In the night sky: Orion
In the night sky: Orion

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In the night sky: Orion

2.3 The death of a star

What happens, though, when a star runs out of fuel?

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NARRATOR 1:
What happens to a star during the rest of its life depends on how massive it is at its birth.
NARRATOR 2:
A star like the sun is in a delicate balance between gravity, which wants to make the star collapse in on itself, and the pressure that pushes outwards that comes from the energy that’s being produced and these fusion reactions happening at its core. However, at some point in the future, the hydrogen runs out. And at that point, the core of the star will start to collapse in on itself under its own weight. It gets denser. It gets hotter, until a point where you can actually start to use the helium atoms themselves as the fuel for the fusion, pushing helium atoms together and making carbon and oxygen the next heavier elements in the periodic table.
NARRATOR 1:
As the star begins to fuse helium it creates more energy, and that causes the outer layers of the star to expand. One day our sun will grow so large it will swallow up the inner planets of the solar system out as far as the Earth. It will become a red giant. For the sun, this will be the beginning of the end.
NARRATOR 2:
What happens is that the outer layers of the star get farther and farther from the middle. The force of gravity that they feel is getting weaker and weaker. And actually, the star loses hold of its outer atmosphere. Its outer atmosphere drifts off out into space. It expands out to become a planetary nebula, and they’re some of the most beautiful objects in the universe.
NARRATOR 1:
Once the outer layers have drifted away, all that is left of the star is its core.
NARRATOR 2:
A white dwarf star is the dead remnant core of a star like the sun at the end of its life. What’s left behind is something that might weigh as much as half the mass of the sun, but it’s only about the size of the Earth. So it’s an incredibly dense object. It’s dead. There's no nuclear fusion going on there anymore. It’s incredibly hot. But then, over millions of years, it will gradually cool down to become a black dwarf.
End transcript
 
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The Sun will gradually run out of fuel but we probably have about another 4.5 billion years’ worth of hydrogen still to go, so there’s no need to worry quite yet!

As the hydrogen gets used up, the rate of energy production will also decrease. This upsets the balance between the outward pressure of hot gas and the inward pressure of gravity that has kept the Sun stable. The core of the Sun will then begin to collapse under its own weight, and as it collapses, the core will begin to heat up.

As the core contracts, it heats up the layers above it, which eventually become hot enough for nuclear fusion to take place in the same way as it did in the core. At the same time, the core continues to contract and get hotter, eventually becoming hot enough for the fusion of helium nuclei into heavier elements.

Three helium nuclei fuse together to form a nucleus of carbon – this is called helium burning, and produces sufficient energy to stabilise the star against further collapse. Because the star has to consume its fuel much faster to prevent it collapsing, this part of its lifecycle is much shorter than the time spent on the main sequence.

How does the appearance of the star change as the different nuclear reactions take place? In a star burning helium in its core and hydrogen in a shell surrounding the core, the rate of energy production rises. At the same time, the radius of the star increases, so its surface area also increases. Although there is now a greater supply of energy, it isn’t sufficient to keep the outer layers of the star at their previous temperature, so the temperature of the photosphere decreases to below 4000 K. The star is now orange-red in colour, and has become a Red Giant.

This isn’t the end, though, for the star! The star loses hold of its outer layers and it becomes a planetary nebula. The remains are no longer burning helium but are still hot, this is known as a white dwarf. This is true for all stars that are born with less than about eight times the mass of the Sun.

You will find out more about planetary nebula in the next section.

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