Ophiuchi A is a rapidly rotating star with a projected rotational velocity of 240 km s−1. It is spinning at about 88.5% of the rate that would cause the star to break up. This gives it an oblate spheroid shape with an equatorial bulge about 20% larger than the polar radius. The polar radius is calculated to be and the equatorial radius . Because of the oblateness and rapid spin, the surface gravity at the pole is higher than at the equator. An effect known as gravity darkening means that the temperature at the poles is also higher than at the equator. The polar temperature is calculated to be and the equatorial temperature . The energy radiated by an oblate star is higher along its axis of rotation because of the larger projected area and the Stefan–Boltzmann law. The axis of rotation of α Ophiuchi is inclined about to the line of sight from the Earth, so that it is being observed from nearly equator-on. The apparent bolometric luminosity seen at this angle is , but the true luminosity of the star is thought to be . Since a star's effective temperature is simply the temperature which would produce its total energy output from a black body, the true effective temperature of is different from the apparent effective temperature of . References External links Ophiuchi, Alpha Ophiuchus (constellation) A-type giants Rasalhague Ophiuchi, 55 086032 K-type main-sequence stars Binary stars 6556 159561 BD+12 3252