The sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma. It is, by far, the most important source of energy for life on Earth. The sun's diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System. About three-quarters of the Sun's mass consists of hydrogen (73%); the rest is mostly helium (25%). The last 2% is made of heavier elements including oxygen, carbon, neon, and iron. The sun is referred to as a Yellow Dwarf star. There are many other Yellow Dwarf stars in the universe. It formed approximately 4.6 billion years ago from a gravitational collapse of matter within an area of a large molecular cloud, a process from which all stars are born. The sun is roughly middle-aged and will remain fairly stable for another five billion years, after which the core of the Sun will experience an increase in density and temperature and transform into a Red Giant. When this happens, the Sun will become large enough to engulf the current orbits of Marcury, Venus, Earth, and possibly all the planets in our Solar System.