The Sun is the star at the centre of the Solar System and the primary source of energy that drives most of its planetary environments. It is a nearly spherical ball of extremely hot ionised gas, or plasma, composed mainly of hydrogen and helium with small quantities of heavier elements. The Sun is classified as a G type main sequence star and is approximately 4.6 billion years old.
Its enormous mass gives it the dominant gravitational influence in the Solar System, while the energy produced by nuclear fusion in its core supplies the radiation that illuminates the planets. The Sun has a diameter of about 1.39 million kilometres and contains approximately 99.8 percent of the total mass of the Solar System. Despite appearing relatively simple from Earth, it has a complex internal structure, magnetic field, atmosphere, cycles of activity and continuously changing surface features.
The Sun is divided into several major structural regions. At the centre is the core, where temperature and pressure are high enough for sustained nuclear fusion. Surrounding the core is the radiative zone, through which energy is transported mainly by the repeated absorption and re emission of photons. Farther outward is the convection zone, where hot plasma rises, cools and sinks in large scale convective motions.
The visible surface is called the photosphere, although it is not a solid surface because the Sun is gaseous and plasma dominated. Above the photosphere lie the chromosphere and the much more extended corona. The corona reaches millions of kilometres into space and becomes especially prominent during total solar eclipses. The transition between the chromosphere and corona contains dramatic changes in temperature and density that are important to solar atmospheric physics.
The Sun's visible light comes primarily from the photosphere, whose temperature is roughly 5800 kelvin. The core is far hotter, with temperatures of roughly 15 million kelvin, while the corona can reach temperatures of around one to several million kelvin. The unexpectedly high coronal temperature is one of solar physics' major research problems and is associated with magnetic processes that transfer energy into the outer atmosphere.
The photosphere exhibits granulation caused by convection, with bright cells of rising hot plasma surrounded by darker regions where cooler material sinks. Sunspots are cooler, darker magnetic regions produced by concentrated magnetic fields emerging through the photosphere. Prominences and filaments consist of comparatively cool dense plasma supported by magnetic structures above the photosphere, while flares are sudden releases of magnetic energy. These phenomena show that the Sun is a dynamic plasma environment rather than a static glowing sphere.
The Sun's gravitational influence determines the large scale orbits of the planets, dwarf planets, asteroids and comets. Its mass creates the central gravitational potential around which the planets travel, and its radiation supplies energy to planetary surfaces and atmospheres. Solar heating drives Earth's climate system, powers the water cycle and supports photosynthesis, which forms the energetic foundation of most terrestrial ecosystems. Solar ultraviolet radiation influences atmospheric chemistry, while the solar wind creates a continuously expanding plasma environment called the heliosphere. The Sun also interacts with planetary magnetic fields, particularly during periods of enhanced solar activity. Changes in solar radiation and solar wind can therefore affect space weather, satellites, spacecraft, radio communication and electrical systems.
The Sun's magnetic field is generated by the motion of electrically conducting plasma inside it. Differential rotation means that different solar latitudes rotate at different rates, while convection and magnetic induction continually distort and amplify magnetic fields. The solar magnetic cycle lasts approximately 11 years when measured by the rise and fall of sunspot activity, although the complete magnetic polarity cycle is approximately 22 years because the global magnetic polarity reverses and later returns to its original orientation.
Solar maximum is associated with greater numbers of sunspots, flares and coronal mass ejections, while solar minimum is comparatively quiet. The activity cycle is not perfectly regular, and scientists continue to study the physical processes that control its timing and amplitude. Magnetic activity is responsible for many of the Sun's most important variable phenomena.
The Sun's atmosphere extends far beyond the visible disc. The corona continuously releases charged particles that form the solar wind. Near Earth, the solar wind normally consists mainly of protons and electrons with a smaller fraction of heavier ions, moving outward at hundreds of kilometres per second. The solar wind carries the solar magnetic field into interplanetary space, creating the interplanetary magnetic field. When solar eruptions produce unusually strong disturbances, they can interact with Earth's magnetosphere and generate geomagnetic storms.
These events can produce auroras, disturb radio communication and satellite operations, increase radiation exposure for astronauts and, in severe cases, induce currents in long electrical conductors. The Sun therefore affects Earth not only through visible light and heat but through a continuous plasma and magnetic connection.
The Sun is currently in the main sequence stage of stellar evolution, meaning its core is converting hydrogen into helium through nuclear fusion. It is expected to remain on the main sequence for several billion more years. As hydrogen in the core is gradually consumed, the core will eventually contract and heat while the outer layers expand. The Sun will then become a red giant, eventually lose its outer layers and leave behind a hot dense white dwarf.
It will not end as a core collapse supernova because its mass is far below the threshold required for that evolutionary pathway. The future evolution of the Sun will profoundly change the Solar System, eventually making Earth uninhabitable long before the Sun becomes a white dwarf. Understanding the Sun therefore connects present solar physics with the long term evolution of our planetary system.
The Sun is important scientifically because it is the nearest star and can be studied in much greater detail than distant stars. Space telescopes, ground based observatories, helioseismology, solar spectroscopy and spacecraft measurements allow scientists to examine its surface, interior, magnetic field, atmosphere and solar wind. Missions that observe the Sun from different locations and distances help measure its magnetic and particle environment.
The Sun also serves as a reference point for understanding other stars, because stellar physics developed from observations of the Sun can be applied to stars with different masses, temperatures and compositions. Although the Sun appears constant on human timescales, its magnetic activity and particle output vary substantially. It is therefore both a relatively stable long term energy source for the Solar System and a highly dynamic astrophysical object.
