How We Came to Know the Cosmos: Space & Time

Discover How We Came to Know the Cosmos

Chapter 12. Red Giants and White Dwarfs

12.1 Red giants

When the Sun runs out of hydrogen to fuse in its core, it’ll no longer produce enough nuclear energy to counterbalance the force of gravity (as discussed in Chapter 11) and so it will contract slightly. This will cause it to increase in temperature and allow hydrogen fusion to begin in the shell around the helium core.

Fusion creates extra radiation pressure and this will cause the Sun to expand. The outer layers of hydrogen will decrease in temperature, which will make them redder, and the Sun will then be a red giant. All main sequence stars that are about 1/5 to 10 times the mass of the Sun will become red giants.[1]

A diagram showing the evolution of a Sun-like star.

Figure 12.1
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A diagram showing the evolution of a Sun-like star from a protostar to a red giant. After this, the core will become a white dwarf while the outer layers will form a planetary nebula.

The helium core of a red giant is so dense that it becomes degenerate. This means that it’s as dense as the Pauli exclusion principle (discussed in Book II) allows and it won’t be able to change in size as it changes in temperature. It cannot expand and cool when it’s heated by the hydrogen fusion in the shell around it, and eventually it gets so hot that the helium nuclei began to fuse into carbon via the triple-alpha process. The carbon can then fuse with helium nuclei to become oxygen and neon.

The triple-alpha process

A diagram of the triple-alpha process.

Figure 12.2
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The triple-alpha process.

Helium fuses into carbon via the triple-alpha process. Here, two helium nuclei (made of two protons and two neutrons) collide to produce a gamma-ray and a beryllium nucleus (made of four protons and four neutrons). This decays into a carbon nucleus (made of six protons and six neutrons), a helium nucleus, and a gamma-ray.

If the carbon nucleus collides with another helium nucleus, it will produce an oxygen nucleus. Neon nuclei are formed if oxygen nuclei fuse with another helium nucleus.

In stars less than about 2.5 times the mass of the Sun, this process is known as a ‘helium flash’.[2] This is because helium fusion occurs in a matter of minutes to hours. The energy released by the helium flash means that the star becomes so hot that it stops being degenerate, it can then expand and cool. These stars are sometimes called horizontal branch stars.

When all the helium has been fused into other elements, radiation pressure decreases, and the star contracts again under gravitation. Much higher temperatures are needed for carbon and oxygen fusion to begin, and so this only occurs in stars over about 10 times the mass of the Sun.

Stars less than about 10 times the mass of the Sun become asymptotic-giant branch stars - red giants with inert, degenerate carbon/oxygen cores, that fuse helium in the shell around the core. This helium fusion causes the star to become unstable and the envelope is ejected as a planetary nebula.

If the remaining core is less than about 1.4 times the mass of the Sun, then it becomes a white dwarf. If it’s more than about 1.4 times the mass of the Sun, then it becomes a neutron star or black hole (discussed in Chapter 13).

A diagram showing that the final fate of stars depends on their mass.

Figure 12.3
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The evolution of stars depends on their mass, with the most massive stars becoming black holes.

12.2 White dwarfs

Over 97% of stars in the Galaxy will become white dwarfs.[3] White dwarfs are stars that have ceased nuclear fusion but still emit light from stored thermal energy.

White dwarfs have a mass that is comparable to the mass of the Sun, but they are compacted to a size comparable to the size of the Earth. They have a very thin hydrogen and helium atmosphere and a crust that is about 50 km thick. It’s thought that there’s a crystalline lattice of carbon and oxygen below this.[4]

A diagram showing that white dwarfs are typically not much larger than the Earth.

Figure 12.4
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White dwarfs are roughly the size of the Earth and the mass of the Sun.

If a white dwarf is part of a binary system - and at least half of all stars are in binaries[5] - it may gain mass from its companion. This extra mass increases the white dwarfs’ temperature but because it’s degenerate it cannot expand and cool. It can get so hot that carbon and oxygen begin fusing extremely quickly, creating explosions on the surface. These types of binaries are known as cataclysmic variables (CVs) or, if matter is constantly fusing, super soft X-ray sources (SSXS or SSS). If enough matter is accumulated then the white dwarf will explode in a Type Ia supernova (also known as a thermal runaway supernova) and eject its companion into space.[6]

If a white dwarf is part of a binary system then the crust may be stripped away, exposing the core. This is what happened to PSR J1719-1438 b, which is known as a diamond planet (discussed in Chapter 28).

When a white dwarf stops producing light it’s known as a black dwarf. White dwarfs are expected to keep radiating for well over 14 billion years, however, and so the universe is not yet old enough to contain any.

A photograph of white dwarfs.

Figure 12.5
Image credit

White dwarfs, an image from the Hubble Space Telescope.

12.3 References

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