The Sun: 380 billion billion megawatts, far more than life needs

A birth in darkness

The collapse of the molecular cloud

About 4.57 billion years ago, a gravitational instability in a giant molecular cloud – composed mainly of hydrogen and interstellar dust – triggered the collapse of matter. Most of the mass concentrated at the center to form a protostar, while a disk of gas and dust, where the planets would be born, flattened around it.

The ignition of nuclear fusion

The progressive compression raised the temperature and pressure at the center of this protostar. When the central temperature reached about 10 million kelvins, thermonuclear fusion of hydrogen nuclei into helium began via the proton-proton chain. This exothermic process released enough energy to counterbalance gravity: the Sun was born, and this equilibrium has lasted for 4.57 billion years.

An unimaginable power

The conversion of mass into energy

The Sun converts about 620 million tonnes of hydrogen per second into about 616 million tonnes of helium. The difference, nearly 4.3 million tonnes, is transformed into energy according to the relativistic equation \( E = \Delta m \cdot c^2 \). The radiated power reaches \(3{,}828 \times 10^{26}\) watts, the nominal value adopted by the International Astronomical Union in 2015. A small fraction of the fusion energy, on the order of 2%, is carried away by neutrinos rather than as light.

Depending on the source, the mass of hydrogen converted each second varies from 600 to 630 million tonnes, depending on the luminosity and fusion efficiency adopted.

A comparison with nuclear power plants

According to the IAEA's PRIS database, 417 nuclear power reactors were in operation in mid-July 2026, for a net electrical capacity of about 380 GW, or \(3{,}8 \times 10^{11}\) W. The Sun's power is therefore about \(10^{15}\) times greater, that is, a million billion times. The ratio remains on the order of \(10^{14}\) to \(10^{15}\) if we count the thermal power of the reactors rather than their electrical power. This energy flux is responsible for the thermal structuring of the entire solar system, keeping the planets in distinct habitable zones.

The energy received by Earth

At 150 million kilometers, Earth intercepts, over its cross-section of \(1{,}27 \times 10^{14}\ \text{m}^2\), a flux of about 1,361 W/m² (nominal total solar irradiance), or \(1{,}7 \times 10^{17}\) W. This represents about \(4{,}5 \times 10^{-10}\) of the Sun's total power, that is, one half-billionth. This tiny fraction is enough to maintain a global average temperature of about 15°C, essential for life. Without the natural greenhouse effect, it would be close to −18°C.

Neutrinos, direct messengers from the solar core

Trapped photons, free neutrinos

The energy produced in the core does not escape directly: photons are absorbed and re-emitted an immense number of times before reaching the surface. Neutrinos, on the other hand, interact so little with matter that they cross the Sun in a few seconds. They are the only direct probe of the zone where fusion takes place, at about 15 million kelvins.

The travel time of photons from the core to the surface depends heavily on the models: estimates range from several tens of thousands of years to more than a hundred thousand years, or even a million years according to some sources.

Borexino and the CNO cycle

Two mechanisms fuse hydrogen into helium: the proton-proton chain, which produces about 99% of the Sun's energy, and the CNO cycle (carbon-nitrogen-oxygen), proposed in the late 1930s and responsible for about 1% of the Sun's energy. In more massive stars, this cycle dominates. In 2020, the Borexino experiment, installed under Gran Sasso in Italy, detected neutrinos from the CNO cycle for the first time, with a statistical significance greater than 5 sigma. The signal is faint: about 7 events per day for 100 tonnes of target.

Measuring metallicity and core rotation

The CNO neutrino flux depends on the abundance of heavy elements in the core. It therefore opens the way to a direct measurement of solar metallicity, still debated. In July 2025, a French team also predicted, through hydrodynamic simulation and topological wave theory, an oscillation with a period of about one hour, sensitive to the rotation of the deep core. Its detection would offer a new window into the star's core.

The increase in its thermal power

The increase in solar luminosity

Since its birth 4.57 billion years ago, the power radiated by the Sun has not remained constant. Stellar evolution models indicate that a yellow dwarf star like the Sun sees its luminosity grow slowly but continuously. The progressive transformation of hydrogen into helium in the core increases the central temperature and the rate of fusion reactions.

