Our Sun is an average-sized star: there are smaller stars and larger stars, even up to 100 times larger. The Sun is composed almost entirely of hydrogen and helium gas. Multimillion-degree temperatures in its dense core sustain nuclear fusion, providing the energy source for sunlight. The Sun usually displays a few dark splotches. These sunspots are regions of concentrated magnetic fields. It is often said that the Sun is an ‘ordinary’ star. The outer layers of the Sun exhibit differential rotation: at the equator, the surface rotates once every 25.4 days; near the poles, it’s as much as 36 days.
The surface of the Sun called the photosphere, is at a temperature of about 5800 K. Sunspots are ‘cool’ regions, only 3800 K (they look dark only by comparison with the surrounding regions). The Sun is about 4.5 billion years old. Since its birth, it has used up about half of the hydrogen in its core. There are eight planets and a large number of smaller objects orbiting the Sun.