The Sun has an equatorial radius of approximately 695,500 kilometers—about 109 times the radius of Earth—and a volume of $1.412 \times 10^{18}$ cubic kilometers, large enough to enclose roughly 1.3 million Earths. Its mass is $(1.988475 \pm 0.000092) \times 10^{30}$ kilograms, or approximately 332,950 times the mass of Earth and 743 times the combined mass of all planets in the Solar System. Despite its immense mass, the Sun's mean density is 1.408 grams per cubic centimeter, roughly one-quarter the average density of Earth, though its central density reaches about 150 to 160 grams per cubic centimeter. Its equatorial surface gravity is 274 meters per second squared, nearly 28 times terrestrial gravity, and its escape velocity is 618 kilometers per second.¹⁴

Situated at a mean distance of 149.6 million kilometers (defined as one astronomical unit) from Earth, sunlight takes an average of 8 minutes and 19 seconds to reach the planet. At this distance, the solar disk subtends an angular diameter of about 0.53 degrees in Earth's sky, nearly identical to the apparent size of the Moon. The Sun's apparent visual magnitude is −26.74, while its absolute magnitude is +4.83. Although often referred to informally as a yellow dwarf, the Sun is brighter and more massive than roughly 85 to 95 percent of the stars in the Milky Way, the vast majority of which are lower-mass red dwarfs. It radiates energy at a luminosity of $3.85 \times 10^{26}$ watts, delivering an average solar irradiance of about 1,340 watts per square meter just outside Earth's atmosphere.

Because the Sun consists entirely of ionized gas (plasma) rather than solid matter, it does not rotate as a rigid body. Instead, it exhibits differential rotation that varies with latitude: the equatorial regions complete a sidereal rotation in 25.05 Earth days at a velocity of nearly 2 kilometers per second, whereas the high polar latitudes take between 34.4 and 36 days to complete a rotation. This rotational shear plays a central role in twisting and amplifying the solar magnetic field. The Sun's rotational axis is tilted by 7.25 degrees relative to the plane of Earth's orbit (the ecliptic) and 67.23 degrees relative to the plane of the Milky Way galaxy, which the Sun orbits at a distance of 24,000 to 28,000 light-years every 225 to 250 million years.

By mass, the Sun's outer layers consist of approximately 73 to 74 percent hydrogen, 25 to 27 percent helium, and less than 2 percent heavier elements—referred to in astrophysics as metals—including oxygen, carbon, neon, nitrogen, magnesium, silicon, calcium, sodium, and iron.¹⁵¹⁶ Because hydrogen has been continuously converted into helium in the deep interior over the past 4.6 billion years, the core contains a significantly higher fraction of helium than the surface layers.

The solar interior is divided into three primary regions—the core, the radiative zone, and the convective zone—distinguished by how energy is produced and transported toward the surface.¹⁷ Throughout the interior, the Sun maintains hydrostatic equilibrium, a state in which the inward gravitational compression of its overlying mass is balanced by the outward thermal and radiation pressure generated by nuclear fusion.¹⁸

Because the Sun is gaseous, it lacks a sharp physical boundary; instead, its atmosphere consists of several distinct layers above the convective zone that transition gradually into interplanetary space.

The lowest layer of the solar atmosphere is the photosphere, which forms the visible "surface" of the Sun from which almost all visible light and infrared radiation escape into space. Measuring several hundred kilometers thick, the photosphere has an effective temperature of approximately 5,780 to 5,800 kelvins and a particle density of roughly $10^{-4}$ kilograms per cubic meter—about 0.01 percent the density of Earth's atmosphere at sea level. Because the opacity of the gas drops rapidly with altitude, reducing emission by a factor of six every 200 kilometers, the edge (or limb) of the Sun appears sharp when viewed from Earth. The photosphere is covered in a shifting cellular pattern of granules and larger supergranules, which are the tops of rising convective cells from the interior below.

Above the photosphere lies the chromosphere, an irregular layer roughly 1,600 kilometers thick where the plasma density drops to one ten-thousandth that of the photosphere. Although temperature initially decreases at the top of the photosphere to a minimum of about 4,400 kelvins, it reverses trend in the chromosphere, rising with altitude from roughly 6,000 kelvins to 20,000 or 25,000 kelvins near its upper boundary. Normally invisible against the glare of the photosphere, the chromosphere can be seen during a total solar eclipse as a reddish rim around the solar disk. Above the chromosphere, a thin, highly irregular transition region separates the lower atmosphere from the outer corona.

