Sun

The **Sun** is the star at the center of the Solar System with a diameter of 1,392,000 km and it is 110 times larger than Earth and 9 times larger than Jupiter. 1 Solar radii is equivalent to the diameter of Sun. The Sun performs a complete rotation on its axis once every 26.8 days, known as the rotational period. The Sun's mass is 1.989 x 10^30 kg or 1 solar mass, which is 300,000 times heavier than Earth. Its mass consists 78.4% of hydrogen and 19.8% helium. There are smaller amounts of oxygen (0.86%), carbon (0.4%), and iron (0.15%) at the surface of the Sun, it is also known as photosphere.

The average surface temperature around the Sun is 5500 °C (5770 K). The core of the Sun has temperatures rising to 15 million °C. Inside the core, the nuclear reactions take place, i.e. hydrogen is turned to helium as the resulting energy spreads as radiation. This causes matter to move, because matter is passed to the irregular surface to the Sun while cooler matter moves towards the centre. The currents result in eruptions on the surface, creating a constant gas vortex. The Sun's atmosphere consists of three layers: the photosphere, the chromosphere and the higher extent of the corona.

## General

The Sun is a perfectly ordinary fixed star with an average size like other main-sequence stars. The distance between Sun and the galactic centre is 7,700 parsecs (25,000 light years). The Sun orbits the centre of the galaxy every 225 million years with a speed of approximately 220 km/s (kilometres per second) on a nearly circular path. Every second, the Sun transforms 4 million tons of matter into electromagnetic radiation that arrives on Earth as light, warmth, X-rays, infrared rays and ultraviolet radiation. This process has been repeated itself over since the birth of Sun, about 4.6 billion years ago, according to physical calculations of astrophysics. On the basis of their calculations, astro-physicists predict that once the Sun entire supply of Hydrogen has been transformed to Helium, as of Sun in helium burning phase, will be 10 times larger and possibly be a giant star. It will then expand into a red giant and subsequently shrink down to the size of a white dwarf that will cool out gradually.

## Structure

The Solar Core is the hottest layer of the Sun where interior temperatures reach 15 million 'C. There are two distinct reactions in which four hydrogen nuclei may eventually result in one helium nucleus: the proton–proton reaction which forms deuterium – which is responsible for most of the Sun's released energy and radiation. In every reactions, a portion of mass is transferred into energy, which energy travels across a radiation zone to the adjoining convection zone, then energy moves up to the photosphere, the visible surface of the Sun from which light and heat is radiated. The temperatures could reach 5500 C in the photosphere. The visible photosphere exhibits variable degrees of brightness, referred to as granules, changes every 10 minutes. The cause was the convection zone of the Sun's interior.

## Solar Activity

The Sun is a magnetically active star; it supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including variations in the number of observed sunspots on the solar surface. Sunspots have been observed since the early 17th century and the sunspot time series is the longest continuously observed (recorded) time series of any natural phenomena. The sunspots on the surface of the Sun, solar flares, and variations in the solar wind that carry heat an energy through the solar system. The effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the solar system, and strongly affects the structure of Earth's outer atmosphere.

## Evolution

The Sun today is roughly halfway through the most stable part of its life. It has not changed dramatically for over four billion years and will remain fairly stable for more than five billion more. However, after hydrogen fusion in its core has stopped, the Sun will undergo dramatic changes, both internally and externally.

The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars. This age is estimated using computer models of stellar evolution and through nucleocosmochronology. The result is consistent with the radiometric date of the oldest Solar System material, at 4.567 billion years ago. Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes, such as iron-60, that form only in exploding, short-lived stars.

The Sun is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy, about 0.03% of the total mass of the Sun. The Sun will spend a total of approximately 10 billion years as a main-sequence star. The Sun is gradually becoming hotter in its core, hotter at the surface, larger in radius, and more luminous during its time on the main sequence.

The Sun does not have enough mass to explode as a supernova. Instead, when it runs out of hydrogen in the core in approximately 5 billion years, core hydrogen fusion will stop and there will be nothing to prevent the core from contracting. The release of gravitational potential energy will cause the luminosity of the star to increase, ending the main sequence phase and leading the star to expand over the next billion years: first into a subgiant, and then into a red giant. The post-asymptotic-giant-branch evolution is even faster. The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionized into a planetary nebula as the exposed core reaches 30,000 K, as if it is in a sort of blue loop. The final naked core, a white dwarf, will have a temperature of over 100,000 K, and contain an estimated 54.05% of the Sun's present-day mass. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to a hypothetical black dwarf.

## Observation and effects

During certain atmospheric conditions, the Sun becomes visible to the naked eye, and can be observed without stress to the eyes. Click on this photo to see the full cycle of a sunset, as observed from the high plains of the Mojave Desert. The Sun, as seen from low Earth orbit overlooking the International Space Station. This sunlight is not filtered by the lower atmosphere, which blocks much of the solar spectrum.
The brightness of the Sun can cause pain from looking at it with the naked eye; however, doing so for brief periods is not hazardous for normal non-dilated eyes. Looking directly at the Sun (sungazing) causes phosphene visual artifacts and temporary partial blindness. It also delivers about 4 milliwatts of sunlight to the retina, slightly heating it and potentially causing damage in eyes that cannot respond properly to the brightness.

During sunrise and sunset, sunlight is attenuated because of Rayleigh scattering and Mie scattering from a particularly long passage through Earth's atmosphere, and the Sun is sometimes faint enough to be viewed comfortably with the naked eye or safely with optics (provided there is no risk of bright sunlight suddenly appearing through a break between clouds). Hazy conditions, atmospheric dust, and high humidity contribute to this atmospheric attenuation.

## See also

* Old version of the Sun
* Star
* Stellar classification
* Solar System
* Timeline of the far future