Starship Asterisk*

dlw wrote:Chris, I did allude to that when I mentioned orbiting. Let me answer with another question: can you place a geostationary satellite in orbit over the north pole?

Of course, you can't place a geostationary satellite in a polar orbit. But you can place a geosynchronous satellite in a polar orbit, and dynamically the two are the same.

I can imagine 300,000 objects orbiting in the same plane - we probably have more that that in our solar system. 300,000 objects in randomly inclined orbits around a common 'core' without occasionally colliding or at least interacting gravitationally is harder to imagine. Gravitational interaction would lead to distorted orbits, etc...

300,000 bodies in randomly inclined orbits are much less likely to come close to each other than they would if they were coplanar. Statistically, it is unlikely that any stars in a globular cluster will ever collide. Like multiple bodies in coplanar orbits, bodies in orbit at different inclinations are inherently chaotic. Nevertheless, it takes tens of billions of years for a globular cluster to evaporate due to angular momentum transfer during near misses. Indeed, the orbits are distorted from simple ellipses by gravitational perturbations... but that doesn't result in significant instability.

Globular clusters don't collapse because each star is in its own orbit around the center of mass of the cluster as a whole. Ultimately, the individual stars are ejected- they don't ever end up in the center.

Thanks. That was what I wondered - whether they were actually in orbital motion around a center of mass or whether they weren't or weren't moving at the required radial velocity and thus would that indicate the presence of dark energy which, as I've read, has a repulsive effect. So I thank you for clarifying that for me.

dlw wrote:Thanks. That was what I wondered - whether they were actually in orbital motion around a center of mass or whether they weren't or weren't moving at the required radial velocity and thus would that indicate the presence of dark energy which, as I've read, has a repulsive effect. So I thank you for clarifying that for me.

Dark energy is repulsive, but is very, very weak. It's effect is only significant over cosmological distances. Within a cluster, or even an entire galaxy, any repulsion it could provide would be infinitesimally small.

FWIW, globular clusters are small enough that they can be well modeled by dynamical simulations, so it's virtually certain that nothing is going on that can't be explained by ordinary (Newtonian) orbital dynamics. How the globulars formed in the first place remains pretty much a mystery, though.