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Sean Pan
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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.
Ken G said: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!
The rotation rate of a star is determined by its initial angular momentum, which is influenced by the amount of mass it has and how it formed. Massive stars have a higher initial angular momentum due to their larger mass, which causes them to spin faster.
The rotation rate of a star can greatly impact its lifespan. Faster rotation causes increased mixing of elements within the star, which can lead to a shorter lifespan due to faster consumption of fuel. However, slower rotation can also lead to a shorter lifespan due to less efficient energy transfer processes.
Yes, the magnetic fields of a star can also play a role in its rotation rate. The interaction between the magnetic field and the material within the star can cause it to spin faster or slower. In some cases, magnetic fields can even cause a star to stop rotating altogether.
Yes, a star's rotation rate can change over its lifetime. As the star ages, it can lose mass through stellar winds, which can cause it to spin faster due to conservation of angular momentum. Additionally, interactions with other stars or planetary bodies can also affect a star's rotation rate.
Scientists study the rotation of massive stars through a technique called spectroscopy. By analyzing the spectrum of light emitted by a star, they can identify shifts in the spectral lines that indicate the rotation rate. This information, combined with other observational data, allows for a better understanding of the rotation and evolution of massive stars.