<P> The Sun is the star at the center of the Solar System . It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process . It is by far the most important source of energy for life on Earth . Its diameter is about 1.39 million kilometers, i.e. 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%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron . </P> <P> The Sun is a G - type main - sequence star (G2V) based on its spectral class . As such, it is informally and not completely accurately referred to as a yellow dwarf (its light is closer to white than yellow). It formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud . Most of this matter gathered in the center, whereas the rest flattened into an orbiting disk that became the Solar System . The central mass became so hot and dense that it eventually initiated nuclear fusion in its core . It is thought that almost all stars form by this process . </P> <P> The Sun is roughly middle - aged; it has not changed dramatically for more than four billion years, and will remain fairly stable for more than another five billion years . It currently fuzes about 600 million tons of hydrogen into helium every second, converting 4 million tons of matter into energy every second as a result . This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat . In about 5 billion years, when hydrogen fusion in its core has diminished to the point at which the Sun is no longer in hydrostatic equilibrium, the core of the Sun will experience a marked increase in density and temperature while its outer layers expand to eventually become a red giant . It is calculated that the Sun will become sufficiently large to engulf the current orbits of Mercury and Venus, and render Earth uninhabitable . After this, it will shed its outer layers and become a dense type of cooling star known as a white dwarf, which no longer produces energy by fusion, but still glows and gives off heat from its previous fusion . </P> <P> The enormous effect of the Sun on Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity . The synodic rotation of Earth and its orbit around the Sun are the basis of solar calendars, one of which is the predominant calendar in use today . </P>

Where does the sun's luminosity come from