When the surface angular velocity is above about 70 % of the critical angular velocity , many interesting features appear which may be tested by interferometric observations , like significant deformation of stars , variation of the effective temperature with the latitude .
Also polar winds become important and equatorial disks may appear .
Near the critical limit , convection is also favored in the outer layers .
In the present paper , we emphasize the need for a proper estimate of the critical velocity since this is the ratio of the actual velocity of the star to that critical velocity which determines the amplitude of the above effects .
We recall the existence of two critical velocities .
The first one , also called the classical critical velocity is the one to consider when the star has an Eddington factor inferior to 0.639 , while the second one is the one to be considered when the Eddington factor is above 0.639 .
The features of the star at these two critical limits may be very different .