Stars

Nebulae are clouds of gas and dust which float through the vastness of space. Over time, parts of these giant clouds will collapse under their own weight, pulling together into a spinning ball of superhot gas called a protostar.

A protostar’s gravity continues to pull in clouds of gases, and clouds of space dust too. It gets bigger and bigger, and gravity increases, accelerating the entire process.

Eventually, a protostar reaches an important milestone: it’s hot enough, and dense enough, for hydrogen atoms at the core of the star to start combining together into helium atoms. This process is called nuclear fusion, and it releases massive amounts of energy.

The process of transforming from a collapsing dust cloud to a shining star takes roughly a million years.

To reach the point at which temperatures are hot enough, and fusion begins, stars need to be sufficiently massive. If a protostar doesn’t get big enough and hot enough for fusion to begin, it becomes a brown dwarf – a failed star.

If a protostar does achieve nuclear fusion, it is said to have entered the main sequence stage of its life cycle. These are a star’s prime years of life, when it’s producing a steady, stable amount of energy.

Our own star – the sun – is a main sequence star. But they can range from about 10 times smaller than the sun, to 200 times as massive.

It will stay in this stage of its stellar life cycle until it runs out of hydrogen, and those fusion reactions are no longer able to happen.

During the main sequence stage, a star is in a state of equilibrium. The fusion reactions at the core of the star exert an outward pressure, and this balances against the inward pressure of the star's gravity.

But eventually, every star will run out of hydrogen fuel. When this finally happens, the outward pressure stabilizing the star is no more, and the force of gravity will cause the star to collapse.

This isn't the end of the story, especially for larger stars. Nuclear fusion will continue, only this time, atoms will fuse together to create magnesium, neon, sodium, and aluminum.

This creation of elements is called stellar nucleosynthesis. This process is responsible for creating almost all the elements on the periodic table.

The bigger a star, the faster it burns. This may seem counterintuitive: bigger stars have more fuel to burn – so why don’t they last longer?

Although more massive stars have a lot of fuel, they burn it up very quickly, thanks to the intense pressure at their core. The most massive stars might live for just a few hundred million years – a brief existence in cosmic terms.

Our own star, being around the middle of the size-range, is likely to live for another 5 billion years. We think it’s currently around 4.6 billion years old, so it is about halfway through its life.

One of the oldest stars ever discovered is the Methuselah Star. Around 190 light years from the Earth, it is estimated to be around 12 billion years old. This estimate makes it almost as old as the universe itself.

Spectral class is a way to categorize different stars according to their surface temperature. These temperatures are measured in Kelvin – you can see the seven classes (hottest at the top) below.

Our sun is a G-class star. It burns at an average surface temperature of approximately 5800 K.

You might notice, from this image, that hotter stellar classes are larger. An O-class star is such a massive body that it’s hard to even imagine.

Stars can also be classified in terms of the current stage of their life cycle. For example, while our sun is a G on the spectral class, it's also in the main sequence stage of its life cycle.

As it gets older, and the equilibrium between fuel and gravity starts to break down, a star might expand into something called a red giant, before squeezing back down into a white dwarf.

A white dwarf will gradually cool over time, before finally becoming a black dwarf – the ghost of a star, which gives off no heat or light. These aren't the only stages of the stellar life cycle, but we'll look at the rest later on.

Another way to classify stars is by luminosity. This is a measure of the energy that the star radiates. The luminosity and temperature of stars can be represented on a Hertzsprung-Russell (HR) diagram.

In the main sequence stage of their life cycle, stars will follow a diagonal line from the top left to the bottom right corner. In other words, the hotter they are, the brighter they are.

But this diagram also shows a few groups deviating from this pattern – like giants and white dwarfs. White dwarfs are hot but dim, while giants are cool but bright.

Whatever the classification of a star, none can last forever.

As we've already talked about, when a main sequence star runs out of hydrogen, the balance between the outward pressure of fusion and the inward pressure of gravity is lost. Gravity now takes over, and the star begins to collapse.

