The Sun: Our Parent Star

What the Sun is: age, size, composition, nuclear fusion, the photosphere and corona, solar activity, Earth’s tiny gravitational wobble — and why only ISO…

Our nearest laboratory

The Sun is so familiar that it can be easy to overlook how extraordinary it is. It is the bright disc that determines the rhythm of our days, drives weather and climate, powers photosynthesis, and holds the Solar System together. Yet it is also a vast, turbulent sphere of superheated plasma: a star whose atmosphere extends far beyond Earth's orbit and whose magnetic activity can disturb radio communications, satellites, power grids, and auroras.

In astronomical terms, the Sun is not an exotic object. It is a fairly typical middle-aged star, classified as a G-type main-sequence star — more specifically, G2V. “Main sequence” means it is in its long, stable phase of life, converting hydrogen into helium in its core. Its apparent ordinariness is one reason it is so valuable: it is the only star close enough for us to study in extraordinary detail.

Age and life stage

The Sun formed about 4.6 billion years ago, from a collapsing cloud of gas and dust in the Milky Way. Gravity pulled most of that material into the centre, where pressure and temperature rose until hydrogen fusion began. The remaining material settled into a spinning disc that eventually formed the planets, moons, asteroids, and comets.

It is roughly halfway through its stable hydrogen-burning lifetime. The Sun is expected to remain on the main sequence for around another five billion years. During that period it will gradually become brighter: although the change is extremely slow on a human timescale, the young Sun was probably about 70 percent as luminous as it is today.

Eventually, after core hydrogen is exhausted, the Sun will expand into a red giant. It will shed its outer layers, leaving behind a dense stellar remnant called a white dwarf. It is not massive enough to end in a supernova; that fate belongs to much more massive stars.

Size and mass

The Sun is enormous by everyday standards:

• Diameter: about 1.39 million km (864,000 miles), roughly 109 Earth diameters.
• Radius: about 696,000 km (432,500 miles).
• Mass: about 1.99 × 10³⁰ kg (2.0 × 10²⁷ metric tonnes, or about 2.2 × 10²⁷ US tons), around 333,000 times the mass of Earth.
• Volume: approximately 1.3 million Earths could fit inside it, although this is a geometric comparison rather than a packing arrangement.
• Surface gravity: about 28 times Earth's gravity.

The Sun contains about 99.86 percent of all the mass in the Solar System. Jupiter accounts for most of the rest. This overwhelming mass is why every planet, dwarf planet, comet, asteroid, and spacecraft is fundamentally moving within the Sun's gravitational domain.

Despite its immense size, the Sun is not solid. There is no surface in the ordinary terrestrial sense. It is composed largely of plasma: gas so hot that atoms are stripped into electrically charged particles — positively charged nuclei and free electrons. Plasma responds strongly to magnetic fields, making the Sun's outer layers dynamic, complex, and often violent.

What the Sun is made of

By mass, the Sun is approximately:

• 73 percent hydrogen
• 25 percent helium
• About 2 percent heavier elements

Astronomers call every element heavier than helium a “metal,” even when it is oxygen, carbon, nitrogen, silicon, or neon rather than a familiar metal such as iron. The Sun's small fraction of heavier elements includes oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, sulphur, and many others.

Hydrogen dominates because the Sun formed from the material left over after the early Universe produced mostly hydrogen and helium. The heavier elements in the Solar System had already been made in earlier generations of stars and spread through space by stellar winds and supernova explosions. In that sense, the iron in blood, calcium in bones, oxygen in air, and silicon in rocks all have a stellar ancestry older than the Sun itself.

The Sun's composition is not perfectly uniform. Nuclear fusion in the core gradually turns hydrogen into helium, so the central region has become richer in helium over time. The outer layers retain a composition much closer to the original solar nebula.

The engine at the core

At the centre of the Sun lies its core, extending to roughly a quarter of the solar radius. This is where the pressure and temperature are high enough for nuclear fusion.

The core temperature is about 15 million degrees Celsius — or roughly 15 million kelvin. Its density is astonishingly high for a gas-like plasma: around 150 times the density of water near the centre. Under these conditions, hydrogen nuclei can occasionally overcome their electrical repulsion and combine.

