The Sun is the only star in our solar system. It is a huge ball of hot plasma made mostly of hydrogen and helium, and its gravity keeps planets, asteroids, and comets in orbit.
Key facts about the Sun
The Sun is about 4.5 billion years old. It formed when a cloud of gas and dust collapsed under gravity.
Earth is about 93 million miles, or 150 million kilometers, from the Sun. This average distance is called one astronomical unit.
The Sun is about 100 times wider than Earth. About 1.3 million Earths could fit inside its volume if empty space between them were ignored.
The Sun contains about 99.8 percent of the solar system’s mass. Its gravity dominates our planetary neighborhood.
The Sun has no solid ground. The bright layer that looks like a surface is called the photosphere.
The photosphere is about 10,000 degrees Fahrenheit, or 5,500 degrees Celsius. The core is far hotter at about 27 million degrees Fahrenheit, or 15 million degrees Celsius.
The Sun shines because hydrogen nuclei fuse into helium in its core. Fusion releases energy that eventually travels outward as light and heat.
Sunlight takes about eight minutes to reach Earth. That means every ordinary view of sunlight shows the Sun as it was several minutes earlier.
The Sun does not rotate like a solid ball. Plasma near its equator turns around in about 25 Earth days, while plasma near its poles takes about 36 days.
Sunspots are cooler magnetic regions on the photosphere. They look dark only because the surrounding surface is brighter and hotter.
The number of sunspots rises and falls in a cycle lasting roughly 11 years. Near the active part of a cycle, flares and other eruptions are more common.
The corona is hotter than the visible surface. NASA lists coronal temperatures up to about 3.5 million degrees Fahrenheit, or 2 million degrees Celsius.
The solar wind is a stream of particles flowing from the Sun’s corona. It helps form the enormous heliosphere around the solar system.
The Sun is about halfway through its life. NASA expects it to expand into a red giant in about 5 billion years and later become a white dwarf.
How does the Sun make light?
Gravity squeezes the Sun’s core until it becomes extremely hot and dense. There, fusion combines hydrogen nuclei into helium. A small amount of mass becomes energy, which supports the Sun against gravity and eventually escapes into space.
Energy does not travel straight from the core to the surface. NASA estimates that radiation takes about 170,000 years to work through the radiative zone, repeatedly interacting with matter, before moving through the convection zone and escaping from the photosphere.
What happens above the surface?
The atmosphere above the photosphere includes the chromosphere and corona. Solar flares release bursts of radiation, while coronal mass ejections can throw large clouds of magnetized plasma into space. These are different events, although they can happen together.
When solar material and magnetic fields disturb Earth’s magnetic environment, they can produce auroras and affect radio, navigation, satellites, and power systems. Scientists call these changing conditions space weather.
A spacecraft that touched the Sun
Parker Solar Probe flies through the corona to measure particles and magnetic fields close to their source. On June 8, 2026, it completed its 28th close approach and again came within 3.8 million miles of the solar surface, matching the distance record it first set in December 2024.
Common myths about the Sun
Myth: The Sun is on fire like a campfire. A campfire burns through a chemical reaction involving oxygen. The Sun is plasma powered by nuclear fusion, not ordinary burning.
Myth: Sunspots are holes. They are cooler parts of the visible surface where concentrated magnetic fields interfere with heat flow.
Myth: The corona cannot be hotter than the surface. Measurements show that it is much hotter. Explaining how the corona receives and keeps that energy is still an active research problem.
Myth: Ordinary sunglasses make it safe to stare at the Sun. They do not. Except during the brief total phase of a total solar eclipse, direct viewing requires a safe solar viewer that complies with the ISO 12312-2 standard. Cameras, binoculars, and telescopes need proper filters fixed to the front of their optics.
Frequently asked questions
Is the Sun the biggest star?
No. NASA describes it as a medium-sized star. Some stars are much larger, while many others are smaller.
Why do sunspots come and go?
