<P> The component of the radial velocity observed through line broadening depends on the inclination of the star's pole to the line of sight . The derived value is given as v e ⋅ sin ⁡ i (\ displaystyle v_ (e) \ cdot \ sin i), where v is the rotational velocity at the equator and i is the inclination . However, i is not always known, so the result gives a minimum value for the star's rotational velocity . That is, if i is not a right angle, then the actual velocity is greater than v e ⋅ sin ⁡ i (\ displaystyle v_ (e) \ cdot \ sin i). This is sometimes referred to as the projected rotational velocity . In fast rotating stars polarimetry offers a method of recovering the actual velocity rather than just the rotational velocity; this technique has so far been applied only to Regulus . </P> <P> For giant stars, the atmospheric microturbulence can result in line broadening that is much larger than effects of rotational, effectively drowning out the signal . However, an alternate approach can be employed that makes use of gravitational microlensing events . 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 . </P> <P> 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 . Stellar magnetic activity is often associated with rapid rotation, so this technique can be used for measurement of such stars . Observation of starspots has shown that these features can actually vary the rotation rate of a star, as the magnetic fields modify the flow of gases in the star . </P> <P> Gravity tends to contract celestial bodies into a perfect sphere, the shape where all the mass is as close to the center of gravity as possible . But a rotating star is not spherical in shape, it has an equatorial bulge . </P>

As a rotating gas cloud contracts what does its rotation rate do