3 ms·

As an aside, is this dynamical friction completely analogous to friction in gases? Do stars reach "thermal equilibrium" in their velocities, and can be somehow

• antognini 11y ago
  It's very similar, but not completely analogous to friction in gasses. But the long-term result is the same. An object moving through a gas will slow down until it comes to rest with respect to the gas. Similarly a massive object moving through a medium of stars will slow down until it comes to rest with respect to the stars. There are many parallels between thermodynamics and orbital mechanics. In fact, a collection of stars can be thought of as a self-gravitating gas where the particles don't collide (they're so far apart relative to their size that collisions are very, very rare). The analogous concept to thermal equilibrium in gravity is called "virial equilibrium." One of the very interesting things about a self-gravitating system in virial equilibrium (like a cluster of stars) is that it has negative specific heat! So if it collapses it loses energy, but the stars move faster so it heats up!