The Sun is the star at the center of the Solar System, held together by its own gravity and powered by nuclear fusion in its core. It contains approximately 99.8 percent of the system’s total mass and dominates the motion of its planets and smaller bodies. Its radiation supplies the energy essential to most life on Earth. Astronomically, it is a G-type main-sequence star, commonly called a yellow dwarf. (science.nasa.gov)
Physical properties
The Sun formed about 4.6 billion years ago. Its mass is approximately 1.99 × 10³⁰ kilograms, about 333,000 times Earth’s mass, and its mean radius is approximately 695,700 kilometers, roughly 109 times Earth’s radius. Its total radiative output, or luminosity, is about 3.8 × 10²⁶ watts. The effective temperature of its visible surface is approximately 5,770 kelvins, whereas the central temperature reaches about 15 million kelvins. These different temperatures reflect the enormous changes in density and physical conditions between the core and atmosphere. (solarscience.msfc.nasa.gov)
Hydrogen and helium overwhelmingly dominate its composition, with small quantities of heavier elements, including carbon, oxygen, and iron. Composition varies with depth because fusion progressively converts hydrogen into helium in the core. Much of the solar material is plasma, in which electrons are separated from atomic nuclei. The Sun has no solid surface: its apparent edge corresponds to atmospheric layers from which visible radiation escapes. (solarscience.msfc.nasa.gov)
Interior and energy production
The solar interior is approximately in hydrostatic equilibrium: the outward pressure gradient balances the inward gravitational force. This balance allows the Sun to remain stable while slowly changing as its fuel is consumed. (science.nasa.gov)
Fusion occurs predominantly through the proton–proton chain, a sequence of reactions whose net result converts four hydrogen nuclei into a helium-4 nucleus. The reactions release energy and neutrinos. The helium nucleus has less mass than the original reactants; this difference becomes energy according to mass–energy equivalence. Nuclear fusion, rather than chemical combustion, accounts for the Sun’s sustained luminosity over billions of years. (cesar.esa.int)
Energy travels outward through two principal regions. In the radiative zone, it is transferred mainly by radiation interacting repeatedly with matter. In the outer convection zone, rising hot plasma and sinking cooler plasma carry energy toward the surface. Convection produces a granular pattern in the visible atmosphere. This layered structure connects energy generation deep inside the Sun with radiation escaping into space. (science.nasa.gov)
Surface and atmosphere
The photosphere is the atmospheric layer responsible for most of the Sun’s visible light. Above it lie the chromosphere, a narrow transition region, and the corona, the extended outer atmosphere. Although the photosphere is comparatively cool, coronal temperatures commonly exceed one million kelvins. Explaining this temperature increase requires processes beyond simple heat transfer from the visible surface. Magnetic waves and magnetic reconnection are major mechanisms investigated in coronal-heating research. (science.nasa.gov)
The Sun’s magnetic field organizes many atmospheric structures. Sunspots are relatively cool, dark regions of the photosphere associated with strong magnetic fields. Prominences consist of comparatively cool, dense material suspended above the surface by magnetic fields. Coronal holes are lower-density regions whose magnetic configuration permits solar material to escape particularly readily. (science.nasa.gov)
Rotation and magnetic activity
The Sun rotates differentially rather than as a rigid body. Near the equator, its rotation period relative to distant stars is approximately 25 days; at high latitudes, it exceeds 30 days. Measurements show that this latitude-dependent rotation extends through the convection zone, while much of the radiative interior rotates more uniformly. (nso.edu)
The solar cycle is an approximately 11-year variation in sunspot numbers and related magnetic activity. Solar maximum brings more active regions and eruptions than solar minimum. The large-scale magnetic polarity reverses between successive cycles, giving a complete magnetic cycle of approximately 22 years. These intervals are approximate, not fixed clock periods. (science.nasa.gov)
A solar flare is a rapid release of energy, expressed especially as enhanced radiation. A coronal mass ejection expels a large cloud of plasma and embedded magnetic field into space. The two phenomena can accompany one another but are physically distinct. (science.nasa.gov)
Influence on Earth and interplanetary space
The continuous solar wind carries charged particles and magnetic fields outward, forming the heliosphere, a vast region extending beyond the planets. Solar-wind disturbances and eruptions produce space weather. Interaction with Earth’s magnetic environment can generate auroras, disrupt radio communications, affect satellites, and induce currents in electrical grids. (science.nasa.gov)
At Earth’s average distance, approximately 150 million kilometers, incoming solar radiation supplies about 1,360 watts per square meter to a surface perpendicular to the rays above the atmosphere. Absorbed sunlight warms the planet, powers evaporation in the water cycle, and supports photosynthesis. Reflection and the redistribution of absorbed energy determine how this input affects different regions. (science.nasa.gov)
Formation, evolution, and observation
The Sun originated when part of a cloud of gas and dust collapsed under gravity. Most material accumulated centrally, while surrounding material formed a disk from which planets developed. In roughly five billion years, depletion of central hydrogen will lead it away from the main sequence and toward the red-giant stage. Later, it will shed its outer layers, leaving a white dwarf that gradually cools. These are evolutionary predictions rather than directly observed future events. (science.nasa.gov)
Solar researchers use spectroscopy to examine radiation and helioseismology to investigate the interior through oscillations measured at the surface. Different oscillation modes probe different depths, revealing internal rotation and constraining models of solar structure. Ground-based observatories and spacecraft also monitor atmospheric features and magnetic changes across the activity cycle. (nso.edu)