How Lower-Main-Sequence Stars Die

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  • 0:01 How Lower-Mass Stars Die
  • 0:43 The Red Dwarfs
  • 3:39 The Medium-Mass Stars
  • 5:26 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
Not every star lives or dies the same way. Red dwarfs and medium-sized stars have key differences that determine their lifespan and how they die. This lesson will tell you why that is the case.

How Lower-Mass Stars Die

Although this may not be a great thing to think about for long, we all know that one day we will cease to exist. Each one of us will have a different reason for no longer being a part of this universe.

Stars, like humans, are born and then one day they die. And exactly like humans, the way stars die varies from one to another, mainly based on structural differences. Some stars die in spectacular explosions and others with barely a whisper. In this lesson, we'll take a look at how lower-main-sequence stars die, namely the medium-mass stars and very low-mass stars, like the red dwarfs.

The Red Dwarfs

Red dwarfs...cool, low mass, and faint stars on the main sequence, die differently from massive stars. Red dwarfs have masses between 0.08 and 0.4 solar masses. As a result, unlike massive stars, they don't have much weight to support, and they do not have to burn through their hydrogen fuel very quickly. They are what a hybrid car is to a Hummer. They're light and very efficient at using their source of energy. This means their hydrogen fuel can take them very far timewise.

But here's another cool thing. So you know that a Hummer burns through its gasoline in its tank very quickly. The tank is like the hydrogen fuel located at the core, or center of a star. A hybrid car not only uses the gasoline tank but also a battery and energy generated from breaking in order to run for much longer. Meaning, it uses energy more uniformly.

Well, red dwarfs use the process of convection to live longer. Convection is a process that involves circulation in a fluid, one where cool fluid sinks and hot fluid rises. In very simple terms, convection is like taking a spoon to a really hot pot of soup cooking on the stove and constantly mixing the soup from the bottom up to distribute the heat a little bit better and more uniformly throughout the pot.

Therefore, you can think of convection as a process that mixes ingredients, like gas in a star, uniformly. In turn, this means that convection allows the red dwarf to use hydrogen fuel throughout the star for energy, not just at its center. This means red dwarfs are not limited to a gas-tank-like core at the star's center, like more massive stars are, for energy.

But this mixing, this convection, also disables a red dwarf's ability to have a helium core that is surrounded by a hydrogen shell. A helium core arises in more massive stars when the hydrogen in their core is converted into helium. This helium core then helps to ignite the hydrogen (the hydrogen shell) surrounding the helium core for energy. This, in turn, is what causes a star to expand into a giant.

In the end, no red dwarf has actually met its end. Our universe is only about 13.7 billion years old. But due to everything you've just learned, a red dwarf can live for over 100 billion years. By extension, this means that no red dwarf ever born has ever died of old age - yet. Talk about good genes!

The Medium-Mass Stars

Unlike red dwarfs, medium-mass stars with less than four solar masses undergo a different process. Perhaps you've already watched the lesson on how giant and supergiant stars form from the main sequence stars. There, you should've learned that when the hydrogen fuel runs out, it becomes a helium core. The helium core only ignites once it becomes hot enough. This helium is then converted into carbon fuel. But stars with less than four solar masses can't ignite the carbon because they can't get hot enough to do so.

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