Starship Asterisk*

Why are all stars, planets, galaxies, natural satellites, and moons always rotating?

If two stars collided head on would they still retain their rotation or part of it?

Frank wrote:Why are all stars, planets, galaxies, natural satellites, and moons always rotating?

A better question might be, "why wouldn't they be rotating?", if you consider that there is no correlation between their various motions. Within any system, much of the material is found to be rotating (and revolving) in the same direction, which is the result of the conservation of angular momentum.

If two stars collided head on would they still retain their rotation or part of it?

Their angular momentum would be conserved. That is, the net angular momentum of the debris from the collision would be the same as the net angular momenta of the two stars.

Frank wrote:Why are all stars, planets, galaxies, natural satellites, and moons always rotating?

When any two objects collide and stick together, an off-center collision (overwhelmingly the most like kind) results in rotation. Two skaters approach each other across a pond; one holds a stick off to the side; the other grabs the end of the stick as they pass; they end up spinning. The same holds true for collections of multiple objects, like the stars, planets, galaxies, natural satellites, and moons when they originally formed.

Frank wrote:If two stars collided head on would they still retain their rotation or part of it?

Angular momentum would be conserved. If two stars of the same mass and equal and opposite spin (or one with double the mass and half the spin, etc. - anything that adds up to equal and opposite angular momentum) collided exactly head-on and merged, their angular momenta would cancel (sum to zero) and there would be no remaining rotation. In any other case, the resulting object would maintain the total angular momentum of its contributing components. Depending on the physical properties of the contributing components and the speed of the collision, different amounts of the original angular momentum would become either a rotation of the entire resulting system or spinning eddies within it. And some (maybe a lot) of the energy of the collision would simply be converted into heat, reducing the resulting angular momentum to compensate.

consider angular velocity of an object: this is real number (from -∞ to +∞) and the only value that describes non-rotating object is 0. now, what would happen if you tried to pick a value for angular velocity at random? it can be calculated that unless probability distribution is delta function (very exotic distribution) the probability to pick 0 is, cough, 0 that does not mean it cant never actually happen, but it means that chances of that happening are next to none, extremely slim, none at all, you get the idea

this is very screwy argument, so to make it simpler, your question is identical to "why all people are not exactly 1m 80cm high".