The Sun is the central star of the Solar System and is a G type main sequence star whose gravitational influence controls the large scale orbital structure of the planetary system. It is composed predominantly of hydrogen and helium, with smaller quantities of heavier elements. Its energy is generated in its core through nuclear fusion, primarily through the proton proton chain, in which hydrogen nuclei are ultimately converted into helium while releasing energy.
The Sun has a radius of roughly 696000 kilometres and contains approximately 99.8 percent of the total mass of the Solar System. Its visible surface has a temperature of approximately 5800 kelvin, while the central temperature reaches roughly 15 million kelvin. Solar energy is emitted as electromagnetic radiation across a broad spectrum and is ultimately responsible for most of the energy driving Earths climate system, weather and biosphere.
The internal structure of the Sun is conventionally divided into the core, radiative zone and convection zone. The core is the region where the temperature and density are sufficient for sustained hydrogen fusion. Energy produced there initially travels outward through radiation in the radiative zone, where photons undergo enormous numbers of absorption and re emission interactions and therefore take a very long time to transport energy over macroscopic distances.
Farther outward, opacity conditions cause convection to become the dominant energy transport mechanism. Hot plasma rises, cools and sinks in large scale convective motions. The visible photosphere lies above the convection zone and is the layer from which most of the optical light that reaches observers originates. Above it are the chromosphere, transition region and corona, which have very different temperatures and physical conditions from the photosphere.
Nuclear fusion in the Suns core is possible because gravitational compression produces enormous temperature and pressure. Individual protons repel one another through the electromagnetic force, but at sufficiently high energies quantum tunnelling allows some nuclei to approach closely enough for the strong nuclear interaction to act. In the proton proton chain, several reactions ultimately convert four hydrogen nuclei into one helium nucleus, with a small fraction of the original mass becoming energy according to the mass energy relationship.
Neutrinos are produced during the fusion process and escape the Sun with relatively little interaction, providing direct information about nuclear reactions occurring deep within the interior. The energy balance between gravitational forces and pressure generated by the hot plasma maintains the Sun in hydrostatic equilibrium over long timescales.
The visible appearance of the Sun is strongly influenced by magnetic fields generated by the motion of electrically conducting plasma. Solar magnetic fields become concentrated, stretched and reorganised through differential rotation and convection. The equatorial regions rotate faster than the higher latitude regions, creating magnetic stresses that contribute to the formation of active regions. Sunspots appear as relatively dark regions in the photosphere because strong magnetic fields suppress convective heat transport there, making them cooler than surrounding photospheric material. Complex magnetic configurations can store substantial amounts of magnetic energy and can produce Solar Flares and Coronal Mass Ejections when magnetic structures reorganise. Solar magnetism therefore links the Suns internal dynamics with its variable atmosphere and space weather effects.
The corona is the outermost major atmospheric layer of the Sun and has a temperature of roughly one to several million kelvin, considerably hotter than the visible photosphere. This counterintuitive temperature structure is associated with mechanisms involving magnetic fields and plasma energy transport, although the detailed balance of coronal heating processes remains an active research topic. The corona is highly structured, with loops, streamers, holes and other features shaped by magnetic fields.
Coronal holes are regions where magnetic field lines are more open to interplanetary space and are important sources of faster Solar Wind. The corona can be observed particularly clearly during total solar eclipses, although spacecraft instruments routinely image it using specialised wavelengths that reveal plasma at different temperatures and densities.
The Sun undergoes an approximately eleven year cycle in the number and distribution of visible magnetic activity, commonly called the solar activity cycle. Sunspot numbers rise and fall, while the magnetic polarity of the global solar field reverses over an approximately twenty two year magnetic cycle. The cycle is associated with changes in the structure of the Suns magnetic field and affects the frequency of Solar Flares and Coronal Mass Ejections. Solar activity also varies on shorter timescales, including days and minutes, and on much longer timescales. Understanding these variations is important because enhanced solar activity can alter the near Earth space environment, affect spacecraft operations and influence radio communication and electrical infrastructure.
The Suns future is governed by its nuclear fuel and stellar evolution. It is currently in the main sequence stage, converting hydrogen into helium in its core. As hydrogen is consumed, the core composition changes and its structure gradually evolves, causing the Suns luminosity to increase over geological time. Eventually core hydrogen will become depleted, after which the Sun will leave the main sequence and undergo major structural changes. It will expand into a red giant, experience substantial mass loss and later end as a white dwarf surrounded for a time by expelled stellar material. These changes occur on timescales of billions of years and are fundamentally different from short term Solar activity.
The Sun is essential to many areas of science because it is the nearest star and therefore provides an unusually detailed laboratory for stellar physics. Helioseismology uses waves and oscillations observed at the Suns surface to infer internal structure and dynamics. Spectroscopy reveals elemental abundances, temperatures, velocities and magnetic fields through characteristic spectral features. Space based solar observatories measure ultraviolet, X ray and other radiation that cannot be observed effectively from the ground because of atmospheric absorption. Solar Physics therefore connects observations of the nearest star with general theories of stellar structure and evolution. Understanding the Sun also has direct practical importance because its radiation and particle emissions interact continuously with Earth and technological systems throughout near Earth space.
