Giant Stars and Helium Fusion

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  • 0:03 Death of Our Sun
  • 0:30 Giant Star Creation
  • 2:32 Giant Star Characteristics
  • 3:19 Helium Fusion
  • 4:38 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will explain to you why a star nearing its end will become a giant star, and how the energy generating processes in its core will change as a result.

The Death of Our Sun

Our sun does not have an infinite supply of energy. One day, as with every living thing here on Earth, it will die. Just as a car, airplane, or your phone's battery has only so much gasoline or juice, a star like our sun only has so much hydrogen that can be converted into helium to generate energy. Once a star's tank is empty, it begins to change quite drastically. This lesson will explain how and why that is so.

Giant Star Creation

To understand exactly how a star changes once it exhausts its fuel supply, we must first understand how it lives its life. A main sequence star, an adult star, such as our sun, produces energy with nuclear fusion rather than gasoline. In this kind of reaction, the star combines hydrogen to make helium. The hydrogen that can be combined into helium is located inside the very hot core of the main sequence star. This core is like a star's gas tank, where the usable fuel is stored.

Furthermore, you can imagine the helium being generated from the nuclear fusion reactions as ash that's left over after burning a piece of wood, our hydrogen. As more and more hydrogen is burned through, more and more helium ash accumulates inside the star's core. Well, as you can only imagine, eventually the gasoline tank, the stellar core, will become filled with helium, will become a helium core, and will contain almost no hydrogen. This means, metaphorically speaking, the star has run out of gasoline. This means that in its core, it no longer has the ability to generate energy through nuclear fusion to oppose the inward force of gravity trying to contract the core.

Therefore, as gravity begins to contract the now powerless core, the core gets hotter and hotter as gravitational energy is converted into thermal energy. The core gets so hot, it's actually able to heat the previously cooler hydrogen existing outside the core where nuclear fusion reactions used to take place. This means hydrogen fusion now occurs in a hydrogen shell, a layer of hydrogen fusing outside of the helium core. This shell expands outward with time, leaving more and more helium ash in its wake and, therefore, increasing the mass of the helium core.

Giant Star Characteristics

As the hydrogen shell expands towards the surface of the star, it heats up the outer layers of the star. This causes the star to expand. Actually, the expansion is very significant. A star the size of our sun can become a giant. These stars have a radius that's 10 to 100 times larger than before. Main sequence stars that are already bigger than our sun can become supergiants; such stars are 1,000 times larger than our sun. As stars become giants or supergiants, their surface areas increase. This means the stars become more luminous. However, as stars expand, their densities decrease, and they become cooler.

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