Sol, also called the sun, is the star of the Sol system. Sol is a G-class star located in the galaxy's Alpha Quadrant. Sol is an average sized star in its main sequence phase, and is the dominant source of light for Earth. According to current definitions, 8 planets and 5 dwarfs planets orbit it, for a total of 13, including the four inner terrestrial planets and the four gas giants, separated by an asteroid belt. Many smaller planetoids also orbit Sol, and many of the system's planets possess moons of varying size and number, such as Earth's satellite, Luna.
Sol is primarily composed of hydrogen and helium gas. Due to the crush of its own gravity, which creates pressure that triggers nuclear reactions at its core, the sun emits energy released as heat and light. Some 600 million tons of hydrogen undergo these reactions every second, and have been doing so for the nearly five billion years of Sol's lifetime. It is estimated that Sol is only halfway through its life span and it is only one of the hundreds of billions of stars in the Milky Way.