At this point, different stars will go through different death cycles depending on their size.

As we've already talked about, when a smaller star collapses, the core becomes extremely hot. As a result of these extreme core temperatures, the gasses on the surface of the star will expand.

The star will swell to many times its previous size, growing up to 1000 times the width of our sun. Because the star’s energy is spread over such massive distances, it grows cooler and, as a result, turns red, forming a red giant.

The red giant continues to heat up at the core – this might last for a billion years. As temperatures increase still further, helium fusion starts. Then the helium runs out, and the star starts to shrink back down again.

Eventually, the star will turn into a white dwarf: an object around the size of Earth, but much hotter and much, much denser. A white dwarf is roughly 200,000 times as dense as Earth.

When a star shrinks down from a red giant to a white dwarf, it sheds off its outer layers. The cloud of dust and gas cast off by the star is called a planetary nebula.

The name is misleading: no planets are involved in a planetary nebula, but the first observers misidentified them as planets, and the name stuck.

If the white dwarf radiates enough energy, it lights up the planetary nebula drifting away from the star, making it glow. These glowing clouds of material, shaped by solar winds and gravity, can be stunningly beautiful.

Astronomers believe that white dwarfs will eventually run out of energy, and turn into black dwarfs instead – small stars that emit no energy whatsoever.

However, this is only a theory, and there are currently no known black dwarfs in the entire universe.

Why not? It could take tens of billions of years, or hundreds of billions of years, for a white dwarf to finish cooling down.

But the universe is less than 20 billion years old. In other words, none of the existing white dwarfs have had enough time to cool into a black dwarf – yet.

When a small star dies, it expands into a red giant, then shrinks back down into a white dwarf. But when a large star dies, it goes out with more of a bang.

We've already talked about stellar nucleosynthesis. This is when larger stars, towards the end of their lives, reach such extreme temperatures and extreme pressures that heavier elements start to form at the core.

But it's game over for the star at a certain point: when silicon starts to fuse into iron.

The fusion of iron doesn’t release energy. It uses it up. And this completely destabilizes the star. The core collapses at unimaginable speed, then explodes outwards. We call this a supernova.

There is more than one way to create a supernova. In addition to supernovae caused by the deaths of massive stars, there are stellar explosions known as type 1a supernovae.

These supernovae involve binary star systems – that's two stars orbiting around each other. For a type 1a supernova to take place, at least one of these stars must be a white dwarf.

Over time, the white dwarf steals mass from its neighboring star, or merges with the star to increase its mass. Once it gains enough mass from its companion, it becomes unstable. This point (around 1.44 times the mass of the sun) is known as the Chandrasekhar mass.

It is believed that fusion switches back on at this particular mass, triggering a runaway reaction and an explosive supernova.

A supernova might leave behind a neutron star – that's a collapsed core, which is usually formed after the deaths of stars up to 25 times the size of our sun.

In a neutron star, electrons and protons are crushed as the core continues to collapse. This creates neutrons, which halt the collapse of the star when it reaches around 20 kilometers across.

With so much matter from the original star now packed together, a neutron star is the densest object in the known universe. A sugar-cube-sized piece of a neutron star would have a mass of around 1 trillion kilograms.

We observe most neutron stars as pulsars. These are spinning neutron stars, which appear to emit very regular radiation pulses. They actually emit two steady, narrow beams of radiation in opposite directions, but because they also spin, this beam of radiation appears to pulse on and off.

If a star is sufficiently massive, the collapsing core doesn’t stop when a neutron star is formed. Instead, the collapse continues on, until all that matter reaches a small, infinitely dense point.

This point is known as a singularity. It's the heart of a black hole.

With such a dense little point at the center, the gravity in a black hole is so unimaginably strong that not even light can travel fast enough to escape it. At the singularity, the gravity of a black hole is infinite.

The edge of the black hole – the point at which its gravity is strong enough to trap light – is known as the event horizon. If anything ever crosses this horizon, it will never be able to escape.