I should probably know by now, but this article http://www.nytimes.com/2001/07/31/science/31OBSE.html prompts me to ask -
Why DO objects like stars and planets rotate?
Do these objects all rotate in the same direction?
Why do they rotate at different speeds?
Well, the one word answer is “Gravity.”
But it’s not quite so simple. Rotation is a form of energy storage so it is conserved. If angular momentum is destroyed, some other type of energy must be created. And while there are forces that slow down rotation, there’s no particular tendency for things to be at exactly zero rotation, so you’re going to see a random distribution of angular momentums and axis orientations. And the forces that cause spinning objects to slow down, tidal forces, are rather weak, so once an astronomical object starts spinning, it’ll keep spinning for a long, long time. These same forces also tend to lock spinning objects into a synchronized spin with their orbit around larger objects, which is why the mMoon always keeps the same face to the Earth.
One of the reasons that things start spinning is that gravity and orbital mechanics works to promote two objects that are gravitaionally bound to rotate around their center. This puts an angular momentum on the system as a whole, and when the two objects collide, this angular momentum is distributed to any surviving objects.
This effect causes what is called an accretion disk to form around things like black holes and coalescing protostars. As matter “falls into” the central object, it’s more likely to not exactly strike it, and the laws of orbital mechanics will make the object loop around and make another pass.
And also, a random collision between two objects isn’t necessarily head on, along a line drawn between the center of masses of the two objects. Thus, these glancing blows will cause objects to spin. Think of striking a cue ball off center to give it “english.”
And since most astronomical objects are made up of recycled material that was once part of another astronomical objects, new ones will be born with some angular momentum anyway.
Thanks for the information. One more question along these lines - If an object’s gravitational rotation is affected by it’s mass, shouldn’t a larger object rotate faster than a smaller one? The article above mentions that our sun rotates 1x every 30 earth days. I remember reading that Jupiter rotates 1x every 10 or so earth hours.
Well, there’s mass and angular momentum that both figure into it. If two objects are the same mass and have the same angular momentum, the one with the larger diameter will rotate slower. The physics 101 example is the ice skater who rotates faster when she pulls her arms in and slower when they’re put out.
But this isn’t really my field. Someone will be along shortly…
Angular momentum isn’t a form of energy; it’s another thing that’s conserved all by itself. The reason that so many things rotate basically comes down to there not being any reason for them to not rotate: If a cloud of gas has any interal motion at all, it’s likely to have at least some angular momentum, and if that cloud collapses into a star or planet, it keeps that angular momentum. The spins of stars, planets, and other such things can be any which way. For more details, I’ve got a Staff Report on the subject.
When you’re dealing with angular momentum, it’s not the mass that you’re interested in, but something called moment of inertia, which is a combination of the amount of mass and how it’s distributed. The figure skater weighs the same with her arms in or out, but her moment of inertia is smaller with her arms in. Just like regular momentum is mass times velocity, angular momentum is moment of inertia times angular velocity (RPMs). The skater’s moment of inertia decreases when she pulls her arms in, and her angular momentum stays the same, so her angular velocity must increase.