How is it that some nebulae are rotating and others are not?

Question

First, I should make it clear that this isn't a question about angular momentum (unless I may have completely missed something). It is my understanding that a nebula must have some inherent initial spin before collapse to form a solar system (and if it doesn't have that spin, only a star is produced). Then, basically, Conservation of Angular Momentum takes hold (and this is where the rotation is more noticeable), thus producing the accretion disc around the star, then planetary bodies, etc.

Now, what determines the initial rotation of a nebula before collapse? Why is it that some nebulae do not have an initial rotation before they collapse?

Answer 1

While there is always Some element of spin (it would be very difficult to have an absolutely non-rotating body, or cluster of bodies) if it is very low, we can treat it as effectively zero, as there are various drag effects as the nebula coalesces.

The reason most objects (whether they are nebula or not) do end up rotating is from the very effect you note - as you move in towards the centre, rotation effects increase due to the Conservation of Angular Momentum, and these outweigh drag effects in a lot of cases.

Answer 2

As @Rory said, there will always be some spin, but it can vary from very little to very much.

The primary source of spin in nebulae (also for galaxies), is torque from the local environment. Nearby nebulae, clouds, stars, etc can exert a net torque on the nebula (or galaxy) giving it angular momentum. This is the primary source of spin for nebulae.

After inflation, there were random over-densities of angular momentum in different regions---just like there were over-densities of mass, etc. This effect is primarily important for larger objects (e.g. galaxy clusters), and I don't think it has any roll on nebulae... but I'm not positive.

Answer 3

Turbulence and tides. The interstellar medium (ISM) suffers a continual injection of energy and momentum from stellar winds and supernovae. This results in turbulence within the ISM that means that any part of the ISM is in motion with respect to any other and this inevitably leads to an inhomogeneous distribution of density and angular momentum. i.e. Even if net angular momentum is zero on large scales, if we zoom in on individual pieces they will have their own positive or negative angular momenta.

To add to this there are torques supplied by gravitational interactions between bodies, even if those bodies don't directly interact. Tidal torques can transfer angular momentum between orbital motion and spin and the fragmentation and distribution of angular momentum between bound pieces is a common feature on many scales (e.g., Barnes & Efstathiou 1987). For example, large clouds orbiting within the tidal field of the galaxy will experience an angular acceleration in their frame of reference as torques act on the tidally deformed cloud to bring its spin and orbital periods into synchronization.

However, to be clear, no rotation of the initial cloud is needed for star or planet formation to occur, and indeed rotation is generally a source of support against collapse.

The molecular clouds that collapse to form stars and star systems are also turbulent on many scales. As clouds collapse, their increasing density leads to a shrinking Jeans mass and they fragment into smaller collapsing pieces. Each of these pieces inherits the angular momentum of that piece of the cloud, collapses further and may fragment again.

Even if the initial cloud had zero angular momentum, the star systems formed would still be spinning. That can be found in theoretical simulations of star formation in turbulent clouds, which rarely give the clouds any initial angular momentum, but produce lots of binary systems, discs etc, which obviously do (e.g. Bate et al. 2009).

That the initial angular momentum is of little importance can be seen by the fact that clusters of stars are not in general found to have aligned spin axes.

Answer 4

I am sure you have since figured this out, since it’s been several years since you asked, but I had this question earlier, and more research answered my question, so I figured for others who may have a similar question, I’ll answer with what I found.

So apparently, some of Earth’s oldest meteorites contain xenon-129, which is a gas, even at relatively low temperatures, and it binds to nothing since it’s noble. This means that it wouldn’t have condensed and mixed with the rock during accretion, and therefore we must conclude that it is a by-product of radioactive decay. The parent isotope is iodine-129 which has a relatively short half-life relative to the age of our solar system (17 million years). Furthermore, it’s a very heavy isotope, so it’s likely from a supernova explosion. Since there was a short time between the explosion and the formation of the solar system, it is believed that such an explosion sent shockwaves through our nebula, therefore triggering the rotation. From there, gravity took over to condense the nebula, and the rest is just the Nebular Theory.

In general, it appears to me that things tend to trigger the events, but it may very well be in part due to thermal fluctuations, though the average temperature of the universe is about 3K, which is barely above absolute zero, and therefore might be negligible because of its low density, though I wouldn’t quote me on that.

Source: The Cosmic Perspective, Bennett, et al.