The Sun as a Star

At a glance

The Sun is a main-sequence star of spectral type G2 V and holds almost all of the mass in the Solar System. The figures below come from NASA’s NSSDCA Sun Fact Sheet. Central values come from standard solar models.

Source: NASA NSSDCA Sun Fact Sheet, last updated 9 May 2024 (checked October 2026). The Earth ratios and the light travel time are calculated from the fact-sheet values.

Sources: [1]

Internal structure and atmosphere

The Sun is a sphere of plasma whose layers are defined by how energy moves through them. In the core, which NASA describes as extending to about 138,000 km from the centre, temperatures exceed 15 million °C and nuclear fusion takes place. Above it lies the radiative zone, where photons are absorbed and re-emitted again and again. NASA estimates that energy takes about 170,000 years to get from the core to the top of the convection zone.

Helioseismology places the base of the convection zone at a depth of 0.287 solar radii, about 0.713 of the way out from the centre, with an error as small as 0.001 radii. Above this boundary, rising and sinking plasma carries the energy. Around it lies the tachocline, a thin shear layer at roughly 0.65–0.75 solar radii. Below the tachocline the interior rotates almost like a solid body. Above it, rotation varies with latitude, from about 25 days at the equator to about 36 days near the poles. Many dynamo models treat this shear layer as a key place where magnetic fields are amplified.

The visible “surface” is the photosphere, only about 400 km thick, at roughly 5,500 °C. Above it, the chromosphere throws up jets called spicules, which can reach about 100 km/s. Then comes the narrow transition region, where the temperature rises sharply, and finally the corona, at about 2 million °C.

Sources: [1] [2] [3] [4]

Energy generation and solar neutrinos

The Sun shines by fusing hydrogen into helium. Most of its energy comes from the proton–proton (pp) chain, in which four protons are turned, step by step, into a helium-4 nucleus, giving off positrons, neutrinos and energy. The CNO cycle, which uses carbon, nitrogen and oxygen as catalysts, dominates in more massive stars. In the Sun, according to standard solar models, it supplies only about 1.3% of the luminosity. Dividing the luminosity by c² shows that the Sun converts roughly 4.3 million tonnes of mass into energy every second.

Photons take a very long time to escape from the core, but neutrinos leave almost at once, which makes them a direct probe of the core. For decades, detectors counted far fewer solar neutrinos than the models predicted, up to two-thirds fewer according to the Nobel committee. This became known as the solar neutrino problem. The Sudbury Neutrino Observatory (SNO) in Canada showed that the neutrinos had not vanished. They had changed “flavour” on the way to Earth. The 2015 Nobel Prize in Physics went to Takaaki Kajita (Super-Kamiokande) and Arthur B. McDonald (SNO) for discovering neutrino oscillations, which show that neutrinos have mass.

Sources: [3] [5]

Helioseismology: looking inside the Sun

Sound waves trapped inside the Sun make its surface move up and down, and Doppler measurements can detect this motion. The oscillations have periods of about five minutes and were first discovered by Leighton and colleagues in 1962. Their power peaks at about 3 mHz, with useful modes between about 1.5 and 5 mHz. Because each mode reaches a different depth, the measured frequencies can be inverted to work out the sound speed, density and rotation throughout the interior.

This is how the depth of the convection zone and the position of the tachocline were measured. The ground-based GONG network has been observing these modes continuously since 1995. The MDI instrument on SOHO has been operating since 1996, and HMI on SDO continues the record. Helioseismology also tests the standard solar model, giving a surface helium abundance close to 0.25. The models use an age of 4.570 ± 0.006 billion years, taken from meteorite dating.

Sources: [3] [4]

Magnetic field, dynamo and Solar Cycle 25

The Sun’s magnetic field is generated by a dynamo, which arises from convection and differential rotation in a conducting plasma. Its most visible sign is sunspots: dark, cooler patches of the photosphere where strong magnetic flux breaks through. Sunspot numbers rise and fall over a cycle of about 11 years. NASA notes that the magnetic poles also swap places about every 11 years, so the field returns to its original orientation after about 22 years.

Solar Cycle 25 began in December 2019. On 15 October 2024, NASA, NOAA and the Solar Cycle Prediction Panel announced that the Sun had entered its solar maximum period. According to SILSO at the Royal Observatory of Belgium, the 13-month smoothed sunspot number peaked at 161 in October 2024, earlier than the panel’s forecast of July 2025. In 2025, Solar Orbiter’s first polar images showed both magnetic polarities mixed together at the south pole, the signature of the field reversal that happens around maximum. As of October 2026, the cycle is in its declining phase, with SILSO’s daily estimated sunspot numbers around 20 in late October.

Sources: [2] [6] [7] [8] [9]

Flares, coronal mass ejections and the solar wind

A solar flare is a burst of radiation, lasting from minutes to hours, released when magnetic energy stored above an active region is suddenly freed. NOAA classifies flares by their peak X-ray flux in the 0.1–0.8 nm band, as measured by the GOES satellites. The classes are A, B, C, M and X, and each is ten times stronger than the one before. This radiation reaches Earth at the speed of light and ionises the upper atmosphere on the sunlit side, which can black out HF radio. On NOAA’s radio blackout scale, an X1 flare counts as R3 (“strong”) and an X20 as R5 (“extreme”).

