The Shape of the Sun
Even when we look at the shape of the Sun, something seems to be amiss. The Sun, like the Earth, rotates on its polar axis. Unlike the Earth it displays “differential rotation," with the equator spinning at 2 km/sec (4,474 mph) and taking 25 days to rotate just once. At the Sun’s poles the rotation is much slower at 34.4 days. Only about 4 per cent of stars rotate at 2 km/sec. Although some stars rotate more slowly many of the stars we see in the night sky rotate much faster with about 27 percent rotating at speeds in excess of 100 km/sec (224,000 mph). Perhaps the fastest spinner is Alpha Arae at an estimated 410 km/sec (a staggering 917,000 mph!).
When a star rotates at high velocity, strange things start to happen. Our Sun is almost a perfect sphere because it rotates so slowly. Faster spinning stars deform into oblate spheroids. Gravity pulls the poles inwards while the equator bulges outwards. The gasses between the poles are compressed and become extremely hot so the poles glow white. Meanwhile the gasses around the equator are more dispersed and cooler and therefore darker, so we end up with a hamburger shaped star with a dark band around its middle. Certainly nothing like the Sun.
Although we do not yet have the technology to directly image fast spinning stars we can get some idea of what they look like by studying the planet Saturn. Despite being considerably larger than the Earth, Saturn spins once on its axis every 10.6 hours, less than half of the Earth’s 24 hours. Like a rapidly spinning star, its poles are pulled inwards so the planet is about 8.5 Earth diameters from pole to pole but 9.5 Earth diameters across its equator. Its oblate shape can clearly be seen in photographs.