Why Does the Sun Keep Shining?

KohakuLook up at a clear sky, and the Sun is quietly shining today, isn't it?

The answer in 30 seconds

Let's explore together why it has kept shining for billions of years.

The Sun does not burn like a fire. Deep in its core, hydrogen nuclei fuse together to become helium in a process called nuclear fusion, releasing light and heat. The core is about 15 million °C, and the Sun uses about 4 million tonnes of hydrogen every second, so it can keep shining for about another 5 billion years.

A partial solar eclipse seen from space, with bright light forming a thin crescent around the Moon. — A partial solar eclipse photographed during the Artemis II mission. The Sun's light peeks around the Moon in a bright arc.Photo: NASA Johnson Space Center / NASA / Public domain / Wikimedia Commons

The Sun Is Not a Fire

The Sun does not shine by burning like a fire. It is a huge ball of hot gas, or plasma, made mostly of hydrogen and helium. In its core, hydrogen nuclei join together to become helium in a process called nuclear fusion. This reaction releases an enormous amount of energy that travels outward as light and heat.

Figure: Nuclear Fusion in the Sun's Core

Fusion in the Core

The Sun's core is about 15 million degrees Celsius (about 27 million degrees Fahrenheit). At that extreme temperature, hydrogen nuclei move fast enough to overcome their natural repulsion and fuse together. The outward pressure from fusion supports the Sun's enormous mass and keeps it from collapsing.

Four Million Tonnes Every Second

To keep shining, the Sun uses about 4 million tonnes of hydrogen fuel every second, and it has done so since it formed about 4.6 billion years ago. Because the Sun is so large, it still has enough fuel to shine for about another 5 billion years.

KohakuIt is not burning like a fire. Deep inside, hydrogen nuclei join together in a process called nuclear fusion.

The Sun's Distant Future

After that, the Sun is expected to swell into a red giant star. It may boil away Earth's oceans and could even expand enough to swallow Earth. That is far in the future, though; today, spacecraft such as Parker Solar Probe and Solar Orbiter are studying how the Sun works.

Want to know more?

The Sun is made of about 92% hydrogen and 8% helium, very different from the mix of elements on Earth.
The Sun's surface temperature is about 5,500 °C, while the core reaches about 15 million °C.
The Sun is a hot gas called plasma, and different parts of it rotate at different speeds.

KohakuThe Sun is expected to keep shining for about another five billion years.

Quick quiz: test yourself

What reaction in the Sun's core produces its light and heat?

Show the answer

Hydrogen fusing into helium (nuclear fusion)
Hydrogen burning with oxygen
Carbon bonding with oxygen (combustion)

Hydrogen fusing into helium (nuclear fusion) The Sun's core is about 15 million °C, so hydrogen nuclei can join together and become helium. This nuclear fusion releases light and heat, unlike ordinary burning.

Sources

Sun: FactsNASA Science
The SunNASA Goddard Space Flight Center
The SunEuropean Space Agency

This article was written after checking the sources above as of 2026-10-03. Explanations may change as research progresses. Photo credits are shown below each photo.