The Sun’s moving plasma creates changing magnetic fields. Sunspots can last from days to months, and their total number changes with the roughly 11-year activity cycle.
What was the Carrington Event?
It was the strongest geomagnetic storm on record. Richard Carrington observed a solar flare on September 1, 1859; telegraph systems malfunctioned, and auroras were reported far closer to the equator than usual.
Will the Sun explode as a supernova?
No. NASA’s account of the Sun’s future has it expanding into a red giant and ending as a white dwarf, not exploding as a supernova.
The Sun is a G2 V main-sequence star about 4.5 billion years old. It looks special because it is close to Earth, but studying it also helps astronomers understand other stars.
Numbers worth knowing
Average Earth-Sun distance: about 93 million miles, or 150 million kilometers.
Diameter: about 865,000 miles, or 1.4 million kilometers, roughly 100 Earth diameters.
Share of solar-system mass: about 99.8 percent.
Core temperature: about 27 million degrees Fahrenheit, or 15 million degrees Celsius.
Photosphere temperature: about 10,000 degrees Fahrenheit, or 5,500 degrees Celsius.
Corona temperature: up to about 3.5 million degrees Fahrenheit, or 2 million degrees Celsius.
Equatorial rotation: about 25 Earth days, compared with about 36 days at the poles.
Galactic orbit: about 230 million Earth years for one trip around the Milky Way, using NASA’s Sun fact-page value.
The average Earth-Sun distance defines one astronomical unit, or AU. Because Earth’s orbit is not a perfect circle, the actual distance changes during the year.
From fusion to sunlight
The Sun remains stable because two effects balance. Gravity pulls its mass inward, while pressure associated with the hot interior and fusion-produced energy supports it outward. This long-lasting balance is called hydrostatic equilibrium.
The proton-proton chain is the main fusion process in a star like the Sun. Its net result converts four hydrogen nuclei into one helium-4 nucleus, while releasing energy, positrons, and neutrinos. The neutrinos escape quickly and can be detected on Earth.
Photons take a very different route. In the radiative zone they repeatedly interact with matter, so NASA estimates that energy takes about 170,000 years to reach the top of that zone. Convection then carries energy through the outer interior before radiation escapes from the photosphere.
Once light leaves the Sun, its trip to Earth takes only about eight minutes and twenty seconds at the average distance. The long interior transport time and the short empty-space travel time describe separate parts of the journey.
The Sun’s layers
The core occupies roughly the inner quarter of the Sun’s radius and contains the fusion reactions that supply its energy. Beyond it, the radiative zone extends to about 70 percent of the radius.
The tachocline is the thin transition between the radiative and convection zones. The radiative interior rotates more nearly as a unit, while the convection zone rotates differentially, so this boundary contains strong shear and is important to solar-dynamo research.
The convection zone is the outermost 30 percent of the radius. Hot plasma rises, cools, and sinks. Above it, the visible photosphere is followed by the chromosphere, transition region, and extended corona.
Sunspots and the magnetic cycle
Sunspots are cooler regions produced by concentrated magnetic fields. NASA says they can last from days to months and typically span 1,000 to 100,000 miles, so a spot that looks tiny in an image may be larger than Earth.
Sunspot counts rise and fall with a cycle of roughly 11 years. Near maximum, solar eruptions become more frequent, and the Sun’s magnetic poles reverse. Returning the global field to its original polarity therefore takes about 22 years, or two sunspot cycles.
Solar Cycle 25 began at the December 2019 minimum. NASA and NOAA announced in October 2024 that the Sun had entered its maximum phase, but they cautioned that the exact peak could be identified only after a sustained decline. NASA’s June 2026 Parker update describes the mission as continuing observations into the declining phase of solar activity.
Flares, CMEs, and space weather
A solar flare is a burst of electromagnetic radiation. A coronal mass ejection, or CME, is a cloud of magnetized solar plasma thrown into space. They often occur together, but one is not simply another name for the other.
