I am carrying out a research work and I am stuck at the first page where there is an introduction to the angular momentum and its relationship in the formation of the solar system. According to that paper, stars are formed by the accretion of interstellar cloud (or nebulla) into a young star and that when central part of the cloud will collapse to the proto-star, the sorrounding cloud will rotate even more faster since angular momentum is conserved in Nebula.* Furthermore, it is also mentioned that '...it is not clear how the angular momentum in the accretion disk is transported out of the system in order to allow the dusts and gases collapse into the proto star.** My question is why is the transfer of mass in this case being looked upon as the transfer of angular momentum? As far as I know angular momentum has something to do with motion rather than the transfer of mass. Also how to visualize physically the transfer of angular momentum. I am stuck!
2 Answers
I think the problem is that, given conservation of angular momentum it's hard to see how stuff gets close to the centre of the system at all: as the cloud collapses its moment of inertia decreases and so its angular velocity should increase. So if you took a big cloud and collapsed the whole cloud down to a star then you'd end up with a star with an enormous rate of spin, or in fact you'd not get a star at all because it just would not collapse far enough.
So what needs to happen is that some of the material collapses to form the star, and in doing so it must transfer angular momentum to the remaining material, so the star can form. And the mechanism for transferring angular momentum seems not to be known.
In the Solar system, for instance, the Sun accounts for essentially all the known mass, but only a tiny proportion of the angular momentum of the whole system, so somehow a lot of the angular momentum got transferred to the planets (which probably really means 'to Jupiter' as to a reasonable approximation it's the only planet).
This is a very hand-wavy argument but I think you can see what I mean.
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$\begingroup$ Thank you for your reply but the thing I didn't understand is the concept of 'the transfer of the angular momentum'. You also said that the cloud must transfer angular momentum to the remaining material in order that the star is formed but is it the transfer of 'material' or transfer of angular momentum? $\endgroup$– Thomson1Mar 30, 2016 at 11:07
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$\begingroup$ @Thomson1: it's really the same thing. Initially all the stuff that makes up the nebula has some angular momentum, and there will be a distribution of angular momentum among the particles/gas (which can be measured/computed). Finally (after the star is formed) there is the same total angular momentum but the distribution is now very different, with almost all of the stuff (in the star) having very little of it and the remaining tiny amount of stuff having hugely more. So somehow all that angular momentum got transferred to a relatively tiny amount of material. $\endgroup$– user107153Mar 30, 2016 at 11:50
Angular momentum transfer works through magnetic fields: if the central region is partially ionized it will create a magnetic field through the dynamo effect. This magnetic field can transfer the angular momentum from matter in the central region to matter in the outer regions (which obviously would have to be ionized as well in order to be affected by the magnetic field). So there is no transport of matter involved here, only transfer of angular momentum from matter in the inner region to matter in the outer region. See http://th.nao.ac.jp/MEMBER/tomisaka/Lecture_Notes/StarFormation/5/node94.html for more
