Sun

The Sun is the star at the centre of our Solar System. It is a huge ball of hot plasma, glowing because of nuclear fusion in its core. This process gives off energy that we receive here on Earth as sunlight, which is essential for life.

The Sun is much bigger and more massive than any other object in the Solar System. Its diameter is about 109 times that of Earth, and it contains about 99.86% of the total mass of everything that orbits it.

The Sun formed about 4.6 billion years ago from a cloud of gas and dust. It continues to shine today by turning hydrogen into helium in its core. In the future, the Sun will change shape, eventually becoming a cool, dim white dwarf.

Etymology

The English word sun comes from old words used in many languages. For example, in Dutch it is zon, and in German it is Sonne. All these words share a common ancient root.

We use words like sunny to describe bright sunlight, and solar in science, from the Latin word sol. The Sun is sometimes called Helios in poems, after an ancient Greek word. Scientists study the Sun in a field called heliology. The symbol for the Sun is a circle with a dot in the middle, used in measuring things like the Sun’s mass, size, and brightness.

General characteristics

The Sun is a bright star that makes up almost all of the mass in our Solar System. It is classified as a G-type star, which means it has a certain temperature and color. The Sun is very massive and bright compared to most other stars.

The Sun spins, but it does not spin evenly. It moves faster at its equator than at its poles. It is made mostly of hydrogen and helium, with a tiny amount of heavier elements like oxygen and iron. These elements were created in earlier stars before the Sun formed.

Structure

See also: Standard solar model

Core

Main article: Solar core

The Sun's core is the central part that stretches from the middle to about 20–25% of the Sun's radius. It is very dense and extremely hot, about 15.7 million kelvins. This is where nuclear fusion happens, turning hydrogen into helium and creating the Sun's energy. The core makes up most of the Sun's power, with almost no fusion happening beyond 30% of the radius.

Radiative zone

Main article: Radiative zone

The radiative zone is the thickest layer of the Sun. It starts above the core and goes out to about 70% of the Sun's radius. Here, energy moves outward mainly as radiation. The temperature drops from about 7 million kelvins near the core to 2 million kelvins farther out.

Tachocline

Main article: Tachocline

Between the radiative zone and the next layer, the convective zone, there is a transition layer called the tachocline. This area has a lot of magnetic activity that helps create the Sun's magnetic field.

Convective zone

Main article: Convection zone

The convective zone stretches from about 70% of the Sun's radius to its surface. In this layer, heat moves outward through convection—hot material rises, cools, and then sinks back down. This creates patterns on the Sun's surface called granulation.

Atmosphere

Main article: Stellar atmosphere

The Sun's atmosphere is the outer layer that extends from the top of the convective zone outward. It includes the photosphere (the visible surface), the chromosphere, and the corona (the outer atmosphere). These layers have very different temperatures and densities.

Photosphere

Main article: Photosphere

The photosphere is the visible surface of the Sun. It is tens to hundreds of kilometers thick and appears slightly darker at the edges than in the center. This layer has a temperature of about 5,772 kelvins and is where most of the Sun's visible light is produced.

Chromosphere

Main article: Chromosphere

Above the photosphere is the chromosphere, a layer about 2,000 kilometers thick. It becomes visible as a colored flash during a total solar eclipse. The temperature in this layer increases with height, reaching up to around 20,000 kelvins.

Corona

Main article: Solar corona

The corona is the Sun's outer atmosphere. It has a very low density but extremely high temperatures, ranging from about 1 million to 20 million kelvins. The corona is where the solar wind begins its journey outward into space.

Heliosphere

Main article: Heliopause

The heliosphere is the area of space where the Sun's solar wind has a greater effect than the material from the rest of space. The solar wind forms a spiral shape as it moves outward and eventually slows down when it meets the interstellar medium.

Light, radiation, and observation

The Sun gives us the energy we need to live. It helps plants grow, lets animals see, and powers Earth’s weather. The Sun is the brightest object in our sky, much brighter than any other star.

Sunlight reaches Earth with different types of light, mostly visible light we can see, some infrared light that feels warm, and a smaller amount of ultraviolet light. Earth’s atmosphere helps protect us by filtering out much of the harmful ultraviolet light.

Magnetic activity

The Sun has a stellar magnetic field that changes across its surface. This magnetic field is weak at the poles but much stronger in areas called sunspots and solar prominences. The magnetic field goes through a cycle roughly every 11 years, called the solar cycle, during which the number and size of sunspots change.