Coronal mass ejections (CMEs) are huge clouds of magnetised plasma thrown out from the corona. NASA notes that they can travel at more than a million miles per hour (about 1.6 million km/h). When one hits Earth’s magnetosphere, it can cause geomagnetic storms, auroras, and disruption to power grids and satellites. These effects are known as space weather.

The Sun also gives off a continuous flow of plasma, the solar wind, which leaves the corona at supersonic speeds and inflates the heliosphere, a bubble extending far beyond the planets.

Sources: [2] [10]

Interactive lab: when does a solar storm reach Earth?

The lab opens on the 1859 Carrington event preset: a CME travelling at about 2,360 km/s reaches Earth in 17.6 hours, while light covers the same path in 8.3 minutes, so the flare’s light arrives hours ahead of the storm. At that speed, satellites at the L1 point can give only about 10 minutes’ warning. Lower the speed to that of the slow solar wind (≈ 400 km/s), or switch the target to Mars or Jupiter, and watch the arrival time stretch into days.

The Sun’s life cycle

The Sun formed along with the rest of the Solar System about 4.6 billion years ago. Meteorite dating puts its arrival on the main sequence at 4.570 ± 0.006 billion years ago. NASA estimates that the Sun is a little less than halfway through its life, with roughly 5 billion years left.

When the hydrogen in the core runs out, fusion will move into a shell around the core and the outer layers will swell, turning the Sun into a red giant. NASA expects this expansion to engulf Mercury and Venus, and possibly Earth. After shedding its outer layers, the Sun will end its life as a slowly cooling white dwarf.

Sources: [2] [3]

Missions and observatories

The ESA/NASA SOHO mission, launched in 1995 to the Sun–Earth L1 point about 1.5 million km from Earth, set the standard for continuous solar monitoring. NASA’s SDO, launched on 11 February 2010, takes multi-wavelength images and magnetograms with its AIA, HMI and EVE instruments. On the ground, the 4-metre Daniel K. Inouye Solar Telescope (DKIST) on Haleakalā, Maui, is the largest solar telescope in the world.

Parker Solar Probe’s 2026 status comes from NASA’s mission page. For Solar Orbiter’s specifications, see ESA’s mission page.

Sources: [9] [11] [12] [13] [14]

Open questions

Coronal heating: The photosphere is at about 5,500 °C, while the corona is around 2 million °C. Leading explanations include magnetic waves and many small reconnection events (“nanoflares”), but how much each contributes is still unknown. Aditya-L1 lists it as a goal, and Proba-3 observes the faint inner corona.

Solar wind acceleration: It is still debated where the fast and slow solar wind come from and how they reach such high speeds. Parker Solar Probe samples the wind close to its source, while PUNCH follows the transition from corona to wind across a wide field.

Cycle prediction: Dynamo theory cannot yet reliably predict the strength and timing of a cycle. Solar Cycle 25 is a case in point: it peaked earlier than forecast. Solar Orbiter’s polar observations, which will improve as its orbit tilts further, are expected to be key for forecasting the next minimum and the next cycle.

Sources: [2] [7] [9] [11] [12] [13] [14]

Working with solar data

The NASA SDO data page provides near-real-time AIA images, HMI magnetograms and daily movies. Helioviewer.org lets you layer images from different instruments, make movies and request data through the Virtual Solar Observatory. Solar Orbiter data are available from ESA’s Solar Orbiter Archive (SOAR), which also supports programmatic access via TAP.

For space weather, NOAA’s SWPC publishes GOES X-ray flux, real-time solar wind data, the Kp index, and 3-day and 27-day forecasts. The official international sunspot number, butterfly diagrams and cycle forecasts are available from SILSO.

Safety: never look at the Sun with the naked eye, through sunglasses, or through binoculars or a telescope without a proper filter. It can cause permanent eye damage. Use only certified solar filters (ISO 12312-2) fitted securely over the front of the instrument, or observe indirectly by projection.

Sources: [8] [10]

Known vs open questions

Established

• The Sun is a G2 V main-sequence star about 4.57 billion years old. The pp chain powers it, and the CNO cycle supplies about 1.3% of its luminosity.
• The convection zone begins at about 0.713 solar radii, and the tachocline lies at roughly 0.65–0.75 solar radii.
• Neutrino oscillations solved the solar neutrino problem (Nobel Prize 2015).
• Solar Cycle 25’s smoothed maximum came in October 2024, and the cycle is declining as of October 2026.

Open

• What heats the corona to millions of degrees?
• Where do the fast and slow solar wind originate, and how are they accelerated?
• How exactly does the dynamo work, and can cycle strength be predicted reliably?
• What triggers individual flares and CMEs, and how far ahead can they be forecast?