The Sun

What the Sun is

The Sun is a huge ball of hydrogen and helium held together by its own gravity.^[1] Astronomers classify it as a G2 V star, a yellow dwarf on the main sequence, and it is about 4.5 billion years old.^[1] It is only an average star in size: stars up to 100 times larger have been found.^[1]

When the solar system formed, most of the material in its cloud of gas and dust was pulled to the centre to make the Sun, which now holds 99.8% of the solar system’s mass.^[1]

The Sun in numbers

• Width: about 1.4 million km (865,000 miles), roughly 100 times wider than Earth and 10 times wider than Jupiter.^[1]
• Mass and volume: more than 330,000 Earths to match its mass, and 1.3 million Earths to fill its volume.^[1]
• Distance: about 150 million km (93 million miles) from Earth.^[1]

Layers, from the inside out

The Sun is made of plasma, a hot gas of electrically charged particles, so it has no solid surface.^[1] Instead it has a series of zones or layers.^[2]

• Core. In the central region, nuclear reactions use up hydrogen to make helium, and these reactions release the energy that finally leaves the surface as visible light.^[2] The core reaches about 15 million °C, is about 138,000 km thick, and has a density of about 150 grams per cubic centimetre.^[1] Its heat creates outward pressure that holds up the Sun’s enormous mass and stops it collapsing.^[1]
• Radiative zone. Above the core, energy moves outward as radiation.^[2] That radiation bounces around for about 170,000 years on its way from the core to the top of the convection zone.^[1]
• Convection zone. The outermost part of the interior begins about 200,000 km down and reaches the visible surface, where its motion shows up as grainy cells called granules and supergranules.^[2] Here large bubbles of hot plasma rise toward the surface.^[1]
• Photosphere. This is the visible surface, the layer we call the Sun’s “surface”.^[2] It is about 250 miles thick, at about 5,500 °C.^[1]
• Chromosphere. In this irregular layer the temperature climbs from 6,000 °C to about 20,000 °C.^[2]
• Transition region. A thin, very irregular layer separates the cooler chromosphere from the hot corona.^[2]
• Corona. The Sun’s outer atmosphere reaches up to 2 million °C, far hotter than the photosphere.^{[1, 2]}

Rotation and magnetism

Different parts of the Sun spin at different speeds: one turn takes 25 Earth days at the equator but 36 at the poles.^[1] The Sun’s magnetic fields rise through the convection zone and break through into the atmosphere, causing solar activity such as sunspots, flares, prominences and coronal mass ejections (huge eruptions of material).^[2] About every 11 years, the Sun’s magnetic poles swap places.^{[1, 6]}

The solar wind

The corona is so hot that the Sun’s gravity cannot hold on to it, so its gas streams away as the solar wind.^{[3, 2]} It flows out in all directions at about 400 km/s.^[3] The speed varies: about 800 km/s over coronal holes and about 300 km/s over structures called streamers.^[3] These changes buffet Earth’s magnetic field and can cause storms in the magnetosphere, the region around Earth controlled by that field.^[3]

The solar wind blows a huge magnetic bubble around the Sun called the heliosphere.^[1] Because the Sun rotates, its magnetic field is carried out in a turning spiral, known as the Parker spiral.^[1]

Sunspots and the solar cycle

Sunspots are dark patches on the photosphere where intense magnetic flux pushes up from inside the Sun.^[4] The strong magnetism can block the flow of heat, so sunspots can be about 1,650 °C (3,000 °F) cooler than their surroundings, which is why they look dark.^[5] An average sunspot is roughly the size of Earth, while the largest groups can be many times bigger.^{[5, 4]}

The number of sunspots rises and falls over about 11 years in the solar cycle.^{[4, 5]} The peak is called solar maximum and the lull solar minimum.^[4] Cycles have been numbered since Cycle 1 in 1755.^[4]

Cycle 25 began in December 2019, at solar minimum.^{[1, 4]} On 15 October 2024, NASA and NOAA announced that the Sun had reached its solar maximum period.^[6] The exact month of the peak can only be identified months or years later, once activity has clearly declined.^[6]

Space weather

Sunspots, solar flares and coronal mass ejections are common at solar maximum.^[1] Rapid changes in the magnetic fields of the active regions around sunspot groups are the most likely source of significant space weather events.^[4] Space weather can affect satellites and astronauts, radio and GPS, and power grids on Earth.^[6] In 2024, a run of large eruptions produced the strongest geomagnetic storm at Earth in two decades.^[6] NOAA’s Space Weather Prediction Center is the US government’s official source of space weather forecasts and alerts.^[6]

Two missions that study the Sun

Parker Solar Probe launched on 12 August 2018.^[7] It is named after Eugene Parker, who in the mid-1950s developed a theory predicting the solar wind.^[7] Its plan is 24 orbits over seven years, with a planned closest approach of about 6.2 million km (3.9 million miles) from the Sun, where it moves at about 700,000 km/h (430,000 mph).^[7] Its carbon-composite heat shield, 11.43 cm thick, can withstand nearly 1,377 °C.^[7] On 14 December 2021, NASA announced that Parker had flown through the corona and sampled its particles and magnetic fields.^[7]

SOHO, the Solar and Heliospheric Observatory, is a joint project of ESA and NASA, launched on 2 December 1995.^{[8, 9]} It circles L1, a point about 1.5 million km from Earth in the direction of the Sun.^{[8, 3]} Planned to last three years, it marked 30 years of continuous operation in December 2025.^[8]

Common misconceptions

“The Sun is a solid ball with a hard surface.” It is plasma; the photosphere is only the layer we see.^[1]

“Sunspots are cold spots.” They are only cooler than their surroundings: NOAA puts a sunspot at about 7,000 °F against about 10,000 °F for the photosphere around it.^[4]

Open questions

Heliophysicists are still working out why the corona is hotter than the layers just beneath it.^{[2, 1]} How and where coronal gas is sped up into the solar wind is also not fully understood.^{[3, 7]} Looking far ahead, scientists expect the Sun to shine for roughly another 5 billion years before it becomes a white dwarf, which would put it a little under halfway through its life.^[1]

What this page does not cover

How nuclear fusion works, the Sun’s energy output, eclipses as a topic, solar observation before telescopes, and why the magnetic cycle happens. It gives no advice on viewing the Sun.

Going further

See Stars for how the Sun compares with other stars, Solar System, Planets and Moons for its family, Magnetism for magnetic fields, and Satellites and Space Exploration for spacecraft.