Sun

The sun is a type G2V star; it is the star at the center of the solar system, and it accounts for over 99% of the mass of the solar system. By mass, it is composed of 72% hydrogen, 26% helium and 2% trace elements of oxygen, carbon, neon, nitrogen, magnesium, iron, and silicon. The eight planets, including Earth, orbit the sun, as do countless other small objects. It provides the Earth with light and heat.

What eventually became the solar system initially existed as a large, rotating cloud of dust and gas, composed of hydrogen and helium produced in the Big Bang as well as small amounts of heavier elements. Around 4.57 billion years ago, the cloud began to contract, perhaps as a result of a shock wave from a nearby supernova. Inertia caused the rotating cloud to flatten into a disk. Most of the mass concentrated in the middle, and began to heat up. Eventually, the kinetic energy of the hydrogen was sufficient to overcome the electromagnetic repulsion between the protons, and fusion began. The resulting solar wind helped clear away much of the material which had not coalesced into planets or other orbiting bodies. . The sun is most likely a third-generation star. In Christian theology, God created the sun on the third day of Creation.

The sun has a diameter of about 1.4 million kilometer; some 1.3 million Earths could fit inside its volume. It is an average-sized, middle-aged star. Its surface temperature is 6000 K, while the temperature of its core is around 13.6 million K. The inner layer of the sun, called the core, is where most fusion takes place. Beyond that is the radiation zone and then the convection zone. Outside that is the photosphere, the visible layer of the sun. It takes anywhere between 17,000 and 50 million years for photons produced in the core to be emitted at the surface, having been absorbed and re-emitted countless times. The atmosphere, including the corona, surrounds the photosphere. The sun is located about halfway between the center of the galaxy and its edge; it takes around 230-250 million years to orbit the galactic center. It is estimated to have made almost twenty of these trips.

The sun produces a vast amount of energy; the Earth receives only a tiny fraction of this energy, yet it powers practically all life (and industry) on Earth, either directly or indirectly. It is also responsible for Earth's climate and weather. Plants and other organisms capture the energy of the sun in a process called photosynthesis. Animals and other heterotrophs obtain food and energy from consuming plants or from consuming animals who have consumed the plants. Only a few microscopic organisms obtain energy directly from chemical reactions; this represents a very ancient metabolic pathway and likely existed prior to the evolution of photosynthesis.

Human civilization, too, is dependent on the sun. Much of humanity's industrial energy needs are obtained through the combustion of fossil fuels, the remains of dead plants and animals. Uneven heating of the atmosphere provides wind power. Also, the sun evaporates water from lower-lying bodies of water; this then falls as rain and flows back down channels, providing hydroelectric power. On a limited basis, solar power is directly utilized. Geothermal energy and nuclear energy are examples of energy sources to which the sun does not directly contribute.