ARE STARS RAPID ROTATORS?

Sun and Other Normal Stars:

Sun:

Rotation Period ~ 25 to 30 days, Mass = 1.99x10³³ g, Radius = 6.96x10¹⁰ cm,
===> (v²/R)/(GM/R²) = 1.5x10^-5!


In order for rotation to be dynamically important, the Sun would have to spin once every few hours.

Other Stars?
As a rule, no. The only normal stars remotely interesting are those known as the Be stars.

Pulsars:

Normal Pulsars:
The fastest known normal pulsar is PSR J0537-6910 with period = 16 msec. Pulsars are rotating neutron stars and so have Mass = 1.4 M(Sun), and are thought to have radii ~ 10 km
===> (v²/R)/(GM/R²) = 8.3x10^-4


Even PSR J0537-6910 which spins at 60 Hz is slowly rotating.


Millisecond Pulsars:

These are a more interesting possibility. The fastest spinining millisecond pulsar has period = 1.56 msec
===> (v²/R)/(GM/R²) = 0.09


At 640 Hz, rotation (and rotationally driven instabilities) are just starting to become interesting. Further, because the equation-of-state of neutron star matter is uncertain (because of uncertainty in the strong interaction), it is plausible that neutron stars have radii as large as 16 km ===> the acceleration ratio is ~ 0.3 and rotation cannot be ignored (e.g., Pringle & Papaloizou 1975).

Overall, however, the understanding of equilbrium stars does not require considering the effects of rotation beyond first order.