The Sun Explained: How Our Star Works, Layer by Layer

The Sun is an ordinary star by galactic standards, but it is the only one close enough to see as a disk. It holds about 99.86 percent of the mass in the solar system and supplies almost all of the energy that drives Earth's climate, weather and living systems. This guide explains what the Sun is made of, how it produces energy, and how that energy travels across space to reach us.

What kind of star is the Sun?

The Sun is a G-type main-sequence star, classified G2V in the Morgan-Keenan system. Main-sequence means it fuses hydrogen into helium in its core, the stable phase that most stars spend the bulk of their lives in. Its visible surface, called the photosphere, sits at about 5,772 kelvin, which gives sunlight its nearly white colour once it is above the atmosphere.

Its diameter is about 1.39 million kilometres, roughly 109 times the diameter of Earth. Its mass is about 333,000 times Earth's mass. Despite that bulk, its average density is only about 1.41 grams per cubic centimetre, about 1.4 times that of water, because most of its volume is hot gas rather than solid material.

The Sun is about 4.6 billion years old. That places it roughly halfway through the main-sequence phase, which is expected to last about 10 billion years in total.

Why the Sun is so stable

Stars of different masses burn at very different rates. A more massive star has a hotter, denser core, so it fuses fuel much faster and lives far shorter. The Sun's mass sets a steady balance between the inward pull of gravity and the outward push of heat and radiation. That balance is called hydrostatic equilibrium, and it is why the Sun changes slowly over billions of years rather than flaring out or collapsing from one year to the next.

How the Sun makes energy

Energy comes from nuclear fusion in the core, where the temperature reaches about 15 million kelvin and the pressure is enormous. Most of the energy is produced by the proton-proton chain. In this chain, four hydrogen nuclei (protons) are combined through several steps into one helium-4 nucleus, two positrons and two neutrinos.

The helium nucleus has about 0.7 percent less mass than the four protons that formed it. That missing mass is released as energy according to Einstein's relation E = mc squared. A small amount of energy per reaction becomes an enormous total because the core runs an immense number of reactions every second.

Why the mass loss matters

Converting mass to energy at this rate means the Sun sheds about 4 million tonnes of mass each second. That figure sounds huge, but it is a negligible fraction of the Sun's total mass, and over its whole life the Sun will have lost well under one tenth of one percent of its mass to radiation.

Neutrinos reach Earth quickly

Neutrinos are produced in the core and barely interact with matter, so they escape almost at once and reach Earth in about 8 minutes. Photons, by contrast, are absorbed and re-emitted countless times on their way out, which is why light from the core takes far longer to emerge than the neutrinos do.

The Sun's layers

The Sun is not a uniform ball. It is organised into layers, each with its own temperature and energy-transport method. The core produces the energy, the radiative zone carries it outward as light, the convective zone carries it outward as moving hot plasma, and the outer atmosphere is much hotter than the surface beneath it.

Layer	Outer edge (fraction of solar radius)	Approximate temperature	How energy moves
Core	0.25	about 15 million K	fusion produces energy
Radiative zone	0.71	about 2 to 7 million K	photons random-walk outward
Convective zone	1.00	about 2 million K at base, falling outward	hot plasma rises and cools
Photosphere	about 1.0 (visible surface)	about 5,772 K	light escapes to space
Chromosphere	above photosphere	about 4,000 K rising to tens of thousands of K	thin, glowing gas layer
Corona	extends millions of km	about 1 to 3 million K	sparse, extremely hot plasma

The coronal heating puzzle

The corona is far hotter than the photosphere beneath it, which seems backwards because heat should flow from hot to cold. The corona reaches millions of kelvin while the surface is near 5,800 K. Researchers still study the mechanisms that deposit this energy, such as magnetic waves and small magnetic reconnection events. The question is a standard open problem in solar physics, and it shows how much of the Sun's detailed behaviour is still under active study.

Sunspots and the solar cycle

Sunspots are regions where strong magnetic fields suppress the convective flow of heat from below. The spots appear darker because they are cooler than the surrounding photosphere, by roughly 1,500 to 2,000 kelvin. They can be as large as Earth or bigger, and they often appear in pairs or groups.

The number of sunspots rises and falls in a cycle of about 11 years. At solar maximum the magnetic field is most tangled, and flares and coronal mass ejections are more frequent. At solar minimum the Sun is quieter. The Sun's overall magnetic polarity also reverses with each cycle, so the full magnetic cycle lasts about 22 years.

Why the cycle matters on Earth

Magnetic activity drives solar flares and coronal mass ejections. When these are aimed at Earth, they can disturb the magnetosphere, create geomagnetic storms, and produce auroras at lower latitudes than usual. The 1859 Carrington Event remains the best-known historical example, when telegraph systems across Europe and North America were disrupted and some operators reported sparks from their equipment.

How sunlight and the solar wind reach Earth

Light from the Sun takes about 8 minutes and 20 seconds to reach Earth, which orbits at a distance of 1 astronomical unit, or 149.6 million kilometres. At that distance the solar constant, the average solar power per square metre above the atmosphere, is about 1,361 watts. Earth's atmosphere and surface absorb and reflect part of that power, and the rest drives weather, ocean currents and photosynthesis.

The Sun also emits a continuous stream of charged particles called the solar wind. It typically moves at about 400 kilometres per second near Earth and forms a huge bubble called the heliosphere that extends well beyond the orbit of Neptune. Earth's magnetic field deflects most of this wind, which is why the planet's surface is shielded from direct particle bombardment.

Key figures at a glance

The table below collects the standard reference values for the Sun. Figures are rounded and taken from widely used solar and astronomical references.

Property	Value
Radius	696,000 km
Mass	about 1.989 × 10^30 kg
Luminosity	about 3.828 × 10^26 W
Effective surface temperature	about 5,772 K
Core temperature	about 15.7 million K
Average density	about 1.41 g/cm³
Rotation at equator	about 25 days
Activity cycle	about 11 years

Key takeaways

The Sun is a stable G2V main-sequence star that converts hydrogen into helium in its core. This fusion releases energy that works its way outward through radiative and convective layers before escaping as light.

The Sun's atmosphere, from the chromosphere to the corona, is hotter than the surface below it, and the reason is still an active research question. Its magnetic activity follows an 11-year cycle that affects space weather near Earth.

Sunlight and the solar wind shape the environment of the entire solar system. Understanding the Sun is the starting point for understanding every planet, moon, asteroid and comet that orbits it.