Sun

The Sun is the star at the centre of our Solar System. It is a huge ball of hot plasma. The Sun shines because of nuclear fusion in its core. This gives us energy called sunlight, which is very important for life on Earth.

The Sun is much bigger and heavier than anything else in the Solar System. Its width is about 109 times that of Earth. It holds about 99.86% of all the mass that goes around it.

The Sun formed about 4.6 billion years ago from a cloud of gas and dust. It still shines today by changing hydrogen into helium. In the far future, the Sun will change shape and become 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.

General characteristics

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

The Sun spins, but not 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 stars before the Sun formed.

Structure

See also: Standard solar model

Core

Main article: Solar core

The Sun's core is the middle part of the Sun. It is very dense and very hot, about 15.7 million kelvins. Here, nuclear fusion turns hydrogen into helium and makes the Sun's energy. The core makes most of the Sun's power.

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 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. It includes the photosphere (the visible surface), the chromosphere, and the corona (the outer atmosphere). These layers have 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 looks slightly darker at the edges. This layer has a temperature of about 5,772 kelvins and makes most of the Sun's visible light.

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 very 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. We can see most of it. Some of it feels warm. A smaller amount is 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 this cycle, 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 fell together and made the Sun and the planets.

Today, the Sun is in its main stage. It mixes hydrogen into helium and makes energy. This energy warms our planet and helps life on Earth. In about five billion years, the Sun will change. It will grow bigger and hotter, becoming a red giant. After that, it will shrink into a cool, dim remnant called a white dwarf.

Gravitational domain and influence

Main article: Solar System

The Sun has eight planets around it. Four are smaller, rocky planets called terrestrial planets: Mercury, Venus, Earth, and Mars. There are two big gas giants, Jupiter and Saturn, and two ice giants, Uranus and Neptune. Many of these planets have moons. Jupiter, Saturn, and Uranus have lots of them.

The Sun’s gravity holds everything in our Solar System together. Its gravity reaches far past the planets, keeping the Solar System stable and in order.

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. It takes roughly 230 million Earth years to complete one full orbit around the galaxy. This journey is called a galactic year. 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. It shifts 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

Long ago, many people thought the Sun was a solar deity or a special force. Early astronomers, like those from Babylon, noticed the Sun did not move in a straight line across the sky, but they did not know why. Later, Greek thinkers such as Anaxagoras thought the Sun was a giant ball of fire, much bigger than places like Peloponnesus. Others, like Eratosthenes, tried to measure how far away the Sun is.

As time passed, scientists learned more. People in ancient China saw 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 on the Sun’s surface, not small objects passing by.

Today, science knows the Sun’s distance very well. Early scientists used events like when planets moved in front of the Sun to calculate distances. By the 1700s, they had good numbers. In the 1800s and 1900s, new tools helped scientists understand the Sun’s light and heat. They found 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 let us 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 fly 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

Many religions and stories have special meanings for the Sun. Ancient people, like the ancient Egyptians, the Inca, and the Aztecs, all looked up to the Sun as something very important. In Hinduism, the Sun is a god named Surya. Big buildings such as Stonehenge in England and Newgrange in Ireland were made to celebrate special days tied to the Sun.

Different places had their own names and tales about the Sun. The ancient Sumerians called it Utu, and the Egyptians honored it as the god Ra. In ancient Greece, people believed the Sun was Helios, who traveled across the sky in a chariot. The Sun has been a source of many festivals and customs over time.