Stars don't last forever. And even though stars all begin in pretty much the same way, the way they end depends on their mass. Little stars live for trillions of years and quietly fade away. Really massive stars have short lives, and go out with a bang. Stars with masses similar to the Sun end in a different way – this is their story.

The Story So Far
Five billion years ago there was a giant nebula. Something disturbed the nebula, and sections of it slowly collapsed into hot, dense lumps. One of the lumps became our Sun when it got hot enough for nuclear fusion in the core (center). The Sun has been shining steadily since then.

What is hydrogen burning?
We often call hydrogen fusion hydrogen burning, but it isn't burning like a campfire. It's a nuclear reaction. The nuclei (centers) of four hydrogen atoms are squeezed together to make helium. The process releases energy, and this energy keeps the Sun shining and the Earth habitable.

The Sun has used up a lot of hydrogen in the last five billion years. But don't worry about it – the Sun is massive and there's still a lot of fuel left. It will keep on shining for a few billion more years.

The hydrogen doesn't last forever – what happens when it runs out?
When the hydrogen in the Sun’s core is gone, the nuclear reactions will stop, and the core will start to shrink. This is the beginning of the end for the Sun, but first it will undergo an amazing transformation.

In starbirth, as the nebula collapses it gets hotter. And when the star is dying, as the core shrinks it will get hotter. In fact, it will release enough heat to start hydrogen burning in the layers around it. Our star will swell into the type of star known as a red giant.

The Sun's expansion will be bad news for the planets. The expanding Sun will swallow up Mercury and Venus. Earth will survive, but not as we know it. The seas will boil away and the crust will melt.

Then there is helium burning.
The core will run out of hydrogen, but it will have lots of helium. Stars can also burn helium, but it needs a higher temperature than for helium burning. When the core is shrinking, the temperature will go up and up. When it gets hot enough, helium burning will start, producing carbon and oxygen and energy. Helium burning produces more heat than hydrogen burning. This means that not only will the core stop shrinking, but the Sun will expand to the orbit of Mars, swallowing the Earth as it does so.

Hydrogen burning keeps a star going for billions of years, but helium burning will last only about 100 million years. When the helium is used up, some stars start burning carbon, but the Sun isn't massive enough for this. When the Sun's helium is gone, the core will start shrinking again.

The outer layers will make a planetary nebula.
The pull of gravity on the Sun's outer layers gets weaker as the core shrinks. The Sun will throw off material from its outer layers, and that will form a nebula. Our star was born from a nebula, and it will leave behind a new nebula. This one will contain the heavier elements the Sun made – all ready for recycling. The oxygen that you're breathing now originally came from a helium-burning star that died billions of years ago.

The nebula is known as a planetary nebula, even though it has nothing to do with planets. In the telescopes of the 18th century, these nebulae looked a bit like planets. William Herschel gave them the name and it stuck.

The end: a white dwarf, and eventually a black dwarf.
After helium burning ends, the core won't shrink into nothing. It will get squeezed into an object somewhat bigger than the Earth, but with the mass of a star. It's called a white dwarf.

Atomic forces will finally stop the collapse, but what's left is very strange. The matter has been squashed into something that's impossible on Earth. If you could collect a teaspoonful of white dwarf material, it would weigh about fifteen tons.

A white dwarf is quite hot, but it has no way of generating heat. So it will slowly cool. When the heat and light are finally gone, all that will be left of the Sun is a black dwarf, a small dark object made of weird matter.

We have observed many white dwarfs, but no one has ever seen a black dwarf. It's not just because they're small, dark and cold. The cooling should take about a trillion years. That's over seventy times the age of the Universe, so there hasn't been time for any black dwarfs to form.