Earth’s atmosphere and magnetic field provide major protection, yet strong space weather can still disturb radio signals, navigation, satellites, astronauts, and power grids. Whether a CME produces a major geomagnetic storm depends on its direction, speed, and magnetic orientation when it reaches Earth.
The 1859 Carrington Event connects solar activity to effects on technology. Carrington saw a bright flare on September 1; about 17 hours later, intense auroras appeared unusually far from the poles and currents disrupted telegraph systems.
Why is the corona so hot?
The visible surface is near 5,500 degrees Celsius, while parts of the corona reach millions of degrees. The temperature therefore rises outward across the thin atmosphere even though the core remains the Sun’s hottest region.
NASA describes two leading families of explanations: Alfvén waves that carry and dissipate magnetic energy, and many small reconnection events often called nanoflares. These ideas are not mutually exclusive, and their relative importance can vary between solar regions.
Parker Solar Probe’s record
Parker Solar Probe launched in August 2018 to sample the corona and solar wind close to their source. Its heat shield points toward the Sun while its instruments measure fields, waves, particles, and plasma.
On June 8, 2026, Parker completed its 28th close approach, again reaching 3.8 million miles from the solar surface. It matched the distance record first set on December 24, 2024, and NASA reported that its systems were operating normally after the pass.
Frequently asked questions
Does the Sun have a solid surface?
No. The photosphere is the layer from which most visible light escapes, but it is plasma rather than solid ground.
Does a high sunspot count mean Earth receives dramatically more visible light?
No. Sunspot count is mainly an activity indicator. Dark spots are accompanied by bright magnetic regions, but the Sun’s total energy output changes only slightly across its normal cycle.
Can scientists predict every solar storm?
No. Observatories monitor active regions and forecasts can provide useful warnings, but the timing, direction, and magnetic structure of individual eruptions remain difficult to predict precisely.
Is it safe to view the Sun through a telescope?
Only with a purpose-built solar filter correctly attached to the front of the optics. Ordinary sunglasses and handheld eclipse glasses placed behind a telescope are unsafe.
The Sun is a G2 V main-sequence star. In that label, G2 describes its spectrum and temperature range, while luminosity class V identifies a hydrogen-fusing dwarf rather than a giant.
Scale, power, and temperature
The International Astronomical Union defines nominal solar conversion constants of exactly 6.957 × 10^8 meters for radius and 3.828 × 10^26 watts for luminosity. These nominal values are convenient units, not claims that the changing physical Sun is measured with infinite precision.
The Stefan-Boltzmann relation, L = 4πR²σT_eff⁴, links luminosity, radius, and effective temperature. Applying it to the nominal radius and luminosity gives a nominal solar effective temperature of 5,772 kelvins. Effective temperature is the temperature a perfect blackbody of the same radius would need to radiate the same total power.
NASA gives a mean Earth-Sun distance near 150 million kilometers and a solar diameter near 1.4 million kilometers. Dividing distance by the speed of light yields an average one-way light time of about 499 seconds, or 8 minutes 19 seconds.
Fusion and energy transport
The Sun’s dominant energy source is the proton-proton chain. The overall reaction turns four protons into a helium-4 nucleus plus two positrons, two electron neutrinos, and energy. Some intermediate steps have alternative branches, so the shorthand net reaction should not be mistaken for one four-particle collision.
The energy release follows from a mass difference between the initial and final products. Neutrinos carry away part of the energy almost immediately, while photons interact repeatedly with dense plasma before their energy reaches the surface.
NASA estimates about 170,000 years for radiative energy to move from the core to the top of the convection zone. This is a statistical transport time, not the uninterrupted travel time of one original gamma-ray photon.
The outer convection zone transports energy by bulk plasma motion. Bright granules in the photosphere show hot rising material; darker lanes mark cooler sinking material.
A layered but fluid star
The fusion core extends through roughly the inner quarter of the solar radius. The radiative zone reaches from about 0.25 to 0.70 solar radii, and the convection zone occupies approximately the outer 30 percent by radius.
