Stellar rotation

Stellar rotation is the angular motion of a star around its axis. This spinning motion is a fundamental part of how stars exist and evolve. It influences a star's physical shape, its internal movements, and its magnetic strength. Scientists study this rotation to understand the life cycles of stars throughout the universe.

By measuring how fast a star spins, astronomers can learn about its age and its internal structure.

When a star rotates, it does not maintain a perfect spherical shape. A force called centrifugal force pushes outward from the center of the star. This force is strongest at the equator. Because of this, the star develops an equatorial bulge, making it look more like an oval. This shape is an equilibrium state where gravity and centrifugal force balance out. The star cannot pull itself into a perfect sphere because the outward force at the equator is too strong.

This effect is known as gravity darkening at the equator.

Stars are not solid objects like planets, so they can experience differential rotation. This means different parts of the star spin at different angular velocities. In our Sun, the equator usually spins faster than the higher latitudes. However, some stars like HD 31993 show the opposite pattern. This happens because of turbulent convection inside the star. Plasma moves toward the surface, carrying angular momentum with it. This movement can redistribute rotation through a process called meridional flow. These differences in speed are important because they help generate a stellar magnetic field.

Astronomers use several methods to measure these rotational speeds. One common way is by analyzing the star's spectrum. If a star is tilted, some parts of its surface move toward us while others move away. The parts moving toward us show a Doppler shift to a higher frequency. The parts moving away shift to a lower frequency. This causes the absorption lines in the spectrum to broaden.

This measurement is often called the projected rotational velocity because it depends on the star's inclination. Another method involves tracking starspots, which are dark areas of magnetic activity on the surface.

Extreme examples of rapid rotation exist in the cosmos. The star Regulus A is a famous case of a fast spinner. Its equator rotates at a velocity of 317 ± 3 km/s. This speed is 86% of the break-up velocity. The break-up velocity is the point where centrifugal force equals gravitational force. If a star spins faster than this, it would break apart. For Regulus A, the equatorial radius is 32% larger than its polar radius. Other rapidly rotating stars include Vega, Pleione, and Achernar.

Stars also experience a process called rotational braking, which slows them down over time. During formation, a collapsing cloud of gas and dust spins faster to conserve angular momentum. To prevent the new protostar from breaking apart, it must slow down within the first 100,000 years. This happens through magnetic braking. The star's magnetic field interacts with its stellar wind. As the wind carries mass away, it also carries angular momentum.

This transfer of momentum acts like a drag on the star's rotation.

In 1972, Andrew P. Skumanich discovered a mathematical relation for how main-sequence stars slow down. This discovery led to the field of gyrochronology. Gyrochronology is the science of determining a star's age by measuring its rotation rate. As stars age, they gradually lose rotation due to the steady loss of mass from their photosphere. This process continues until the star approaches a state of very little rotation. However, the effectiveness of this braking changes as stars evolve into different stages, such as ultracool dwarfs or white dwarfs.