The Sun and How Stars Work
A research overview, from the solar core to the death of stars
September 2026
1. The Big Picture: A Star Is a Balancing Act
The single most important idea in stellar physics is that a star is a giant ball of gas locked in a tug-of-war. Gravity constantly tries to crush it inward, while pressure from its heat constantly pushes outward. When those forces balance, a state astronomers call hydrostatic equilibrium, you get a stable star that can shine for billions of years.
The Sun is a fairly ordinary example. It is about 4.6 billion years old, roughly 1.4 million km across, and holds about 99.8% of all the mass in the solar system. By mass it is roughly three-quarters hydrogen and one-quarter helium, with a light sprinkle of everything else.
The balance is also self-correcting. If the core gets slightly too hot, it expands and cools, and fusion slows. If it cools too much, gravity squeezes it, it heats up, and fusion speeds up again. In effect, the Sun has a built-in thermostat.
The Sun at a Glance
| Property | Value |
|---|---|
| Age | About 4.6 billion years |
| Diameter | About 1.4 million km (roughly 109 Earths) |
| Share of solar system mass | About 99.8% |
| Composition (by mass) | About 73% hydrogen, 25% helium, 2% heavier elements |
| Core temperature | About 15 million °C |
| Surface (photosphere) temperature | About 5,500 °C |
| Corona temperature | 1 to 3 million °C |
| Mass converted to energy | About 4 million tonnes per second |
| Light travel time to Earth | About 8 minutes 20 seconds |
| Rotation period | About 25 days at the equator, about 35 days near the poles |
2. The Engine: Nuclear Fusion in the Core
The core is about 15 million °C, and the pressure is so extreme that matter there is more than 100 times denser than water, even though it is still a plasma rather than a solid or liquid.
The main process is the proton-proton chain. In a few steps, four hydrogen nuclei fuse into one helium nucleus. The helium weighs about 0.7% less than the four hydrogen nuclei that made it, and that missing mass becomes energy, exactly as E = mc² predicts. The Sun does this on a staggering scale, converting around 4 million tonnes of matter into energy every second.
| Why the Sun doesn't explode Protons repel each other, and even at 15 million degrees they lack the energy to touch by classical physics. They fuse only through quantum tunneling. The first step also requires a proton to turn into a neutron via the weak nuclear force, which is extremely unlikely, so a typical proton in the core waits billions of years before fusing. That slowness is exactly why the Sun burns steadily for so long. |
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A second route, the CNO cycle, uses carbon, nitrogen, and oxygen as catalysts to turn hydrogen into helium. It supplies only about 1% of the Sun's energy but dominates in stars heavier than the Sun. It was purely theoretical until 2020, when the Borexino detector in Italy directly detected CNO neutrinos, confirming the process Hans Bethe and Carl Friedrich von Weizsäcker proposed in the 1930s.
3. How the Energy Gets Out
Sunlight reaches Earth in about 8 minutes, but the energy in that light was created in the core anywhere from tens of thousands to over a hundred thousand years earlier.
In the inner radiative zone, the plasma is so dense that a photon travels only a tiny distance before being absorbed and re-emitted in a random direction, stumbling outward like someone trying to leave a packed concert. About 70% of the way out, the gas becomes opaque enough that it begins to churn like boiling soup. This is the convection zone. The rice-grain texture seen in close-up images of the surface, called granulation, is the tops of these convection cells, each roughly the size of Texas.
The visible surface, the photosphere, is about 5,500 °C. It is not solid; it is simply the layer where the gas becomes transparent enough for light to escape into space.
4. How We See Inside the Sun
Neutrinos
Neutrinos produced by fusion barely interact with matter, so they escape the core in about two seconds and reach Earth directly, giving a live feed from the Sun's engine. From the 1960s to the 1990s, experiments detected only about a third of the expected neutrinos. This solar neutrino problem was solved when scientists showed that neutrinos change flavor as they travel, which proved neutrinos have mass and earned the 2015 Nobel Prize in Physics.
Helioseismology
The Sun rings like a bell, vibrating with millions of sound waves bouncing through its interior. By measuring how the surface ripples, scientists map the interior's temperature, density, and rotation, much as seismologists study Earth using earthquakes.
An Open Puzzle: The Solar Metallicity Problem
Improved models of the Sun's surface suggested it contains fewer heavy elements ("metals" to astronomers) than previously thought. But solar models built on those lower values no longer match helioseismology data. CNO neutrinos are one of the best independent tests, because their numbers depend directly on how much carbon and nitrogen is in the core. Borexino's final measurement showed mild tension with the low-metallicity values, leaning toward the older, higher estimates, but the question remains open.
5. The Atmosphere and the Coronal Heating Mystery
Above the photosphere sit the chromosphere, a thin transition region, and the corona, the ghostly halo visible during a total solar eclipse. The corona is over a million degrees, hundreds of times hotter than the surface below it. Explaining why is the coronal heating problem, unsolved for about 80 years.
The two leading ideas are that energy is carried upward by magnetic waves called Alfvén waves, or released by countless small magnetic explosions called nanoflares. Both likely contribute. NASA's Parker Solar Probe, flying directly through the corona, discovered "switchbacks": sudden S-shaped kinks in the magnetic field. Research suggests many switchbacks form naturally as Alfvén waves grow while traveling outward, feeding turbulence that heats the plasma.
| Parker Solar Probe in 2026 Parker completed its 29th close approach on September 4, 2026, matching its records of 3.8 million miles from the Sun and 430,000 mph, and imaged structures near the Sun's north pole. A result published in spring 2026 studied magnetic reconnection, the snapping and rejoining of magnetic field lines that powers solar storms. Unexpectedly, protons were accelerated into a spread-out beam while heavier ions were driven in a narrow straight line, which will help refine models of how solar storms are powered. |
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6. Magnetism, Sunspots, and the Solar Cycle
The Sun does not rotate as a solid body. Its equator spins once every ~25 days while the poles take ~35. That uneven rotation, combined with convection, twists and stretches the magnetic field. This process is the solar dynamo. Where tangled magnetic bundles break through the surface, they block some of the rising heat and create cooler, darker patches called sunspots.