The Sun's magnetic field stretches far out into space, carried by the solar wind. This creates what is known as the interplanetary magnetic field. The magnetic field and solar activity, such as solar flares and coronal mass ejections, can affect Earth. These events can create beautiful auroras and sometimes disrupt radio communications and electric power.

Main article: Solar cycle Main article: Coronal heating problem

Life phases

Main articles: Formation and evolution of the Solar System and Stellar evolution

The Sun formed about 4.6 billion years ago from a giant cloud of gas and dust. This cloud collapsed under its own gravity, forming the Sun and a disk of material that became the planets.

Today, the Sun is in its main stage of life, fusing hydrogen into helium in its core. This process creates the energy that warms our planet and powers life on Earth. Over the next five billion years, the Sun will continue this steady energy production before eventually changing dramatically. It will grow larger and hotter, eventually expanding into a red giant before shrinking down to a cool, dim remnant called a white dwarf.

Gravitational domain and influence

Main article: Solar System

The Sun has eight planets orbiting it. These include four smaller, rocky planets called terrestrial planets: Mercury, Venus, Earth, and Mars. There are also two large gas giants, Jupiter and Saturn, and two ice giants, Uranus and Neptune. Many of these planets have their own moons, with Jupiter, Saturn, and Uranus having especially large systems.

The Sun’s gravity holds everything in our Solar System together. Its gravitational influence extends far beyond the planets, affecting objects even hundreds of thousands of times farther away. This gravity helps keep the Solar System stable and organized.

Overall location

The Sun, along with the entire Solar System, travels around the center of the Milky Way galaxy. It moves at a speed of about 230 kilometers per second, taking roughly 230 million Earth years to complete one full orbit around the galaxy. This journey is known as a galactic year, and the Sun has completed this orbit about 20 times since it formed.

As the Sun orbits the galaxy, it also moves up and down relative to the galactic plane, shifting about 99 parsecs away from the plane every 83 million years. The Sun's path is influenced by the varying mass distribution within the Milky Way, including its spiral arms.

Observational history

See also: The Sun in culture

In ancient times, many cultures believed the Sun was a solar deity or a magical force. Early astronomers, like those from Babylon, noticed the Sun’s path across the sky wasn’t straight, though they didn’t know why. Later, Greek thinkers such as Anaxagoras suggested the Sun was a huge ball of fire much larger than countries like Peloponnesus. Others, like Eratosthenes, tried to measure how far the Sun is from Earth.

As time went on, scientists began to learn more. Observers like those in ancient China watched dark spots on the Sun, called sunspots. With new tools like telescopes, astronomers could see these spots better. In the 1600s, Galileo Galilei showed that sunspots are actually on the Sun’s surface, not little objects passing by.

Modern science now knows the Sun’s distance quite well. Early scientists used events like when planets pass in front of the Sun to calculate distances. By the 1700s, they had fairly good numbers. In the 1800s and 1900s, new tools helped scientists understand the Sun’s light and heat. They discovered that the Sun’s energy comes from a process called nuclear fusion, where tiny particles called hydrogen combine to form helium and release huge amounts of energy.

See also: Solar observatory and List of heliophysics missions

Spacecraft have allowed us to study the Sun from far away. Early probes like the Pioneer satellites, launched in the 1950s and 1960s, measured the stream of particles the Sun sends out, called the solar wind. Later, spacecraft such as Helios and the Skylab space station gave us new information about the Sun’s outer layers.

More recent missions, like the Solar and Heliospheric Observatory, have watched the Sun constantly, helping us learn about its activity and even discovering many small comets that zoom close to the Sun. Other probes have traveled to areas above Earth’s usual path around the Sun to study its poles.

Religious aspects

Main article: Solar deity

Solar deities have been important in many religions and mythologies. Ancient civilizations such as the ancient Egyptians, the Inca, and the Aztecs all worshipped the Sun. In Hinduism, the Sun is considered a god named Surya. Many ancient structures, like Stonehenge in England and Newgrange in Ireland, were built to mark important solar events such as the solstices.

Different cultures had their own names and stories for the Sun. The ancient Sumerians called it Utu, while the Egyptians worshipped it as the god Ra. In ancient Greece, the Sun was seen as Helios, who drove a chariot across the sky. The Sun has inspired many festivals and traditions throughout history.