The Sun is almost 5 billion years old. But how was it formed?

The Sun is a gigantic ball of burning gas located 150 million kilometers away from us. It was created 4.6 billion years ago. Prior to that time our Sun, planets and moon did not exist. The Universe contained billions and billions of stars, but our Sun was not one of them. So how was our Sun formed?

The Solar System

The Solar System is made up of our Sun and all its orbiting bodies, e.g. planets and asteroids. It is located in one of the spiral arms in the Milky Way, which is the galaxy we live in. The Milky Way is made up of 200 billion stars, some of which are dead and some are still burning their fuel.

The Milky Way is 10 billion years old, which is much older than our Sun. This tells us that our galaxy (i.e. a collection of billions of stars) had already been created, when the Sun was born. Many of the stars in the Milky Way are like our Sun and the creation of the Sun is not at all unique. Star formation happens continuously throughout our galaxy, and our Sun was created like most other stars.

The beginning: Gas and dust collapse

The Milky Way consists of stars, gas and dust. The gas and dust in-between the stars make up a medium called the interstellar medium (ISM), and the ISM has been around for as long as the galaxy. The ISM contains a lot of turbulent motion, which makes the gas and dust move randomly around in their region. Sometimes the gas and dust moves towards the same place to form a dense cloud, and this makes the cloud collapse due to gravity.

This is what happened around 5 billion years ago when the Sun was about to get born. The Sun’s birth started with a collapsing cloud of gas and dust.

It takes around 10 million years for a cloud to collapse and become a star like our Sun. During this time the cloud decreases in radius and increases in density, almost like two hands squeezing a snowball. Slowly, this process forms a so-called protostar with a spinning disk of gas and dust around it.

Our Sun started out as a relatively cold protostar and it did not emit any light. Luckily for us, the protostar continued to grow by eating the surrounding gas.

The formation process of the Solar System. A cloud of gas and dust collapses into a core (a-b), which continues to contract into a protostar. The protostar grows by eating gas from the surrounding disk (c) and eventually triggers nuclear fusion in the core (d). At some point the star stops growing (e) and planets are formed from the remaining material (f). The entire process takes around 10 million years for a Sun-like star. Illustration: Astronomicca.

The early years: ProtoStars grow by eating surrounding gas

As the protostar grows in size by eating gas from the disk, the internal gas reaches temperatures above 10 million degrees Celsius. At these temperatures atoms can overcome the Coulomb barrier, so that Hydrogen nuclei begin to fuse into Helium nuclei. This process is known as nuclear fusion and it is a characteristic of any ‘regular’ star, including our Sun.

When nuclear fusion is triggered the object is no longer called a protostar but instead it becomes a T-Tauri star, which is characterized by its ability to fuse atomic nuclei. Smaller bodies like planets and comets do not fuse nuclei in their core. This is something that is reserved for the stars.

The fusion processes inside a star creates energy in the form of photons (light particles). The photons travel throughout the star and are eventually emitted from the surface as what we recognize as sun-rays. The T-Tauri star “eats” the inwards falling gas and so it can continue to grow in size. This mechanism of a star growing in size by eating the surrounding inwards falling gas is called ‘accretion’.

Finally a star: stop growing and start glowing

The T-Tauri star continues to grow as long as there is a surrounding disk of gas to eat. At some point the accretion comes to a halt. For most stars this happens when they reach the size of our Sun, but reality is of course more complicated than simply reaching a certain size. The point of halt depends on both the size of the disk (how much “food” can the star “eat” from the disk?) and the radiation pressure (how much light does the star emit?). Even though light has zero mass it still carries with it a pressure that eventually can prevent gas to fall onto the star.

When accretion comes to a halt the surrounding gas can no longer fall onto the surface of the star. The gas will instead remain a disk around the star. The star is now called a ‘main-sequence star’ and is a regular Hydrogen-burning star like our Sun. It emits light from nuclear fusion in the core. The Sun has now officially been born.

The remaining disk around the Sun still rotates around the central star. As the disk rotates gas and dust will start to “clump” together and form smaller spherical bodies, that are not nearly dense enough to trigger nuclear fusion in the core. These bodies will continue to rotate around the central star.

Our Sun has many of these bodies – and eight of them we as humans have put in the category ‘planet’. Planets form exactly this way: from left-over material from the host star’s birth, in our case the host star is our Sun. This means that all the planets in the Solar System (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) were formed from a disk around our Sun. A disk that a bit less than 5 billion years ago served “food” to our Sun.