The corona is the extended outer atmosphere of the Sun, stretching millions of kilometers into space. Despite having a very low particle density of $10^{15}$ to $10^{16}$ particles per cubic meter in the low corona, its average temperature ranges from 1 million to 2 million kelvins, with the hottest active regions reaching 8 million to 20 million kelvins. In the outer reaches of the corona, past a boundary known as the Alfvén critical surface, the thermal and magnetic acceleration of the plasma overcomes the Sun's gravitational pull and magnetic containment. The coronal gas then expands outward at supersonic speeds as the solar wind, a continuous stream of charged particles—consisting of both a slow component traveling at roughly 354 kilometers per second and a faster component—that carves out a vast magnetic bubble called the heliosphere, which extends well beyond the orbits of the planets into interstellar space.²¹

The Sun possesses a complex, dynamic magnetic field generated by the movement of electrically conducting plasma inside the convective zone and at the tachocline, a process known as the solar dynamo. Magnetic flux tubes rise through the convection zone and erupt through the photosphere into the chromosphere and corona, driving a wide range of solar activity. Where concentrated bundles of magnetic field lines pierce the photosphere, they inhibit the upward convective flow of heat from the interior, creating cooler, darker regions on the surface known as sunspots. Above and around these active regions, magnetic reconnection—the sudden rearrangement and snapping of stressed magnetic field lines—triggers solar prominences (colossal loops of glowing plasma suspended in the atmosphere), intense bursts of electromagnetic radiation called solar flares, and coronal mass ejections (CMEs), which hurl billions of tons of magnetized plasma into the Solar System.²²

Solar magnetic activity follows a periodic rhythm known as the solar cycle, which lasts roughly 11 years. Over the course of each cycle, the Sun transitions from solar minimum, when sunspots and eruptions are scarce, to solar maximum, when sunspot numbers and energetic outbursts peak, before subsiding again. Near the peak of each 11-year cycle, the Sun's global north and south magnetic poles reverse polarity, meaning a complete magnetic return to the original polarity configuration takes approximately 22 years.

When solar flares and CMEs are directed toward Earth, their high-energy radiation and magnetized plasma interact with Earth's magnetosphere and upper atmosphere, producing space weather. While these interactions generate auroras at high latitudes, severe geomagnetic storms and solar energetic particle events can disrupt high-frequency radio communications, degrade satellite navigation signals, damage spacecraft electronics, pose radiation hazards to astronauts, and induce electrical currents capable of damaging terrestrial power grids. Variations in the heliospheric magnetic field across the solar cycle also modulate the flux of galactic cosmic rays entering the Solar System, with heightened solar activity acting as a turbulent magnetic barrier that suppresses cosmic ray intensity at Earth, a phenomenon that includes rapid, short-term drops known as Forbush decreases.²³

The Sun formed approximately 4.5 to 4.6 billion years ago from the gravitational collapse of a dense region within a giant molecular cloud of gas and dust. As the collapsing fragment contracted over hundreds of thousands of years, conservation of angular momentum caused it to spin faster and flatten into a circumstellar disk, while gravitational potential energy heated the central protostar. Once the core temperature and pressure rose high enough to ignite sustained hydrogen fusion, outward radiation pressure halted further gravitational contraction, and the Sun settled onto the main sequence, while the surrounding disk coalesced into the planets and smaller bodies of the Solar System.

The Sun is roughly halfway through its main-sequence phase, which is projected to last a total of about 10 billion years. As hydrogen is steadily converted into helium in the core, the core slowly contracts and heats up to maintain pressure equilibrium, causing the Sun's fusion rate and luminosity to gradually increase over geological timescales. In approximately 5 billion years, the supply of hydrogen at the very center of the Sun will be exhausted, disrupting the hydrostatic equilibrium that has stabilized it on the main sequence.

Once core hydrogen fusion ceases, the inert helium core will contract under gravity while hydrogen fusion shifts outward into a spherical shell surrounding the core. This shell burning will cause the Sun's outer layers to expand dramatically and cool at the surface, transforming the star first into a subgiant and then into a luminous red giant. During this red giant expansion, the Sun will grow large enough to engulf the inner planets, potentially including Earth. Eventually, the compressing core will become hot and dense enough to ignite the fusion of helium into carbon and oxygen, initially in a rapid runaway event known as the helium flash before settling onto the horizontal branch. Because the Sun lacks the mass required to fuse carbon into heavier elements or explode as a core-collapse supernova, the exhaustion of its core helium will mark the end of its nuclear life. The unstable outer envelope will be expelled into space to form an expanding glowing shell of ionized gas called a planetary nebula, leaving behind the exposed, degenerate core as a white dwarf—an Earth-sized remnant with a large fraction of the Sun's original mass that will slowly cool and fade over billions of years.

