3. Models of Globular Clusters


Messier 36
M 36
The spatial structure of a cluster, i.e. the density function, does not stay the same, but evolves slowly with time as a result of simple dynamical principles: Encounters between stars perturb the orbits, and exchanges of energies occur, which affect the evolution of the cluster as a whole. By the mid-1950's it became obvious that globular clusters cannot be stationary objects, but face different stages throughout their lives since formation. Theoretical models have been developed to understand the physical processes, and two different approaches became spectacularly successful.

The first model by Richard Michie and Ivan King was based on the so-called "lowered Maxwellian" distribution of velocities. The velocities of the stars in a cluster decline exponentially to a high cutoff, beyond which they have enough kinetic energy to escape. If they cross a certain boundary, the tidal radius, these high-velocity stars are no longer gravitationally bound to the cluster and join the galaxy's stellar halo. Thus, globular clusters are to evaporate slowly, with the galactic tides constantly stripping away their stars.


The second model developed by Michel Hénon at about the same time did something entirely different: His simulations showed up a slowly changing equilibrium leading to an accelerated infall of the core. The reason lies in the random motions of the stars that are considered to balance the stars' mutual attractions. In the crowded centre, close encounters happen quite often and produce fast-moving escapers. These stars are pulled out of the core and take away some kinetic energy. The core needs to find a new equilibrium and shrinks a little. This increases the gravitational binding energy, and the remaining stars have to move a bit faster (they "heat up") to compensate for the higher potential energy. Then, however, they are more likely to be pulled out. The process repeats itself and accelerates into an uncontrollable runaway. The event the cluster is heading to is called the "gravothermal catastrophe" or "core collapse". Gravothermal catastrophe
Gravothermal catastrophe

Virtually the same process of core collapse happens in protostars, when the gas cloud falls in, but with one crucial difference: In protostars the core becomes hot and dense enough to ignite thermonuclear reactions. These reactions provide energy that compensates further shrinking. In globular clusters different energy source stops the collapse.




2. Some observational characteristics   |   4. The core collapse
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