 'Particle' density means a vast range in mass bridging the number of atoms per cubic centimeter ascending up to the number of stars per cubic parsec (3.26 cubic light years). On these scales, atoms and stars are mathematically considered 'particles' for computation models. In both cases individuals are tiny compared with the sample volume of space they occupy, and interact as point sources with specific properties that affect other particles in the same way. Think of a swarm of bees flying through a giant gnat cloud and you get the drift.

Turning this into numbers can get moderately complex, but it's piffle compared with calculating the effects of energy density. Gravity, light, kinetics (the energy of large objects moving in a direction), thermal (tiny objects moving at random), magnetic fields, even sonic turbulence. Space has sound, and there are screams in space. We can't hear them, but we can see them—shock waves of stellar gas expulsions give planetary nebula their shapes; turbulent shock waves make nebulae like M17, Orion, and Carina look like shredded cirrus clouds at sunset; and spiral arms are density concentrations not much different than the gong of a bell rung once a long time ago. The speed of sound in a collapsing supernova is 72,000 kms/sec. If we could hear these, Kepler's Music of the Spheres would be a music of rips, tears, shrieks, wheezes, gurgles, and a loud bang when a supernova shock front hits a snoozy little hydrogen cloud out there peaceably doing not much. These descriptions would horrify a young Ph.D with fresh diploma in hand, but after reading page after page of what these folks write, I'll take bees, gnats, John Cage, and pine boughs in a crosswind (the behavioural analogy of spirals in galaxies), any day of the week.

Whether the descriptions be florid or mathematical, the basic problem remains: particle densities span a large continuum, and so do energy densities. You can have low-particle, low energy density at one end and find yourself in the middle of an intergalactic void; and high-particle, high-energy density on the other and be in the middle of a supernova. Star clusters occupy a span somewhere in the middle. On one side are associations so tenuous they barely hang together (Lupus-Centaurus or Chamaeleon), and a Class I globular like NGC 2808 so tight no force in the universe can sunder it. N2808 could eat 100 simultaneous supernovae and spit out carpet tacks. The 'open' or 'globular' appellations are merely a lone reliable milepost on the long road between not much and a lot, nowhere and singularity, roots and wings.