How white dwarfs are born

How a Sun-like star becomes a compact Earth-sized ball as it runs out of fuel.

An artist's impression of a white dwarf star. Credit: 7activestudio / Getty Images
Published: June 6, 2022 at 9:15 am
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As a Sun-like star runs out of fuel, it expands and loses its outer layers before becoming an Earth-sized ball called a white dwarf.

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You may have heard of a white dwarf star before, but what exactly is happening as a star like our own Sun turns into a compact white dwarf?

Understanding more about white dwarfs and how they are created could enable astronomers to gain a greater understanding as to what will happen to our Solar System in the future and what will happen when our Sun dies.

Let's explore the steps that a Sun-like star goes through in order to become a white dwarf.

White dwarf formation: step-by-step

A diagram showing the steps a star goes through in order to become a white dwarf

1. Burning hydrogen

A Sun-like star is powered by nuclear fusion, which turns hydrogen into helium with the release of energy. This can go on for around 10 billion years. During this time it is said to be a ‘main sequence’ star.

2. Shell burning

A star is said to move off the main sequence when it transitions from burning hydrogen to fusing helium. This often happens in layers surrounding the dead core and so astronomers refer to it as ‘shell burning’.

3. Red giant

The star gets through its helium one hundred times faster than it did its hydrogen. This creates huge amounts of outwards pressure and it surges out into its planetary system. Its surface would reach out to about where Earth is now.

4. Planetary Nebula

Helium burning is unstable. It sends shockwaves through the star and convulsions shake it apart. A few of these events, each 100,000 years apart, create nested shells called a planetary nebula.

5. White dwarf

A core of carbon and oxygen – the products of helium fusion – is left at the centre. Astronomers refer to this as a white dwarf and it contains about half the mass of the original star packed into an Earth-sized ball.

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This guide originally appeared in the June 2022 issue of BBC Sky at Night Magazine.

Authors

Astronomy author and writer Colin Stuart
Colin StuartScience communicator

Colin Stuart is an astronomy author and speaker.

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