Activity follows a roughly 11-year cycle, at the end of which the Sun's magnetic field flips so its north and south magnetic poles swap. Returning to the original orientation takes another 11 years, so the full magnetic cycle lasts about 22 years.
In 2025, ESA's Solar Orbiter tilted its orbit out of the plane of the planets and became the first spacecraft to image the Sun's poles. It found the south pole in magnetic disarray, with both polarities present at once, a short-lived state that occurs only around solar maximum while the field is flipping.
| Where we are now Solar Cycle 25 very likely peaked in October 2024, and the Sun is now in its declining phase, expected to continue until the cycle ends around 2030. Historically, some of the strongest flares and geomagnetic storms happen during the declining phase, so significant space weather and auroras remain possible. Large flares and coronal mass ejections can disrupt satellites, GPS, radio, and power grids. |
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7. How All Stars Work
Birth
Every star begins as a cold, dusty molecular cloud that collapses under its own gravity, often triggered by a nearby supernova or shockwave. As it shrinks it heats up, forming a protostar surrounded by a spinning disk, the birthplace of planets. When the core reaches about 10 million degrees, hydrogen fusion ignites and a star is born.
Mass Decides Everything
Heavier stars need hotter, denser cores to support their stronger gravity, so they burn fuel dramatically faster. A star twice the Sun's mass is roughly ten times brighter and lives only a fraction as long.
Red dwarfs (under about half the Sun's mass) are the most common stars in the galaxy. They burn so slowly they can shine for trillions of years, so none has ever died of old age. Below about 8% of the Sun's mass, objects cannot sustain hydrogen fusion and become brown dwarfs, "failed stars" between planets and stars.
Sun-like stars spend about 10 billion years on the main sequence, the stable hydrogen-burning phase. The Sun is a little under halfway through.
Massive stars (about 8 to more than 100 times the Sun's mass) burn through their fuel in just a few million years, shine blue-white, and die spectacularly.
8. How the Sun Will Die
The Sun brightens by roughly 10% every billion years, so in about a billion years Earth will be too hot to keep liquid oceans, long before the Sun itself dies.
In about 5 billion years, the core's hydrogen runs out. The core contracts and heats, hydrogen burns in a shell around it, and the outer layers swell into a red giant that may engulf Mercury, Venus, and possibly Earth. The core eventually reaches about 100 million degrees and fuses helium into carbon and oxygen. When that fuel is spent, the Sun gently sheds its outer layers as a glowing planetary nebula.
What remains is a white dwarf: an Earth-sized ember holding about half the Sun's mass, so dense that a teaspoon would weigh several tonnes. It will cool and fade for trillions of years.
9. How Massive Stars Die, and Why You Exist
Massive stars keep going after helium, fusing carbon, then neon, oxygen, and silicon in onion-like layers, each stage faster than the last. The final silicon-burning stage lasts about a day, and it ends at iron.
Iron is a dead end: fusing it consumes energy instead of releasing it. With nothing left to hold it up, a core more massive than the Sun collapses to the size of a city in under a second, rebounds, and blows the star apart in a supernova that can outshine an entire galaxy. What remains is a neutron star, so dense a sugar cube of it would weigh as much as a mountain, or, for the heaviest cores, a black hole.
| We are made of star stuff The Big Bang produced only hydrogen, helium, and a trace of lithium. Nearly every other element was forged in stars. The carbon in your cells and the oxygen you breathe came from stellar cores, much of the iron in your blood came from exploding stars, and heavy elements like gold and platinum come largely from colliding neutron stars, an event astronomers observed directly in 2017 through both gravitational waves and light. |
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10. Open Questions Worth Watching
Several major puzzles remain unsolved: why the corona is so hot, exactly how the solar dynamo works and why the strength of each cycle is still hard to predict, the solar metallicity problem, and how the solar wind is accelerated to around a million miles per hour. Parker Solar Probe, Solar Orbiter, the Daniel K. Inouye Solar Telescope in Hawaii, and next-generation neutrino detectors such as SNO+ are all working on these questions now.
Sources
- NASA Science, Parker Solar Probe blog: 29th close approach (Sept. 10, 2026) and earlier encounter updates.
- NASA Science, "Parker Solar Probe Finds Explosive Surprises on Sun" (April 15, 2026), on magnetic reconnection results published in the Astrophysical Journal.
- European Space Agency, "Solar Orbiter gets world-first views of the Sun's poles" (2025).
- Max Planck Institute for Solar System Research, "First images of the Sun's south pole" (2025).
- Borexino Collaboration, "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun," Nature (2020), and the later Phase-III CNO measurement.
- Cerdeño et al., "CNO Neutrino Grand Prix: The race to solve the solar metallicity problem" (arXiv:1712.06522).
- Space.com, "Will 2026 bring strong auroras? What the sun's recent activity tells us."
- UK Met Office blog, "Declining phase for Sun's activity, but what's next?" (2026).
- Research literature on switchbacks and Alfvén waves from Parker Solar Probe data (e.g., arXiv:2104.08321, arXiv:2101.09529).