The tachocline is a thin shear layer between the more uniformly rotating radiative interior and the differentially rotating convection zone. It is important in many dynamo models, but NASA describes its precise role in generating the global magnetic field as an active subject of simulation, not a settled one-location answer.
The photosphere is not a material boundary like Earth’s crust. It is the optical layer from which most visible photons escape, and its apparent edge exists because opacity changes quickly with height.
The corona is tenuous but can reach about 2 million degrees Celsius, far above the photosphere’s roughly 5,500 degrees. The core, at about 15 million degrees Celsius, remains hotter than both.
Rotation, magnetism, and the solar cycle
The Sun’s plasma rotates differentially: NASA gives about 25 days at the equator and 36 days near the poles. Helioseismology, which infers internal structure and flow from oscillations, shows that differential rotation extends through the convection zone.
The solar dynamo converts kinetic energy in moving conductive plasma into magnetic energy. Differential rotation, turbulent convection, and meridional circulation are all implicated, but present models simplify an exceptionally broad range of scales.
In a Babcock-Leighton picture, differential rotation helps build a toroidal field, tilted bipolar active regions emerge, and the dispersal and poleward transport of their magnetic flux rebuilds a reversed poloidal field. It is an influential framework, not a complete first-principles solution to the solar cycle.
The familiar sunspot cycle lasts roughly 11 years, but the global magnetic polarity returns to its starting orientation after about 22 years. Individual cycles vary in strength, duration, and the timing of their peak.
Sunspots and eruptions
Sunspots are magnetically concentrated regions in which inhibited heat transport leaves the photosphere cooler and dimmer than its surroundings. They are not holes, and they do not prove that the entire Sun has cooled.
A solar flare is an abrupt release of electromagnetic radiation associated with magnetic reconnection. A CME ejects magnetized plasma. One can accompany the other, but their radiation and material propagate differently and can reach Earth on different schedules.
Geomagnetic effects depend strongly on the arriving magnetic field’s orientation. A fast Earth-directed CME is not guaranteed to produce the most severe possible storm if its field couples weakly to Earth’s magnetosphere.
The Maunder Minimum without a climate myth
The Maunder Minimum names the approximately 1645 to 1715 interval when observers recorded unusually few sunspots. It overlapped part of the Little Ice Age, but overlap alone does not establish sole causation.
NASA’s climate review says there is little evidence that the Maunder Minimum started the Little Ice Age by itself. The colder interval began earlier, and volcanic aerosols, ocean variability, land-use changes, and reduced solar activity all enter the historical explanation.
A future grand solar minimum would not cancel modern human-caused warming. NASA summarizes modeling that finds only a modest, temporary cooling effect compared with the warming driven by greenhouse-gas increases.
Coronal heating and Parker Solar Probe
The coronal-heating problem asks how energy crosses and dissipates in the solar atmosphere strongly enough to maintain million-degree plasma above a much cooler photosphere. Leading mechanisms include dissipation of Alfvénic waves and many small reconnection events called nanoflares.
Those mechanisms are not exclusive. Reconnection can launch waves, and different magnetic environments may divide the heating budget differently, so one observation need not eliminate an entire family of models.
Parker Solar Probe reduces the uncertainty caused by measuring solar-wind plasma only after it has traveled to Earth. On June 8, 2026, its 28th close approach again reached 3.8 million miles from the surface, matching the record first achieved on December 24, 2024.
Frequently asked questions
Is the Sun an average star?
“Average” depends on the comparison. The Sun is a dwarf rather than a giant and NASA calls it medium-sized, but it is more massive than the small red dwarfs that dominate the stellar population.
Does G2 V mean the Sun is literally yellow?
No. It is a spectral and luminosity classification. Apparent color also depends on observing conditions, the detector, and how an image maps wavelength to color.
Did the Carrington Event destroy telegraph networks worldwide?
