The Sun didn't exist five billion years ago. But the material to make it did. There was even enough to make a number of stars and still have material left over for planets, moons and other small bodies. What was this material, and how did it end up as stars and planets?

It starts with a giant nebula.

Stars are made from nebulae, thinly spread-out clouds of gas in space. A nebula has about a hundred gas particles in every cubic centimeter. That is very thin. The air you're breathing has a quadrillion times as many. (A quadrillion is “1” followed by 15 zeroes.)

However the nebula was gigantic, several light years across. Since a light year is nearly 10 trillion kilometers (6 million miles), that really is a lot of material. The problem is how to give it a big squeeze and collapse it into hot dense pieces. This is a job for gravity.

Gravity works slowly, but surely.

Mass is the measure of how much matter (material) something contains. And wherever there is matter, there is gravity at work. More matter means something is more massive. If it's more massive, it has a bigger gravitational pull on other matter.

Let's think about our nebula. If there's a way of getting some of the material bunched together, it would begin to attract other matter to it. And this effect would get bigger as more material comes together. In fact, this is exactly what happens when something disturbs the nebula, for example, a supernova explosion. That gets the collapse going, and over several million years gravity does the rest.

The whole nebula doesn't collapse at once. Any place where matter starts bunching together pulls in more and more matter until that part of the nebula collapses. Each piece of nebula could become a star if it attracts enough other matter. This means that stars usually form in groups. Sometimes they stay together long enough to make a star cluster that we can observe, like the Pleiades. Even though it's on its own now, the Sun once had companions.

During the gravitational collapse, several things happen that will turn the pieces of nebula into stars. They get smaller, but denser. As they collapse, they also get hotter.

Protostar and protoplanetary nebula.

Let's move on a few million years to see what's happening to the piece of nebula that will be the Sun one day. It's still pulling in new matter and it's heating up nicely. It's also rotating (spinning). Instead of being a big shapeless mass, the center gets rounded because gravity pulls its mass together. Any heavenly body with enough mass is spherical. This includes planets and large moons, as well as stars.

The collapsing piece of nebula is hottest and densest in the middle. But it doesn't pull in all of the matter. Since it's spinning, some of the material spins out into a flat disk surrounding the center. The hot central part becomes a protostar and we call the disk a protoplanetary disk. The protostar is going to be the Sun. Earth and the other planets, moons, etc. are made out of the material in the disk.

The protostar has to get extremely hot to be a real star.

At some point, the protostar's collapse slows down. Gravity still pulls material inward, but the heat this produces causes an outward push.

The Sun today is stable. The inward pull of gravity is balanced by the outward push of heat from its center. But this heat isn't the heat of gravitational collapse – that wouldn't be stable. The Sun shines by the energy of nuclear fusion, because a star is a giant nuclear reactor. But in order to get its nuclear reactions going, the center of the protostar needs to get to 10 million °C (18 million <°F).

In order to be a star, the Sun had to get its nuclear reactions going, and it had to get into balance. When the pull of gravity and the outward pressure of the heat from fusion balanced, a star was born! As long as the star's nuclear reactor is using hydrogen for fuel, it stays balanced. We say that it's on the main sequence.

How long will the Sun stay on the main sequence?

A star spends most of its existence on the main sequence. However the amount of time it spends there depends on its mass. A red dwarf with half the Sun's mass may last for 80 billion years or more – that's nearly six times the age of the Universe so far. Really massive stars last for only a few million years. The Sun is nearly half way through its lifetime of around ten billion years.