The dominant process is the proton-proton chain. In simplified form, four hydrogen nuclei ultimately become one helium nucleus. The helium nucleus weighs slightly less than the four original hydrogen nuclei, and that tiny difference in mass is converted into energy according to Einstein's familiar relationship:

The amount of mass converted each second is large in human terms — several million tonnes — but tiny compared with the Sun's total mass. That is why the Sun can shine steadily for billions of years.

The energy created in the core begins as high-energy gamma radiation. It does not travel directly to the surface in a straight line. Instead, photons are repeatedly absorbed and re-emitted, scattering through the dense solar interior. The journey outward can take tens of thousands to hundreds of thousands of years, depending on how the process is modelled. Once the energy reaches the outer convection zone, rising and sinking plasma carries it more efficiently toward the visible surface.

A separate product of fusion is the neutrino. These nearly massless particles interact only weakly with matter, so vast numbers pass through Earth — and through us — every second. Solar-neutrino observations provide a direct test of fusion in the Sun's core.

The visible surface

The layer we call the Sun's “surface” is the photosphere. It is the region from which most visible light escapes into space. It is only a few hundred kilometres thick, extremely thin compared with the Sun's overall radius.

The photosphere has a temperature of roughly 5,500 degrees Celsius, often quoted as about 5,778 K. This is much cooler than the core, but still hot enough for the Sun to radiate intensely across visible, infrared, and ultraviolet wavelengths.

Seen close up, the photosphere has a grainy pattern called granulation. Each bright granule is a cell of hot plasma rising from below, typically around 1,000 km across. Darker lanes mark cooler material sinking back down. Granules are short-lived, commonly evolving over minutes, while larger convective patterns — supergranules — can span tens of thousands of kilometres.

Sunspots are another familiar photospheric feature. They look dark only by comparison with the hotter surrounding surface. A sunspot is still extremely bright and hot, often around 3,000 to 4,500 degrees Celsius. It appears dark because strong magnetic fields inhibit convection, preventing heat from rising as efficiently from beneath.

Why the Sun looks yellow

From space, the Sun is essentially white, not yellow. It emits light across a broad range of visible wavelengths, and the combination appears white to the human eye.

From Earth's surface, atmospheric scattering changes its appearance. When the Sun is high in a clear sky, it can appear white or slightly yellow. Near sunrise and sunset, sunlight passes through much more atmosphere. Shorter blue wavelengths are scattered away more strongly, leaving the direct solar beam richer in red, orange, and yellow light.

The Sun's spectral classification as a “yellow dwarf” is therefore somewhat misleading. It is neither particularly small for a star nor intrinsically yellow in the way a painted object is yellow. The term survives as a convenient historical description of its spectral type.

The atmosphere and corona

Above the photosphere are several atmospheric layers. The chromosphere, visible as a reddish rim during a total solar eclipse, rises through temperatures of roughly 4,000 to tens of thousands of degrees. Beyond it lies a narrow transition region, where temperature rises very sharply.

Then comes the corona: the Sun's extended outer atmosphere. During totality, it can be seen with the naked eye as a pearly white halo, structured by streamers, loops, and rays shaped by magnetic fields. For more on what that crown looks like and why it hides in plain sight, see The Ghostly and Elusive Solar Corona .

The corona is remarkably hot: typically around 1 to 3 million degrees Celsius, with some active regions becoming substantially hotter. This produces one of solar physics' most famous puzzles. How can the corona be hundreds of times hotter than the photosphere below it?

Heat usually flows from hot to cold, and the photosphere is only around 5,500 degrees Celsius. The corona's extreme temperature therefore cannot be explained by simple upward leakage of thermal heat from the visible surface. Instead, the energy must be transferred and released through magnetic processes.

Why the corona is so hot

The leading explanation is that the Sun's magnetic field stores and releases energy in the corona. The exact balance of mechanisms remains an active research topic, but two related ideas are central.

First, magnetic fields can become twisted, stretched, and tangled by the constant motion of plasma beneath the photosphere. When magnetic field lines rearrange themselves — through a process called magnetic reconnection — stored magnetic energy can be rapidly released as heat, particle acceleration, and radiation. Large reconnection events power solar flares; countless smaller ones may collectively provide a substantial part of the corona's heating.

Second, turbulent motion in the Sun can launch magnetic waves, especially Alfvén waves, along magnetic field lines. These waves can carry energy from lower layers into the corona. If their energy dissipates there, it heats the coronal plasma.

