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Western Slope Skies - How Stars Become White Dwarfs

Hubble image of the faintest stars, including white and red dwarfs in an ancient star cluster.
Hubble image of the faintest stars, including white and red dwarfs in an ancient star cluster.

Some may enquire about what would happen to the Sun once it dies, or other stars like the Sun. Before there is panic, thankfully the Sun will not be on track to die for the next five billion years. Stars are classified by their mass, and their mass determines what they will be once they die. There are three mass ranges. The first are low mass stars, they are less than two times the mass of the Sun. The Sun fits into this category. The second is intermediate mass stars. Their mass ranges between two to eight times the mass of the Sun. Lastly, there are high mass stars, and they are greater than eight times the mass of the Sun.

When a star dies what is left behind is called a stellar remnant. The type of stellar remnant is dependent upon the star’s mass. A remnant is left behind after a dying star has exhausted its nuclear fuel and blows apart the outer layers. Since the Sun is considered a low mass star, its stellar remnant is a white dwarf. This also applies to intermediate mass stars. This means that white dwarf stars are the most common stellar remnant. In contrast, high mass stars have two options, either they become a neutron star or a black hole. A neutron star is a ball of neutrons and a black hole is an object whose escape speed is equal to or exceeds the speed of light. High mass stars have the possibility to become black holes, because unlike the other types of stellar remnants, that have electron or neutron degeneracy pressure to keep them together, black holes have nothing to balance the crush of gravity.

What causes the death of a star and how does it become a white dwarf? A star begins dying when the fusion of hydrogen into helium in its core stops. This leads to the topic of the life cycle of low mass stars. The first is maintaining fusion of hydrogen into helium. The second is when hydrogen fuel runs out causing the core to contract from lack of fusion pressure, therefore leaving an inert helium core. From there, the helium core contracts and heats up, resulting in the helium fusing into carbon. Fusion pressure inflates the star into a red giant, causing the core to contract, however the star does not reach temperatures high enough to fuse past carbon. This stage is referred to as double-shell fusion and ends with a pulse that ejects the hydrogen and helium gas layers of the star into space, creating a planetary nebula. Once the outer layer has ejected, the remaining core of a dead star is composed mostly of carbon. The star is now a white dwarf and electron degeneracy pressures help counter the crush of gravity. This is what the Sun will become in about 5 billion years.

You’ve been listening to Western Slope Skies, produced by the Black Canyon Astronomical Society and KVNF Community Radio. This feature was written and recorded by Carolyn Aldrich, an astronomy student of Dr. Catherine Whiting at Colorado Mesa University.

Works Cited

“White Dwarf.” ESA/Hubble | ESA/Hubble, esahubble.org/wordbank/white-dwarf/. Accessed 10 May 2024.

Whiting, Dr. Catherine. “Life Cycles of Stars.” 2024.

Whiting, Dr. Catherine. “Stellar Remnants.” 2024.

Whiting, Dr. Catherine. “Sun.” 2024.