Human cultures have tracked the Sun's motion across the sky since antiquity to regulate calendars and agriculture, often venerating it as a deity.²⁴ Modern scientific study of the Sun began in the 17th century with the invention of the telescope, and systematic daily recording of sunspot activity commenced in 1755, establishing the numbered sequence of solar cycles still used by astronomers.²⁵ In the early 19th century, dark absorption lines—known as Fraunhofer lines—were discovered in the solar spectrum.²⁶ In 1859, German scientists Gustav Kirchhoff and Robert Bunsen demonstrated that these lines correspond to specific chemical elements absorbing light in the solar atmosphere, founding the science of astronomical spectroscopy.

For decades, the prominence of heavy-element lines such as iron led astronomers to assume the Sun had the same chemical composition as Earth's crust.²⁷ This paradigm was overturned in 1925 by British-born astronomer Cecilia Payne (later Payne-Gaposchkin) in her doctoral dissertation at Harvard College Observatory.²⁸ Applying Indian physicist Meghnad Saha's ionization theory to stellar spectra, Payne demonstrated that spectral differences among stars reflect differing surface temperatures rather than elemental compositions, and proved that the Sun and other stars are composed overwhelmingly of hydrogen and helium. Although initially dismissed as spurious by leading astrophysicist Henry Norris Russell, independent studies confirmed her findings within a few years.

In the second half of the 20th century and early 21st century, the Sun stood at the center of three major scientific controversies in physics and astronomy²⁹³⁰:

In the 2020s, solar observation has been led by a fleet of space-based observatories—including the long-running Solar and Heliospheric Observatory (SOHO), NASA's Solar Dynamics Observatory (SDO), NASA's Parker Solar Probe, and the European Space Agency's (ESA) Solar Orbiter. Launched in February 2020, Solar Orbiter carries ten remote-sensing and in-situ instruments into an elliptical orbit that brings it within 42 million kilometers of the Sun (less than one-third the Earth–Sun distance), utilizing Venus gravity assists to incline its orbit above the ecliptic plane and capture the first high-latitude views of the Sun's polar regions and miniature coronal "campfires".

NASA's Parker Solar Probe, launched in August 2018 and named after astrophysicist Eugene Parker, has flown closer to the Sun than any previous spacecraft.³³³⁴ Protected by a heat shield capable of withstanding extreme thermal radiation, the probe crossed the Alfvén critical surface for the first time in April 2021 at a distance of 18.8 solar radii (13 million kilometers), becoming the first human-made object to enter the solar corona. Following its seventh and final gravity-assist flyby of Venus in November 2024, Parker Solar Probe achieved a record-breaking perihelion on December 24, 2024, skimming just 6.1 million kilometers (3.8 million miles) above the solar surface at a speed of 692,000 kilometers per hour (430,000 miles per hour), followed by additional close passes at this distance in March and June 2025.³⁵ Observations from Parker Solar Probe and Solar Orbiter have revealed that sudden S-shaped magnetic reversals known as "switchbacks"—which help accelerate the fast solar wind—originate near magnetic funnels at the solar surface rather than deep inside the corona itself, helping to refine models of coronal heating and wind acceleration.

These close-range missions have coincided with Solar Cycle 25, which began at solar minimum in December 2019. Although the international Solar Cycle 25 Prediction Panel initially forecasted in 2019 that the cycle would be relatively weak—comparable to Solar Cycle 24, with a predicted smoothed sunspot peak of roughly 115 in mid-2025—solar activity ramped up far faster and stronger than anticipated.³⁶ Sunspot counts averaged 31 percent higher per day than at equivalent points in the previous cycle, making Solar Cycle 25 the most active in two decades. The cycle reached its unsmoothed monthly sunspot peak of 216 in August 2024 and its definitive 13-month smoothed maximum of 161 in October 2024. This elevated maximum phase generated numerous major space weather events, including an X9.0 solar flare in October 2024, an extreme 15.7 percent Forbush decrease in galactic cosmic rays during May 2024, and five Ground-Level Enhancement (GLE) radiation events between 2021 and November 2025, with solar activity gradually declining toward the next solar minimum expected around 2030.³⁷