What is the Sun made of? How our star works

The Sun is made of about 73% hydrogen and 25% helium by mass, with the last 2% a mix of heavier elements such as oxygen, carbon, neon and iron. It isn't solid, liquid or a fire. It is plasma: gas so hot that its atoms have been stripped of electrons. In its core, hydrogen fuses into helium, and that reaction is what makes the Sun shine.

In short

• The Sun is a ball of hydrogen and helium plasma, 109 times Earth's width and about 333,000 times its mass.
• It shines by nuclear fusion in its core, not by burning: each second it turns about 4 million tonnes of matter into energy.
• It is about 4.6 billion years old, roughly halfway through its life, and will swell into a red giant in about 5 billion years before shrinking to a white dwarf.

What is the Sun made of, exactly?

The answer depends on whether you count by weight or by number of atoms, because a helium atom weighs about four times as much as a hydrogen atom. NASA's Sun fact sheet gives the count by atoms at its visible surface; the share by mass is the figure most textbooks quote.

Element	Share by mass	Share by number of atoms
Hydrogen	about 73%	about 91%
Helium	about 25%	about 8.9%
Everything else (oxygen, carbon, neon, nitrogen, iron and more)	about 2%	about 0.15%

So nine of every ten atoms in the Sun are hydrogen. The heavier elements, which astronomers call metals even when they aren't, were made in earlier generations of stars that exploded before the Sun formed. The iron in your blood came from the same kind of event.

Two discoveries made this picture. Helium was found in the Sun before it was found on Earth: astronomers saw an unknown yellow line in sunlight's spectrum in 1868 and named the element after Helios, the Greek Sun god. And in 1925, Cecilia Payne, then a doctoral student at Harvard, showed from stellar spectra that stars are made mostly of hydrogen. Many astronomers at first doubted her, because they expected stars to have Earth's mix of elements.

How does the Sun shine?

The Sun shines because its core fuses hydrogen into helium, and the helium weighs slightly less than the hydrogen that made it. The missing mass, about 0.7%, becomes energy, by Einstein's E = mc². This is nuclear fusion, and it needs extreme conditions: the core is about 15 million °C and so dense that hydrogen nuclei, which normally repel each other, collide hard enough to fuse.

How much mass does that take? Here is the arithmetic, with figures from NASA's fact sheet.

1. Power out. The Sun gives off about 3.83 × 10²⁶ watts, that is, joules every second.
2. Mass needed. E = mc² rearranged is m = E ÷ c². The speed of light squared is about 9.0 × 10¹⁶. So 3.83 × 10²⁶ ÷ 9.0 × 10¹⁶ ≈ 4.3 × 10⁹ kg: about 4.3 million tonnes of matter turned into energy every second.
3. Hydrogen used. Since only 0.7% of the fused hydrogen's mass becomes energy, the core fuses about 4.3 million ÷ 0.007 ≈ 600 million tonnes of hydrogen each second.
4. Over its life. At that rate for 4.6 billion years (about 1.45 × 10¹⁷ seconds), the Sun would have turned roughly 6 × 10²⁶ kg into light: about 100 times the mass of Earth.
5. As a share. The Sun's mass is about 2 × 10³⁰ kg, so that is about 0.03% of it. The young Sun was dimmer than today's, so the real figure is a little lower.

An enormous loss by human standards is a rounding error for a star. That is why the Sun can shine steadily for about 10 billion years.

What are the Sun's layers?

The Sun has no solid surface, but it has distinct layers, each with its own way of moving energy outward. Temperatures here are from NASA's Sun facts.

Layer	What happens there	Temperature
Core	Fusion; roughly the inner fifth to quarter of the Sun's radius	about 15 million °C
Radiative zone	Light is absorbed and re-emitted countless times, wandering outward	cooling outward from millions of degrees
Convection zone	Hot plasma rises, cools and sinks, like boiling soup	cooling to about 5,500 °C at the top
Photosphere	The visible surface, where light finally escapes into space	about 5,500 °C
Chromosphere and corona	The thin outer atmosphere, seen during a total eclipse	corona up to 2 million °C

The light you see today left the photosphere about 8 minutes and 20 seconds ago. But the energy it carries was made in the core long before: NASA estimates it takes about 170,000 years to work its way from the core to the top of the convection zone, mostly because, in the radiative zone, each bit of light travels only a tiny distance before it is absorbed and sent off in a random direction.

Why is the Sun's outer atmosphere hotter than its surface?

Nobody is fully sure, and it is one of the main open questions in solar physics. The corona is hundreds of times hotter than the photosphere below it, the opposite of what you'd expect moving away from a heat source. The leading ideas involve the Sun's tangled magnetic fields: waves carrying energy up along them, or countless small magnetic explosions, sometimes called nanoflares. NASA's Parker Solar Probe, which flies through the corona, was built partly to settle this.

The same magnetic fields drive the solar cycle. About every 11 years the Sun's magnetic poles swap, and sunspots, flares and eruptions rise and fall with it. NASA and NOAA announced on 15 October 2024 that the Sun had reached the maximum period of the current cycle, Solar Cycle 25. Eruptions from the Sun that reach Earth are what light up the northern lights.

What will happen to the Sun?

The Sun formed about 4.6 billion years ago from a collapsing cloud of gas and dust, the same cloud that made the solar system. It is a middle-aged, fairly ordinary star, one of a few hundred billion in our galaxy.

In about 5 billion years, the core will run low on hydrogen. The Sun will then swell into a red giant, large enough to swallow Mercury and Venus and perhaps Earth, according to NASA. After that it will shed its outer layers as a glowing shell of gas and leave behind its hot core, a white dwarf about the size of Earth, which will slowly cool. It is far too small to explode as a supernova or to collapse into a black hole; those fates need stars at least several times heavier. The Sun is one star among the countless ones that fill the universe.

Check yourself

• Why is the Sun about 73% hydrogen by mass but about 91% hydrogen by number of atoms?
• Where does the energy of sunlight come from, and why doesn't the Sun run out quickly?
• Why does light made in the core take so long to reach the surface, but only 8 minutes to reach Earth?

Sources

• Sun: facts — NASA
• Sun fact sheet — NASA Space Science Data Coordinated Archive
• NASA, NOAA: Sun reaches maximum phase in 11-year solar cycle — NASA, 2024
• Sun — Wikipedia