Angular Momentum in a Solar Nebula

[Comeback City]

TL;DR Summary

Why does a solar nebula begin to spin as it contracts?

Hello all! Hope everyone's been doing well!

My question relates to the nebular theory of solar system formation. It is generally accepted that via the nebular hypothesis, matter in a nebula contracts on its own gravity and begins to spin, but I'm having trouble understanding why it must begin to spin. If angular momentum must be conserved, and we see a nonzero amount of angular momentum in the solar system, there must have been an equal amount of angular momentum that existed in the system initially compared to what is seen now, yet I cannot think of a reason this could occur. I don't know any of the fundamental mathematics of Big Bang Theory yet, but it seems obvious that no angular momentum could have simply existed from the beginning (maybe this is where I am wrong, if localized quantities of angular momentum cancel with others to net to zero in some way?)

My instincts lead me to believe that the answer comes down to random chance, since separate stellar systems tend to follow no pattern of which direction they begin to spin when forming from nebulae.

To sum up, I feel as if there must exist angular momentum in the nebula before it begins to collapse, but I am confused as to where this originates. There's a good chance I'm missing something obvious Any help would be appreciated!

Thanks again,

Comeback

 

[hmmm27]

Why does a solar nebula begin to spin as it contracts?

With utmost respect... why wouldn't it ? That being said there are (probably, I'm not an astrophysicist) systems out there which didn't spin enough, and are just a sun.

 

[PeroK]

Summary: Why does a solar nebula begin to spin as it contracts?

Hello all! Hope everyone's been doing well!

My question relates to the nebular theory of solar system formation. It is generally accepted that via the nebular hypothesis, matter in a nebula contracts on its own gravity and begins to spin, but I'm having trouble understanding why it must begin to spin. If angular momentum must be conserved, and we see a nonzero amount of angular momentum in the solar system, there must have been an equal amount of angular momentum that existed in the system initially compared to what is seen now, yet I cannot think of a reason this could occur. I don't know any of the fundamental mathematics of Big Bang Theory yet, but it seems obvious that no angular momentum could have simply existed from the beginning (maybe this is where I am wrong, if localized quantities of angular momentum cancel with others to net to zero in some way?)

My instincts lead me to believe that the answer comes down to random chance, since separate stellar systems tend to follow no pattern of which direction they begin to spin when forming from nebulae.

To sum up, I feel as if there must exist angular momentum in the nebula before it begins to collapse, but I am confused as to where this originates. There's a good chance I'm missing something obvious Any help would be appreciated!

Thanks again,

Comeback

If you have a large system of particles then you would need some sort of coincidence for the angular momentum about the centre of gravity to be zero. If a nebula is formed from matter from several sources, then it's very unlikely the total AM will be zero.

The AM may be relatively small, but as the system contracts the angular velocity will increase to conserve AM.

 

[Comeback City]

If a nebula is formed from matter from several sources, then it's very unlikely the total AM will be zero.

This makes sense simply due to asymmetry.

If you have a large system of particles then you would need some sort of coincidence for the angular momentum about the centre of gravity to be zero.

With utmost respect... why wouldn't it ?

Respectfully again, I don't believe this answers the question. I see how contraction is obviously inevitable, yet I don't quite understand why angular momentum is inevitable in this case. What did come to my mind from @PeroK was this...

Most nebulae likely form from more than one source, making asymmetry unlikely. This would imply that neighboring systems may have opposite directions of angular momentum to cancel out in a sense (though they don't necessarily have to be neighboring, this would still be based on random chance.) Overall, there should not be any net angular momentum in the universe, unless I am gravely misunderstanding things.

Also, I did not seem to account for conservation of energy in my initial question, so perhaps an answer lies there? Yet this in itself wouldn't account for the direction of angular momentum.

 

[mathman]

Since the nebula starts with a non-zero AM to start with, it becomes more noticeable as the nebula contracts (conservation of AM).

 

[Comeback City]

Since the nebula starts with a non-zero AM to start with, it becomes more noticeable as the nebula contracts (conservation of AM).

@PeroK @hmmm27

I finally found some time to ask my astrophysics professor this question, and he managed to clear it up for me. I explained how it did not make sense to me that something with zero initial angular momentum could ultimately spin, thus creating a nonzero angular momentum. With that in mind, my question was more along the lines of, "Where does the angular momentum of the nebula come from, since it must be present?"

His answer was roughly what I thought it might be, though clearly in more depth than I would have pictured. He explained that the universe's initial angular momentum is somewhat irrelevant to the question. Rather, random motion and gravitational interactions of matter in the early universe made it practically inevitable for local clusters of matter to gain nonzero angular momentum. He also mentioned turbulence as an important factor resulting from inflation in the early universe, which I did not quite understand, but will do some digging into and find an understanding of it. If anyone knows any good sources for this, that would be awesome, but if not I will find some on my own.

Basically, localized angular momentum is inevitable, and thus accounts for why galaxies and stellar systems have angular momentum, but this does not speak much (if at all) to universal angular momentum.

 

[phinds]

Basically, localized angular momentum is inevitable, and thus accounts for why galaxies and stellar systems have angular momentum, but this does not speak much (if at all) to universal angular momentum.

"Universal AM" cannot mean that the entire universe has angular momentum because that would imply that there is a center for it to revolve around and there isn't.

 

[Comeback City]

"Universal AM" cannot mean that the entire universe has angular momentum because that would imply that there is a center for it to revolve around and there isn't.

I agree, and will admit for some reason I did not think about it like that. Before I heard the answer my professor explained to me, I suppose I was trying to think of ways angular momentum could exist initially for these systems, but even saying Universal AM was flawed.