Eclipse and solar activity in narrow band hydrogen-alpha from my back garden in North London, June 2021. ZWOASI174, 100quad with ERF, Quark Chromosphere.

The Sun, 93 million miles away is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma heated to incandescence by nuclear fusion reactions in its core, radiating the energy mainly as visible light and infrared radiation. It is by far the most important source of energy for life on Earth. Its diameter is about 1.39 million kilometres. It accounts for about 99.86% of the total mass of the Solar System. Roughly 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.

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.

The enormous effect of the Sun on Earth has been recognized since prehistoric times. The Sun was thought of 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 Gregorian calendar, the predominant calendar in use today.