Massive Stars Spin Faster: The Reason Explained

[Sean Pan]

It is said that massive stars spin faster than less massive ones and I am always wondering why.Could someone please tell me the reason? Thanks a lot.

 

[zhermes]

Hi Sean Pan, welcome to PhysicsForums. The basic idea is that more massive stars formed from larger molecular clouds. Larger molecular clouds had more angular-momentum, and that angular-momentum is (largely) conserved in the star-formation process. Thus you end up with a faster spinning star.

 

[Sean Pan]

Thanks, but there are many processes in the forming of stars that can reduce the angular momentun of the centural stars. Maybe I should have paid more attention to its initial angular momentum, but other factors should also be considered. Since star forming last a very long time, I think the final state may not depend largely on its original states.

 

[Ken G]

We would expect all stars to spin rapidly, because we believe there is always ample angular momentum in the molecular cloud. So the question is not so much why do massive stars spin faster, it is why do low-mass stars spin slower. This is an ongoing research question, but one idea is that they tend to have a strong magnetic coupling with the gas that is forming them, and this coupling involves magnetic field lines that connect the rotating star to gas that is very far away from the star, which is in orbit. Kepler's laws say that the farther away gas is, the longer is its orbital period, so you have a rotating star with a short rotation period connected to gas with a long orbital period, and this tends to rob the star of angular momentum (and send it out to that gas way out there). Then you need a mechanism to get much of the high-angular-momentum gas to escape the system, and you can "spin down" your star (since this can happen with an accretion disk, it is also called "disk locking"). I'm not sure what the present status is of understanding how reliable this mechanism is, but no doubt many questions remain unanswered. For one thing, we might imagine that high-mass stars could also lose angular momentum in similar ways, so then we'd be back to asking why they spin so fast. It is thought that high-mass stars are even more likely to form in close binaries, which can then merge and convert the orbital angular momentum of the merging stars into spin. But that can happen to low-mass stars too, so then we are back to asking why low-mass stars spin so slowly! If you look at young low-mass stars, you find the younger they are, the faster they spin, so they are losing rotational angular momentum long after than have formed. Here interactions between magnetic fields and the winds from the stars are thought to play a key role, but you then have to explain why the winds are so strong in young stars. So you see, there is plenty of grist for the research mill here!

 

[Drakkith]

Regardless of the mechanism for shedding angular momentum, I would guess that one of the basic reasons is that more massive stars simply have much more momentum to shed to slow down to a given rotation rate.

 

[Sean Pan]

We would expect all stars to spin rapidly, because we believe there is always ample angular momentum in the molecular cloud. So the question is not so much why do massive stars spin faster, it is why do low-mass stars spin slower. This is an ongoing research question, but one idea is that they tend to have a strong magnetic coupling with the gas that is forming them, and this coupling involves magnetic field lines that connect the rotating star to gas that is very far away from the star, which is in orbit. Kepler's laws say that the farther away gas is, the longer is its orbital period, so you have a rotating star with a short rotation period connected to gas with a long orbital period, and this tends to rob the star of angular momentum (and send it out to that gas way out there). Then you need a mechanism to get much of the high-angular-momentum gas to escape the system, and you can "spin down" your star (since this can happen with an accretion disk, it is also called "disk locking"). I'm not sure what the present status is of understanding how reliable this mechanism is, but no doubt many questions remain unanswered. For one thing, we might imagine that high-mass stars could also lose angular momentum in similar ways, so then we'd be back to asking why they spin so fast. It is thought that high-mass stars are even more likely to form in close binaries, which can then merge and convert the orbital angular momentum of the merging stars into spin. But that can happen to low-mass stars too, so then we are back to asking why low-mass stars spin so slowly! If you look at young low-mass stars, you find the younger they are, the faster they spin, so they are losing rotational angular momentum long after than have formed. Here interactions between magnetic fields and the winds from the stars are thought to play a key role, but you then have to explain why the winds are so strong in young stars. So you see, there is plenty of grist for the research mill here!

Thanks a lot for your very detailed analysis!