It caused severe disturbances, shocks, sparks, and some fires in telegraph equipment, but broad phrases such as “destroyed the world’s network” overstate a varied historical record.
Will the Sun become a supernova?
No. NASA’s stellar-evolution summary has the Sun becoming a red giant and then a white dwarf. Its future does not include a core-collapse supernova.
Solar physics is unusually constrained for stellar astrophysics: the Sun’s radius, luminosity, spectrum, oscillation modes, surface fields, neutrino fluxes, and local wind can all be measured with detail impossible for distant stars. That abundance of evidence also exposes small disagreements that an introductory model can hide.
What a standard solar model actually does
A standard solar model evolves a one-solar-mass star to the solar age while solving spherical hydrostatic structure, energy generation, energy transport, and composition change. It is calibrated by adjusting initial helium, initial heavy-element abundance, and a convection parameter so that the model matches the present luminosity, radius, and surface composition ratio at the adopted age.
Its inputs include an equation of state, radiative opacities, nuclear reaction rates, element diffusion, atmospheric boundary conditions, and a prescription such as mixing-length theory for convection. “Standard” therefore names a framework and input choices, not a model free of empirical calibration or uncertainty.
The International Astronomical Union’s nominal solar radius, 6.957 × 10^8 meters, and luminosity, 3.828 × 10^26 watts, are exact conversion constants. They prevent unit inconsistencies; they do not freeze the measured physical Sun at exact invariant values.
NASA divides the interior into a fusion core, radiative zone, tachocline, and convection zone. The approximate radial boundaries near 0.25 and 0.70 solar radii are useful summaries, not discontinuities in all physical quantities.
The pp chain and the CNO contribution
In the dominant pp I termination, two helium-3 nuclei combine to form helium-4 and return two protons. The earlier weak interaction p + p → d + e+ + νe sets the slow pace that allows the Sun to shine for billions of years.
The pp II and pp III branches use helium-4 as an intermediate target and produce neutrinos with different energy spectra. Detectors can therefore test more than one weighted average of the solar core.
The CNO cycle also converts four protons into helium while carbon, nitrogen, and oxygen nuclei act as catalysts. It supplies only a minor share of the present Sun’s power but becomes increasingly important in hotter, more massive main-sequence stars.
In 2020, the Borexino Collaboration reported the first experimental evidence for solar neutrinos from the CNO fusion cycle. The result established that the catalytic branch operates in the Sun; it did not overturn the pp chain’s dominance.
Why the solar-neutrino problem was a particle-physics discovery
Early radiochemical experiments detected fewer electron neutrinos than standard solar calculations predicted. Changing the solar core enough to remove the deficit would also have conflicted with other solar observations.
Super-Kamiokande measured direction and energy information from solar neutrino scattering, while the Sudbury Neutrino Observatory used heavy water to distinguish electron-neutrino sensitivity from sensitivity to the total active-flavor flux. The total was compatible with solar production while the electron component was depleted.
Neutrino oscillation means a neutrino produced in an electron-flavor state can later be detected as another flavor. Matter in the Sun modifies the flavor evolution through the MSW effect, so the energy-dependent survival probability is not simply a fixed vacuum-average fraction.
The solution required neutrinos to have nonzero mass and mix, physics absent from the minimal Standard Model. Solar neutrino measurements thereby became simultaneous tests of stellar interiors and particle physics.
Helioseismology and the tachocline
Helioseismology infers internal sound speed, density, rotation, and the convection-zone boundary from the Sun’s global oscillation frequencies. Inversions are model-dependent, but their agreement with much of the standard model is one of the model’s strongest successes.
The radiative interior rotates much more nearly as a solid body than the differentially rotating convection zone. The thin transition around 0.70 solar radii is the tachocline, where radial and latitudinal shear can stretch magnetic field.
Calling the tachocline “the place where the dynamo is generated” is too categorical. NASA describes it as a presumed seat whose role remains under investigation, while dynamo research also considers convection-zone and near-surface contributions.
