Red Giant, Horizontal & Asymptotic Giant Branch Stars

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  • 0:03 A Red Giant
  • 0:35 The Red-Giant Branch
  • 2:21 Horizontal Branch Stars
  • 3:26 Asymptotic Giant Branch Stars
  • 4:38 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will outline how stars of low to intermediate mass die as seen on the H-R diagram. We'll cover important key terms such as red giant branch stars, asymptotic branch stars, and horizontal branch stars.

A Red Giant

Perhaps you've come across the concept that our sun will one day become a red giant. A red giant is a very luminous, expanded star of a red color. It's simply a later stage of a star's evolutionary life cycle, one that will come about in our own sun when it runs out of hydrogen fuel in its core, its gas tank.

What you may not know is that sun-like stars will evolve into more than just a red giant with time. In fact, our sun will become two different kinds of red giant stars.

The Red-Giant Branch

A main sequence star, an adult star lying on the main sequence of the H-R diagram, generates energy by converting hydrogen to helium in its core. Eventually, as with any non-renewable fuel, the star will run out of hydrogen and will begin to sputter and die. How exactly a star will die will depend on its mass. This lesson will explore how low to moderate mass stars like our sun die.

As such a star begins to die after running out of hydrogen fuel, it will first swell up to become a red giant. This means its luminosity, the total energy a star radiates every second, will increase while its surface temperature will decrease. Thus, such a post-main-sequence star will move up and right along something called a red-giant branch (RGB) on the H-R diagram.

After the hydrogen core is used up, it becomes a helium core. Helium can also be used for fuel. When the helium core of the star contracts, it heats up, and this allows for energy-producing helium fusion to begin.

When helium fusion begins, the core will expand, and this will cool down the core. The cooling of the helium core will also cool down the hydrogen-burning shell surrounding the helium core. This will cause the hydrogen shell to put out less energy every second. This means the luminosity of the star will decrease for a little bit after helium fusion begins in the core.

Horizontal Branch Stars

Since less energy is being released by the star, its outer layers will contract as well. When they contract, they increase in temperature, and thus the star's surface temperature increases once again.

H-R Diagram
HR diagram

This means that on our H-R diagram, the star will move to the left on the H-R diagram as it increases in temperature. But at the same time, the luminosity of the star remains basically the same at this point in time.

This means the star moves to the left horizontally on the H-R diagram on something appropriately called the horizontal branch. Thus, such a star is known as a horizontal branch star, a star that is increasing in temperature but not changing its luminosity.

Such stars have cores composed of helium where helium is fused to produce energy. This core is in turn surrounded by a hydrogen-burning shell, just like we discussed before.

Asymptotic Giant Branch Stars

Eventually, even the helium core will run out of fuel, and it will be converted into a dead end for stars of this size, a carbon core. This inert carbon core cannot be used for energy production, and the star reaches its last stages of life.

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