Magnetic braking (astronomy)
Magnetic braking is a theory explaining the loss of stellar angular momentum due to material getting captured by the stellar magnetic field and thrown out at great distance from the surface of the star.
It plays an important role in the evolution of binary star systems.
At the dense center of this disk a protostar forms, which gains heat from the gravitational energy of the collapse.
As the collapse continues, the rotation rate can increase to the point where the accreting protostar can break up due to centrifugal force at the equator.
Thus the rotation rate must be braked during the first 100,000 years of the star's life to avoid this scenario.
One possible explanation for the braking is the interaction of the protostar's magnetic field with the stellar wind.
Ionized material captured by the magnetic field lines will rotate with the Sun as if it were a solid body.
[1][2] The same effect is used in slowing the spin of a rotating satellite; here two wires spool out weights to a distance slowing the satellites spin, then the wires are cut, letting the weights escape into space and permanently robbing the spacecraft of its angular momentum.
, or "energy density", while rotating together with the Sun as a solid body: Since magnetic field strength decreases with the cube of the distance there will be a place where the kinetic gas pressure
Due to the high conductivity of the stellar wind, the magnetic field outside the sun declines with radius like the mass density of the wind, i.e. decline as an inverse square law.
The amount of solar mass needed to be thrown out along the field lines to make the Sun completely stop rotating can then be calculated using the specific angular momentum: It has been suggested that the Sun lost a comparable amount of material over the course of its lifetime.
[5] In 2016 scientists at Carnegie Observatories published a research suggesting that stars at a similar stage of life as the Sun were spinning faster than magnetic braking theories predicted.
In a study published in Nature Astronomy in 2021, researchers at the University of Birmingham used a different approach, namely asteroseismology, to confirm that older stars do appear to rotate faster than expected.
