<P> From an observation of a transit of Venus in 1032, the Persian astronomer and polymath Avicenna concluded that Venus is closer to Earth than the Sun . In 1672 Giovanni Cassini and Jean Richer determined the distance to Mars and were thereby able to calculate the distance to the Sun . </P> <P> In 1666, Isaac Newton observed the Sun's light using a prism, and showed that it is made up of light of many colors . In 1800, William Herschel discovered infrared radiation beyond the red part of the solar spectrum . The 19th century saw advancement in spectroscopic studies of the Sun; Joseph von Fraunhofer recorded more than 600 absorption lines in the spectrum, the strongest of which are still often referred to as Fraunhofer lines . In the early years of the modern scientific era, the source of the Sun's energy was a significant puzzle . Lord Kelvin suggested that the Sun is a gradually cooling liquid body that is radiating an internal store of heat . Kelvin and Hermann von Helmholtz then proposed a gravitational contraction mechanism to explain the energy output, but the resulting age estimate was only 20 million years, well short of the time span of at least 300 million years suggested by some geological discoveries of that time . In 1890 Joseph Lockyer, who discovered helium in the solar spectrum, proposed a meteoritic hypothesis for the formation and evolution of the Sun . </P> <P> Not until 1904 was a documented solution offered . Ernest Rutherford suggested that the Sun's output could be maintained by an internal source of heat, and suggested radioactive decay as the source . However, it would be Albert Einstein who would provide the essential clue to the source of the Sun's energy output with his mass - energy equivalence relation E = mc . In 1920, Sir Arthur Eddington proposed that the pressures and temperatures at the core of the Sun could produce a nuclear fusion reaction that merged hydrogen (protons) into helium nuclei, resulting in a production of energy from the net change in mass . The preponderance of hydrogen in the Sun was confirmed in 1925 by Cecilia Payne using the ionization theory developed by Meghnad Saha, an Indian physicist . The theoretical concept of fusion was developed in the 1930s by the astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe . Hans Bethe calculated the details of the two main energy - producing nuclear reactions that power the Sun . In 1957, Margaret Burbidge, Geoffrey Burbidge, William Fowler and Fred Hoyle showed that most of the elements in the universe have been synthesized by nuclear reactions inside stars, some like the Sun . </P> <P> The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968 . These probes orbited the Sun at a distance similar to that of Earth, and made the first detailed measurements of the solar wind and the solar magnetic field . Pioneer 9 operated for a particularly long time, transmitting data until May 1983 . </P>

Since the sun is in hydrostatic equilibrium the size of the sun