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Forces Keeping Stars in a Stable Equilibrium

Forces Keeping Stars in a Stable Equilibrium
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  • 0:03 Remaining Stable and Balanced
  • 0:41 Hydrostatic Equilibrium
  • 3:05 Pressure, Density, Temperature
  • 4:44 Nuclear Reactions
  • 5:52 Relating Luminosity to…
  • 6:52 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will outline for you the forces and properties that keep stars stable. They include temperature, pressure, gravity, density, and nuclear reactions.

Remaining Stable & Balanced

A person who is well-balanced can be someone who is truly good at balancing on something like a tightrope over a canyon. Or that terminology could mean they are emotionally and mentally stable. Or perhaps it points to someone who is good at a lot of different things. In any of those cases, the person is at a sort of equilibrium, where the balance keeps them steady and centered.

Stars must also find a balance in their lives in order to maintain a sort of equilibrium. This lesson will explain how stars find this balance and inner peace at their core.

Hydrostatic Equilibrium

You might be thinking that my use of the term 'inner peace' is just a cute metaphor but actually, the inner core of a star plays a big role in this equilibrium.

A star's stability is a balance of forces and properties, including gravity, density, temperature, and pressure.

For this lesson's purpose we can, somewhat oddly, compare a star to an ancient stone pyramid. I know you think I'm crazy, but it'll help you understand this lesson much better.

An Egyptian pyramid is made up of layers upon layers of stones. The stone all the way at the very tip of the pyramid does not have to support more than the weight of air pressing down upon it, which is nothing really. As we move downwards from the tip of the pyramid to the base, each successive layer has to support the weight coming from the layer above it.

While a star doesn't have concrete layers like a pyramid, the same exact principle applies nonetheless, and it's a good conceptual tool for this lesson.

A star's deeper, inner layer must support the weight of the layers above pressing inwards as a result of the force of gravity. In order to maintain stability, a deeper layer must counteract this with gas pressure pushing upwards and outwards - gas pressure because a star's insides are made up of gas! But you knew that already.

Anyways, you can think of the gas pressure as your muscles holding up a bench-press weight above your chest, a weight that's trying to push down onto your chest to compress the chest and make it difficult for you to breathe.

This stable balance, the outward pressure of hot gases balancing the inward pull of gravity, is called the hydrostatic equilibrium. The word hydrostatic comes from hydro-, meaning water and implying a fluid such as a gas, and -static, implying stability. All in all, what the term hydrostatic equilibrium encompasses is the fact that the fluids in a star, its gases, are not expanding and not contracting when it is stable.

Pressure, Density, Temperature

The pressure of a gas in such an equilibrium depends on the gas's temperature and density.

At the surface layers of a star, there's very little weight pressing inwards. Again, just imagine our pyramid from before. There aren't a lot of stones pressing down on the top layers. For a star, this means that the gas pressure counteracting the little weight from above doesn't need to be very high in order to achieve stability.

As we go ever deeper, the pressure of the gas has to be higher and higher in order to maintain stability. This, by extension, means that the temperature and density of a gas in that layer has to be higher as well.

This means that for a star to be stable, a star's inner core must have a high temperature, density, and pressure to support its own weight.

This is easy to remember. Let's think of another pyramid to illustrate why. Let's look at a human pyramid.

Human pyramid
human pyramid

The bottom layer of people has to support the entire weight of a pyramid. There are more people standing at the bottom layer, meaning they are crowded together, i.e. denser. The pressure is much higher on them to maintain the massive weight above them when compared to outer layers of the human pyramid. And because they are working extremely hard to contract their muscles to lift everyone above them, they start to sweat as they become hotter and hotter from all that exercise.

Nuclear Reactions

Moving on…

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