Solar luminosity increases on average by about 10% per billion years. Thus, at its formation, the Sun radiated only about 70% of its current power, or:

• At birth: about \(2{,}7 \times 10^{26}\ \text{W}\)
• Currently: about \(3{,}83 \times 10^{26}\ \text{W}\)
• In 2 billion years: about \(4{,}6 \times 10^{26}\ \text{W}\)

The faint young Sun paradox

With only 70% of its current power, the young Sun should have left Earth frozen. Yet geological traces indicate the presence of liquid water as early as the Archean. The explanation generally put forward is an atmosphere richer in greenhouse gases. This gradual increase in luminosity has thus modified Earth's radiative balance over geological time, influencing the evolution of the atmosphere and the biosphere.

Consequences for Earth's climate

In the long term, this progressive warming will make Earth's surface uninhabitable well before the end of the Sun's life. According to a modeling study published in 2021 in Nature Geoscience, Earth's oxygen-rich atmosphere should persist for only about 1.08 ± 0.14 billion years. It would be followed by a brutal deoxygenation, even before the onset of a runaway greenhouse effect and the massive loss of the oceans.

This deadline is an estimate from a model. It remains sensitive to assumptions about Earth's geochemical cycles.

Solar matter and the solar wind

The composition of the solar plasma

The Sun is a sphere of plasma whose mass composition is dominated by hydrogen (about 74%) and helium (24 to 25%). The rest, about 1.4 to 1.6%, consists of metals in the astrophysical sense: oxygen, carbon, iron, neon, etc. The exact value of this metallicity is still debated depending on the spectroscopic analysis methods.

Conditions in the core

The core extends to about a quarter of the solar radius. The temperature there reaches 15 million kelvins and the density about 150 g/cm³, or 150 times that of water. Thermal agitation is such that electrons are torn from atomic nuclei, forming a fully ionized plasma.

The solar wind

Part of the coronal matter escapes continuously as charged particles – electrons and protons – constituting the solar wind, which propagates at speeds of 300 to 800 km/s. The corona, heated to one or two million degrees while the surface is only about 5,800, remains one of the great enigmas of solar physics.

The consequences of the solar wind

The polar auroras

By interacting with Earth's magnetosphere, the solar wind induces currents of energetic particles that penetrate the polar atmospheric layers. These interactions excite nitrogen and oxygen molecules, producing the aurora borealis and australis, mainly in green (557.7 nm), red (630.0 nm) and blue wavelengths.

Comet tails

Comets, for their part, reveal the direction of the solar wind through the formation of an ion tail, always oriented opposite the Sun.

The Sun under surveillance: cycle 25 and space probes

Solar cycle 25 in its declining phase

On October 15, 2024, NASA, NOAA and the international prediction panel announced that the Sun had reached the maximum of its 25th 11-year activity cycle. This cycle exceeded the initial forecasts of 2019, which predicted a modest peak of about 115 (smoothed sunspot number) around July 2025. Since then, activity has entered its declining phase. Powerful eruptions remain possible, but the next minimum is expected around 2030-2031.

Parker Solar Probe, closest to the corona

On December 24, 2024, NASA's Parker Solar Probe passed 6.1 million kilometers from the Sun's surface, at about 692,000 km/h. This is the closest passage and the fastest object ever achieved by humanity. This distance and speed have been matched during seven flybys since, the last of which took place on September 4, 2026, the 29th passage of the mission. The mission has been extended until 2029 and will observe the Sun during the declining phase of the cycle.

Solar Orbiter, first images of the poles

Launched in 2020, the Solar Orbiter probe (ESA-NASA) inclined its orbit to 17° in 2025. It thus obtained the first images of the Sun's poles, outside the ecliptic plane. It observed a "disordered" magnetic field at the south pole, with mixed north and south polarities, a phenomenon typical of solar maximum, when the magnetic field reverses. Before it, only the Ulysses probe had flown over the poles (1990-2009), without imaging instruments. The orbit's inclination will increase to 24° on December 24, 2026, then to 33° from June 2029.

A still bright future

The red giant phase

Currently in the main sequence phase, the Sun produces its energy by fusing hydrogen into helium in its core. It has already consumed about half of this fuel, and this stable phase will last another 5 billion years or so. Then the core will collapse on itself, causing the outer layers to expand: the Sun will become a red giant, engulfing Mercury, Venus and probably Earth.

Earth's fate depends on the models: the Sun's mass loss will push the orbits outward, while tidal effects could bring the planet closer to the swollen star.

The white dwarf

Too low in mass to explode as a supernova, the Sun will end its life as a white dwarf, a dense remnant the size of Earth, leaving behind a planetary nebula. Until then, it will continue to supply Earth's biosphere with energy.