Stellar rotation causes a star's shape to be flattened at the poles and bulged at the equator. The CHARA Array can measure the size and shape of rapidly rotating stars. Because the equator is further from the center of the star, it will appear cooler than the poles, an effect called "gravity darkening".

Rapid rotators imaged with the MIRC beam combiner at the CHARA Array.  Image courtesy of John Monnier.


References:

Regulus -- Che et al. 2011, ApJ, 732, 68

Rasalhague -- Zhao et al. 2009, ApJ, 701, 209

Altair -- Monnier et al. 2007, Science, 317, 324

Alderamin -- Zhao et al. 2009, ApJ, 701, 209

Beta Cas -- Che et al. 2011, ApJ, 732, 68