The modern picture is likely not a single mechanism operating everywhere. Small impulsive heating events, often called nanoflares, magnetic reconnection, wave dissipation, and the complex geometry of active-region magnetic fields may all contribute. Space missions such as NASA's Parker Solar Probe and ESA's Solar Orbiter are designed partly to investigate this problem by studying the solar wind and magnetic environment far closer to the Sun than previous spacecraft.

A variable, magnetic star

The Sun may look calm from Earth, but it is variable on timescales from seconds to millennia. Its best-known rhythm is the approximately 11-year solar cycle, during which the number of sunspots rises and falls.

At solar minimum, the Sun tends to have fewer sunspots and less frequent major activity. At solar maximum, active regions, flares, prominences, and coronal mass ejections become more common. The cycle is driven by the solar dynamo: the interplay between the Sun's rotation, convection, and magnetic fields.

The Sun does not rotate as a rigid solid body. Its equator rotates in about 25 days, while regions near the poles take roughly 35 days. This differential rotation helps stretch and wind magnetic fields, contributing to the magnetic cycle. About every 11 years, the magnetic polarity pattern reverses, making the full magnetic cycle approximately 22 years.

Solar flares release bursts of electromagnetic radiation, while coronal mass ejections can hurl billions of tonnes of magnetised plasma into space. If directed toward Earth, they can cause geomagnetic storms. These may produce spectacular auroras but can also affect satellite operations, navigation systems, high-frequency radio, and electrical infrastructure.

The Sun's overall brightness varies much less than its visible activity might suggest. Across the normal solar cycle, total solar irradiance changes by only about a tenth of one percent. That is small compared with everyday weather variation, but it is scientifically important for understanding Earth's upper atmosphere and long-term climate influences.

Earth makes the Sun wobble

A pleasingly counterintuitive fact is that Earth does not orbit the exact centre of the Sun. Strictly speaking, Earth and the Sun both orbit their shared barycentre — the centre of mass of the two-body system.

Because the Sun is so much more massive than Earth, that barycentre lies well inside the Sun. But it is not precisely at the solar centre: it is roughly 450 km from the centre, about 300 miles. Earth's gravity therefore makes the Sun perform a tiny monthly-and-yearly gravitational dance around the shared centre of mass.

This is a genuine wobble, though it is minute compared with the Sun's radius of about 696,000 km. Jupiter has a far larger effect: the Sun–Jupiter barycentre can sometimes lie outside the visible solar surface. When all the planets are included, the Sun follows a continuously changing path around the Solar System's overall barycentre.

The same principle is central to the discovery of exoplanets. A planet orbiting a distant star tugs gravitationally on that star. If the system is viewed at a favourable angle, the star's tiny back-and-forth motion can be detected through shifts in its spectrum — the radial-velocity method. Light from a star moving slightly toward us is shifted toward shorter wavelengths; light from a star moving away is shifted toward longer wavelengths.

The effect from an Earth-like planet is extremely small, which makes such detections technically demanding. Large planets close to their stars are easier to find because they induce stronger stellar motion. Nonetheless, the basic idea is exactly the same as the Sun's tiny response to Earth: a planet is never merely orbiting a motionless star; both objects orbit their common centre of mass.

A star worth watching

The Sun is our nearest laboratory for stellar physics. It lets astronomers observe convection, magnetic fields, plasma behaviour, fusion, stellar winds, flares, and planetary interactions on scales that no distant star can match. Its activity also matters directly to daily life, from satellite-based navigation to power resilience and radio communication.

For casual skywatchers, the Sun is also one of the most accessible astronomical objects — but only with proper precautions. Never look at it directly through a camera, binoculars, telescope, or unaided eyes without certified solar filters designed for that exact purpose. Ordinary sunglasses, exposed film, smoked glass, and improvised filters are unsafe . Darker everyday sunglasses do not reduce the risk — they can give a false sense of security while still allowing damaging radiation through. Only filters and viewers that meet the ISO 12312-2 standard are widely regarded by professionals as safe for direct solar viewing during partial phases.

During a total solar eclipse, the brief period of totality reveals the corona — the very hot outer atmosphere normally drowned out by the brilliant photosphere. For a few minutes, the familiar daylight Sun becomes visibly what it truly is: a dynamic star with a vast magnetic atmosphere, extending outward into space and continually shaping the environment of every world that orbits it. Read The Awe and Splendor of Totality for what that moment feels like — and plan your protection before eclipse day.