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

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?

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.

• Why would it be very difficult to have a non-rotating body? Also, in thinking about it today, I came up with a potential solution to my own question. If all the particles in the cloud have a temperature (which is almost certain, since no known records of absolute zero exist), then they all have a motion, and therefore a vector quantity. Thus when the nebula reaches its critical mass (or density/volume), the vector sum of the particles gives it the angle of rotation (conserved during the collapse) and therefore noticeable spin. Is this the case? If not, where have I gone wrong?
– Nate
Feb 28, 2012 at 4:19
• I think you have partially answered your own question. There are all sorts of motion from very large to very small scale. The probability of the sum of these resulting in a net zero rotation is low. Feb 28, 2012 at 8:36

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.

Ваша теория не верна,от слова-СОВСЕМ!!! Если рассматривать "схлопывание" туманности на нашей СС,то такие скорости как орбитального,так и вокруг своей оси,случайным импульсом не создать. Первое что противоречит такому сценарию,это отсутствие жестких взаимодействий,то есть при ударе волны по облаку газа и пыли,произойдет деформация формы только периферийной части. Все вращения газа, происходят по асинхронному методу.вы,так же писали что скорость вращения туманностей всегда увеличивается к центру скопления.Так вот смею,Вас,заверить что это остаточное явление вращения от диска аккреций черной дыры-центра бывшей галактики! Смотрите сами,размеры туманностей от 1 до 100 н световых лет,что явно созвучно с размерами"ДА". Как пишут астрофизики,в таких туманностях идет активное звезд образование и это не удивительно. Ибо сам диск аккреций,очень насыщен самыми разнообразными газами,пылью и не только,как говорится имеются все условия для формирования солнечных систем.Да и стартовая скорость также имеется.А вот там где она практически отсутствует и появляются шаровые скопления звезд.

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• Hi, please answer and ask questions here in English so that more of us can understand what you're saying. Feb 21 at 2:37

Turbulence.

No rotation of the initial cloud is need for star formation to occur, and indeed rotation is a source of support against collapse.

The molecular clouds that collapse to form stars and star systems are turbulent on many scales. That turbulence is injected by energy and momentum from stellar winds and supernovae. Different pieces of the cloud move with respect to each other and any box within the cloud has its own angular momentum. 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.

Having said all that, we might expect most molecular clouds to have some rotation. Any object of significant size will have torques exerted on it due to the spatially varying gravitational tidal field of the Galaxy.

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.