The Sun

The heart of our Solar System

The Sun is the star at the centre of our Solar System. It's an enormous, glowing ball of hot gas that provides light and heat to the Earth. In fact, the Sun is so big that it makes up about 99.86% of the total mass of the entire solar system! If the whole Solar System was a cake, and you cut it into 1000 tiny slices, only 1½ of those slices would be used to make the planets, moons, asteroids, and other objects.

The Sun is around 4.6 billion years old and has been shining for about half its lifetime. It has a diameter of about 1.39 million kilometers (865,000 miles), making it 109 times wider than Earth. An incredible 1.3 million Earths could fit into the same volume as the Sun!

The Sun is incredibly hot, with a relatively cool surface temperature of 5,500 degrees Celsius (about 9,932 degrees Fahrenheit). Dive deeper into the Sun, and it gets a lot hotter; its core reaches temperatures of about 15 million degrees Celsius (27 million degrees Fahrenheit).

The Sun achieves these huge temperatures because its mass, at about 330,000 times that of Earth, produces the immense gravity needed to kick-start nuclear fusion—the process by which the Sun creates all its heat and light. That immense gravity also holds the Solar System together, and without the Sun, the planets would not have formed. No need to worry about all this burning of energy, though, as the Sun is expected to continue shining for another 5 billion years before its nuclear fuel is exhausted.

The Sun consists of several layers, each playing a crucial role in its structure and energy production. These layers include:

• The Core: this is where nuclear fusion occurs, generating the Sun's energy.

• Radiative zone: this zone surrounds the core, and allows energy to move outward through radiation.

• Convective zone: where hot gas rises to the surface, cools, and then sinks back down.

• Photosphere: this is the visible surface of the Sun.

• Chromosphere: a thin layer above the photosphere.

• Corona: the outermost layer of the Sun, which extends millions of kilometers into space and is even visible during solar eclipses.

NASA/JPL/University of Arizona, Public domain, via Wikimedia Commons

Key stats

How the Sun works

The Sun works through a process called nuclear fusion, which happens in its core. In nuclear fusion, four hydrogen atoms smash together to form one helium. This process releases a tremendous amount of energy, because some of the mass of the hydrogen atoms is converted into energy according to Einstein's famous equation, E=mc², where E is energy, m is mass, and c is the speed of light. This energy powers the Sun and radiates out into space, providing the light and heat that sustain life on Earth.

The energy produced in the Sun's core takes a long time to reach the surface. It moves outward through the different layers of the Sun, but this process can take thousands, or even millions, of years because the energy is constantly absorbed and re-emitted as it moves towards the Sun’s surface.

The Sun constantly produces an incredible amount of energy, converting about 600 million tons of hydrogen into helium every second, releasing energy equivalent to about 4 trillion 100-watt light bulbs! A tiny fraction of this energy lands our planet Earth, which is just the right distance from the Sun to allow liquid water to exist and with it, life.

NASA/JPL-Caltech/SwRI/MSSS/Betsy Asher Hall/Gervasio Robles, Public domain, via Wikimedia Commons

Goldilocks star

The Sun is a G-type main-sequence star, also known as a yellow dwarf. It's a medium-sized star—not too big and not too small. Stars come in different sizes, colours, and temperatures. Hotter stars appear blue or white, while cooler stars appear red or orange. The Sun's yellow colour indicates that it has a moderate temperature.

Most stars in the universe are much smaller red dwarfs, which have far longer lifespans than the Sun. They burn their fuel more slowly and can live for trillions of years. One of the oldest known stars is Methuselah, which is an incredible 13.7 billion years old—3 times older than our sun, and almost as old as the Universe itself.

Then there are stars that are much bigger and brighter than the Sun. Initially these massive stars burn bright and hot, and are blue in colour. These blue giants are stars many times larger than the sun, and the resulting increase in gravity allows for a more energetic burn, resulting in their blue colour. Such stars only live for a few million years, after which they start to cool, as the hydrogen fuel is extinguished and they are forced to burn heavier elements. At this point they swell to enormous sizes and turn red, into what we call red supergiant stars.

One example is Betelgeuse, a star at least 700 times larger than the Sun, and nearing the end of its life. Incredibly, over 400 million Suns would fit within Betelgeuse. In the scale model of the Dirleton Solar Walk, our Sun—with a diameter of 38 cm—would be dwarfed by an enormous Betelgeuse around 300 meters in diameter! To visualize this, consider that nearby hill Berwick Law would roughly represent one hemisphere (half a sphere) of Betelgeuse.

Betelgeuse is fast approaching its death in an immense explosion called a supernova which will take place within the next 100,000 years. During a supernova, the stars core collapses under gravity, triggering an enormous release of energy that blasts the outer layers into space. When Betelgeuse does go supernova, the light will illuminate the night sky as brightly as the moon for a short period. Potentially it will even be visible during the day.

We just don’t know when this will happen. In fact, Betelguese may have already exploded, but because it is over 400 light years away, the light from the supernova has not yet reached us!

NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, Public domain, via Wikimedia Commons

Sun storms and solar activity

The Sun isn't just a big ball of constant light and heat; it also has dynamic and sometimes violent activities called solar storms. One of these phenomena are called solar flares, sudden bursts of radiation that occur when stored magnetic energy in the Sun's atmosphere accelerates particles. These solar flares can send streams of charged particles out into space, known as solar wind.

Sometimes, the Sun ejects even larger amounts of material in events called coronal mass ejections (CMEs). These are huge bubbles of gas and magnetic fields that can travel through space and impact planets. A single CME can release as much energy as 1 billion hydrogen bombs!

When these charged particles from the Sun reach the Earth, they interact with our planet's magnetic field, which protects us from most of the harmful radiation. However, these interactions can also create beautiful displays of light called auroras, or the Northern and Southern Lights. These colourful lights are seen near the poles and are caused by particles colliding with the Earth's atmosphere.

Sunspots are another fascinating aspect of solar activity. These are cooler, darker areas on the Sun's surface caused by magnetic activity. They appear in pairs or groups and can be seen with special telescopes which are designed to view the Sun safely. Sunspots are indicators of the Sun's magnetic activity and can affect space weather, which in turn can impact satellite operations, communication systems, and even power grids on Earth.

NASA/JPL-Caltech/ASU, Public domain, via Wikimedia Commons

Solar eclipses

A solar eclipse occurs when the Moon passes between the Earth and the Sun, casting a shadow on Earth. There are different types of solar eclipses:

• Partial: where only part of the Sun is covered

• Annular: where the Sun appears as a ring around the Moon

• Total: where the entire Sun is blocked out by the Moon

Total solar eclipses are rare and spectacular events, and they can only happen during a new moon when the Sun, Moon, and Earth are perfectly aligned. During a total solar eclipse, the sky becomes dark as if it were night, and the Sun's outer atmosphere, called the corona, becomes visible as a glowing halo.

NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill, Public domain, via Wikimedia Commons

Eclipses remind us of the precise dance of celestial bodies in our Solar System, and provide valuable opportunities for scientists to study the Sun's corona and other features that are otherwise difficult to observe.