Stellar rotation
The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface.
These differences in the rate of rotation within a star may have a significant role in the generation of a stellar magnetic field.
The magnetic field of the star interacts with the wind, which applies a drag to the stellar rotation.
These occur when a massive object passes in front of the more distant star and functions like a lens, briefly magnifying the image.
The more detailed information gathered by this means allows the effects of microturbulence to be distinguished from rotation.
[4] If a star displays magnetic surface activity such as starspots, then these features can be tracked to estimate the rotation rate.
However, such features can form at locations other than equator and can migrate across latitudes over the course of their life span, so differential rotation of a star can produce varying measurements.
[8] Surface differential rotation is observed on stars such as the Sun when the angular velocity varies with latitude.
When turbulence occurs through shear and rotation, the angular momentum can become redistributed to different latitudes through meridional flow.
[12][13] The interfaces between regions with sharp differences in rotation are believed to be efficient sites for the dynamo processes that generate the stellar magnetic field.
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.
The expanding wind carries away the angular momentum and slows down the rotation rate of the collapsing protostar.
"As the expected life span of a star decreases with increasing mass, this can be explained as a decline in rotational velocity with age.
However, the discovery of rapidly rotating brown dwarfs such as the T6 brown dwarf WISEPC J112254.73+255021.5[24] lends support to theoretical models that show that rotational braking by stellar winds is over 1000 times less effective at the end of the main sequence.
Tidal interactions in a close binary system can result in modification of the orbital and rotational parameters.
Thus the force of gravity produces a torque component on the bulge, resulting in the transfer of angular momentum (tidal acceleration).
The accreting companion can spin up to the point where it reaches its critical rotation rate and begins losing mass along the equator.
[27] After a star has finished generating energy through thermonuclear fusion, it evolves into a more compact, degenerate state.
During this process the dimensions of the star are significantly reduced, which can result in a corresponding increase in angular velocity.
A white dwarf is a star that consists of material that is the by-product of thermonuclear fusion during the earlier part of its life, but lacks the mass to burn those more massive elements.
It is a compact body that is supported by a quantum mechanical effect known as electron degeneracy pressure that will not allow the star to collapse any further.
Once the white dwarf reaches this mass, such as by accretion or collision, the gravitational force would exceed the pressure exerted by the electrons.
If the beam sweeps past the direction of the Solar System then the pulsar will produce a periodic pulse that can be detected from the Earth.
The energy radiated by the magnetic field gradually slows down the rotation rate, so that older pulsars can require as long as several seconds between each pulse.
[30] A black hole is an object with a gravitational field that is sufficiently powerful that it can prevent light from escaping.
This rotation causes the space within an oblate spheroid-shaped volume, called the "ergosphere", to be dragged around with the black hole.
