Sciences on Moon

[QuantumJo]

Why do they spin?
A star is formed when gas and dust compress due to gravitational forces and collapses under its own weight. What I wonder about is why does the new star start to spin. What force is responsible for this? I have done some research on angular momentum but I am still not clear on what starts the rotation.

[jtmckinley]

Originally Posted by QuantumJo

Why do they spin?
A star is formed when gas and dust compress due to gravitational forces and collapses under its own weight. What I wonder about is why does the new star start to spin. What force is responsible for this? I have done some research on angular momentum but I am still not clear on what starts the rotation.

http://www.pbs.org/deepspace/experts/week1b.html

The guy in the link above says tidal forces between the various objects in the cloud, which is probably correct.

If I had to guess off the top of my head, it might have something to do with the curvature of space around massive objects, but I'm just guessing here, I didn't look it up or do any calculations. If a particle has an initial velocity and enters a gravity well of another object, unless it was initially heading directly at the center of mass, the trajectory would curve toward the center of mass so if it was captured I think it would spiral into the gravity well. Think of a marble being flung into the top of a horn shaped tube but not right down into the hole, a bit off to the side, it spirals around as it goes down to the bottom. I'm guessing that as a bunch of them come together, the angular momentum of the resulting collapsed cloud would be non-zero and the resulting object would be spinning no? The rate of spin would then maybe be proportional to the non-uniformity of the original cloud? I'm not an astrophysicist though, so I might be full of it .

[VAXman(Admin)]

Originally Posted by QuantumJo

Why do they spin?
A star is formed when gas and dust compress due to gravitational forces and collapses under its own weight. What I wonder about is why does the new star start to spin. What force is responsible for this? I have done some research on angular momentum but I am still not clear on what starts the rotation.

It's time for some tensor calculus! How's your grasp of the curvature tensor?


Seriously, I doubt you'd be ready to grasp that. The spinning is easily explained when you understand the conservation of angular momentum. L = r X p where r is the radius from where the particle is in space relative to an origin and p is its linear momentum. Then a particle turns the angular momentum (L)is a pseudovectorial component perpendicular to the plane of the radial component r and the tangential momentum vector (dp/dt). This can easily be demonstrated in a gyroscope. As a mass acrues, it will create a gravitational "well" and a particle flying by will be caused to curve by it. In the beginning this will be a rather random event but as the mass acrues more "participating particles", the sum effects of one group particles will outweigh the others; eventually, there will be a concensus rotation in one direction. Also, since the momentum must be conserved, as the mass grows and pulls in more mass, that mass will spin faster and faster. It's like having a weight on a string you are spinning about your head. To maintain it spinning in a plane above your head at a distance, it will move slower. As you pull the string in making it shorter, the mass will move faster and faster. A pirouetting ice skater is another great example.