# Thread: What causes primodial stars to spin

1. ## What causes primodial stars to spin

I've been reading Einsteins Theory of Relativity and more in particular the principal of Equivalence and gravitational theory.

I can conceptualize the idea of dust and gas clumping together in a nebula. The more gas and dust the greater the gravitational effect the clump has on the area surrounding it, until the mass becomes so great that it causes fusion to begin.

What I am wondering is what causes the initial mass to begin spinning. Is it a 'whirl' in the gas that starts the initial clumping and it (excuse the pun) spins off from there, or a uneven distribution of the matter causes the in-falling matter to hit at an angle to the main mass?

2. Conservation of momentum.

The same thing that causes a figure skater to spin faster when she pulls her arms closer to her axis of rotation will magnify any tiny bit of rotation in the cloud of gas when it collapses into a star.

3. Established Member
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Jan 2008
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Originally Posted by phunk
Conservation of momentum.

The same thing that causes a figure skater to spin faster when she pulls her arms closer to her axis of rotation will magnify any tiny bit of rotation in the cloud of gas when it collapses into a star.
That's about the size of it.

4. I think it's also a mistake to think that anything has to start it spinning. Basically, everything in the universe has rotation. An interesting way to look at it is this: saying that something isn't spinning is really just a way of saying that its rotation is precisely zero. And on average, this is infinitely unlikely to happen.

Going out on a limb, but I wonder if maybe the total rotation of all of the mass of the universe would add up to zero?

5. Originally Posted by rommel543
What I am wondering is what causes the initial mass to begin spinning. Is it a 'whirl' in the gas that starts the initial clumping and it (excuse the pun) spins off from there, or a uneven distribution of the matter causes the in-falling matter to hit at an angle to the main mass?
Any existing cloud will have had a number of things happen to it to give it some initial spin from the days of its primordial formation (ie Big Bang). Early supernova, bipolar stellar flows, gravitational stresses from normal and dark matter, galactic density waves, etc, all contribute to dynamic behavior for clouds.

Within some clouds are supersonic flows that create shockwaves that also add to the dynamic fun. Once the cloud begins to condense, many hundreds, thousands, and sometimes hundreds of thousands of clumps of gas and dust become separate from the original cloud. These clumps will form the individual stars, or multiple star systems. Accretion disks soon form and the protostar should be spinning extremely fast, but they don't, which has been a mystery. The evidence is mounting favoring a few ways the protostar sheds its angular momentum to the disk, otherwise stars can't form. Of course, evidence supports the idea that stars do form.

6. I have a lazy mind. Painting massive images of coalescing masses converging and compressing... No, I have what works for me, I will share. ie;
Nothing in space is still. Every thing has motion in regard to every thing else. Any body in motion will interact with other bodies in motion. If its a galactic mass or just a few molecules. Size does not mater at all. Its all got radial motion in relation to the surrounding mass. Drawn together as friends or by gravity it's all going round. It just does... Stir the coffee, watch the froth... (only good for the demonstrations) Its not science, but it helps perception., and then drink the coffeeee...

7. "Going out on a limb, but I wonder if maybe the total rotation of all of the mass of the universe would add up to zero?" end quote...

If we can conclude that it is expanding and, moving away at some velocity. For the same reason you used. Then its probably rotating. I do not see zero. How can we ever know ? I have no idea.

8. Order of Kilopi
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Dec 2004
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11,218
Stars form from giant clouds of gas and dust. In order for any part of the
cloud to collapse into a star, it has to be sufficiently dense. If it is dense,
it is opaque. That means the interior is dark. The surface of the cloud
radiates heat and the interior cools. That is what initiates the collapse.

Even a very cool cloud has heat, though. The gas and dust particles are
in random motion. Every particle has its own motion in some random
direction relative to all the particles around it. That random motion
means that the cloud has internal angular momentum. Because the
motions of the particles are thermal, and therefore random, and because
there is an enormous number of particles in the cloud, you couldn't just
look at the cloud and see that it has a net angular momentum in some
particular direction. Even if you could see every individual particle, the
motion would be too complex to determine the net angular momentum.

As part of the cloud becomes cooler, it shrinks, making that part of the
cloud denser, which makes collisions between particles occur more often.
When collisions happen frequently enough, the shrinking part of the cloud
collapses into a thin disk. The orientation of the disk is random, and not
predictible from the motions of the particles before the collapse. It is
determined by vast numbers of chance collisions. But the distribution and
speeds of particles within the disk depend on the angular momentum of the
particles in the cloud before it collapsed. If the plane of the disk happens
to align with the direction of greatest angular momentum of partcles, the
disk will have many particles far fom the center. If the plane of the disk
happens to be far from aligning with the angular momentum of the cloud
particles, more particles will end up at or near the center.

I can't explain in detail why the collapse ends with a rotating disk, but in
general: When two particles collide and do not stick together, they change
each other's speed and direction of motion. A small light particle's motion
is affected more than a large, heavy particle. If a particle going in one
direction hits a lot of particles going in another direction, it will end up going
in the same direction and same speed as all the other particles. When two
particles collide and stick together, their new speed and direction of motion
is an average of the two individual particle's motions. Particles moving in
opposite directions which collide and stick together will tend to cancel each
other's angular momenta and fall toward the center of the disk. Particles
which are moving in nearly the same direction collide more gently and end
up moving in about the same direction as before the collision. This all ends
up as a disk with a big lump of matter at the center. The particles are far
closer together in the thin disk than they were in the cloud, so collisions are
far more frequent. Particles end up in very nearly circular orbits around the
central mass, so collisions are low speed, making it more likely for particles
to stick together. The disk is very much like the rings of Saturn, just far, far
bigger with far, far more material in it.

That help?

-- Jeff, in Minneapolis

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