Internal gravity modes would probe the deep radiative interior and core more directly than acoustic p modes, but robust identification of individual solar g modes remains difficult. Claims of a fully mapped core rotation should therefore be treated cautiously.
Babcock-Leighton dynamos and what they leave open
In Babcock-Leighton models, differential rotation converts large-scale poloidal flux into toroidal flux. Buoyant magnetic structures emerge as systematically tilted bipolar regions, and their dispersal plus cross-equatorial cancellation and poleward transport regenerate a reversed poloidal field.
This framework links observed active-region tilts and surface-flux transport to polar-field reversal. It can reproduce important cycle features, but implementations depend on parameterized emergence, turbulent diffusion, meridional flow, pumping, and nonlinear saturation.
The 11-year sunspot cycle is one half of the approximately 22-year magnetic-polarity cycle. Hemispheric asymmetry, stochastic tilt scatter, variable flows, and uncertain deep transport help prevent exact cycle-to-cycle repetition.
Global simulations cannot yet resolve all relevant solar scales at realistic diffusivities and stratification. NASA’s tachocline simulations explicitly note that computational constraints keep their input parameters far from actual solar values, so extrapolation and observational tests remain essential.
The solar abundance problem
Three-dimensional, non-LTE photospheric analyses revised inferred carbon, nitrogen, and oxygen abundances downward by roughly 30 percent relative to older compilations. Standard models built with these lower metal abundances agree less well with helioseismic sound-speed, convection-zone-depth, and surface-helium constraints.
That mismatch is often called the solar abundance problem or, more broadly, the solar modeling problem. It is not evidence that helioseismology simply measured the wrong Sun or that spectroscopy alone supplies an uncontested correction.
Opacity is a leading suspect because metals influence radiative transport near the base of the convection zone. Revised opacity calculations, composition changes, diffusion, and additional mixing can improve selected diagnostics, but the reviewed models do not yield one generally accepted complete solution.
The problem illustrates parameter degeneracy: changes in composition, opacity, and mixing can produce overlapping structural effects. Multiple helioseismic inversions and neutrino fluxes are needed to separate them.
Coronal heating is an energy-budget problem
The photosphere radiates at an effective temperature near 5,772 kelvins, yet coronal plasma reaches megakelvin temperatures. The question is how magnetic and mechanical energy is transported upward, converted, and dissipated at the required rates in different environments.
Wave-heating models send Alfvénic disturbances into the atmosphere and dissipate them through turbulence, phase mixing, resonances, or mode conversion. Reconnection models release stress stored in braided magnetic fields through events spanning a broad energy distribution, including hypothesized nanoflares.
The categories interact: reconnection can generate waves, while waves and turbulence can promote small-scale current sheets. Evidence for one mechanism in one region does not by itself establish the global partition of coronal heating.
Parker Solar Probe samples fields, plasma, and particles before the solar wind has mixed over the full Sun-Earth distance. Its June 8, 2026 perihelion was the mission’s 28th close approach and again matched the 3.8-million-mile distance record first set in December 2024.
Precision traps in solar trivia
“A photon takes exactly 170,000 years to leave the Sun” is too literal. NASA’s value describes an estimated energy-transport timescale through the radiative interior. Absorption and re-emission erase the identity of an original core photon.
“The solar cycle is exactly 11 years” is false precision. Eleven years is a useful average for sunspot activity, while real cycles vary and the complete magnetic-polarity cycle spans roughly twice that interval.
“The Sun’s surface is 5,772 K” mixes definitions. The nominal effective temperature follows from nominal luminosity and radius; local photospheric temperatures vary with depth and magnetic structure.
“Detecting CNO neutrinos means the CNO cycle powers most sunlight” is wrong. Borexino detected the branch, while the pp chain remains dominant in the present Sun.
“The abundance problem has one known opacity fix” overstates the literature. Opacity is a leading contributor, but composition, equation-of-state, diffusion, and macroscopic-mixing uncertainties remain entangled.
