This pressure as your muscles holding up

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.

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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…A star’s energy, by way of nuclear reactions, is produced thanks to the fact that it is so hot in its core.

These nuclear reactions make just enough energy to balance the inward pull of gravity I discussed before. I say ‘just enough’ for a reason.This is because if a star were to make too much energy, it would cause its layers to expand outwards.

This would cause the temperature and density in the core to drop. This, in turn, would cause our nuclear reactions to slow down back to equilibrium.Conversely, if our nuclear reactions were to start falling, meaning they would make too little energy, the star would contract a bit. This would increase the star’s central density and temperature, which in turn would pull the nuclear reactions back to a stable level.Such a concept, where a star has its own pressure-temperature regulator, is known as the pressure-temperature thermostat.

Relating Luminosity to This Lesson

Now, other lessons have pointed out to you the fact that the more massive the star, the more luminous it is.

Recall that luminosity refers to the total energy a star radiates in one second.A more massive star, like a red giant, is more luminous than a smaller object, like a white dwarf. But why?Well, you now have all the knowledge you need to answer this question.A gigantic star, like a gigantic pyramid, has a humongous amount of weight pressing down on its interior layers.

That’s not hard to understand. But how will we counter this weight based on everything you’ve learned so far? The core must be hot and the pressure must be high, that’s how. As a result, this means the star makes more energy to support its own weight.

Lesson Summary

You can imagine a star as a series of layers. The inward force of gravity is balanced out by the outward force of pressure to keep the star stable.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.The pressure of a gas pushing outwards and counteracting the weight from above depends on the gas’s temperature and density. This means that the inner layers are very dense and very hot in order to support all of the weight above them, when compared to outer layers needing to support relatively little weight.A star’s energy, from nuclear reactions, is produced in its core thanks to the high heat of the core itself. In turn, the energy produced by the nuclear reactions helps to balance the inward pull of gravity.

Learning Outcomes

After you have finished with this lesson, you’ll be able to:

  • Describe the relationship between pressure and gravity that helps keep a star stable
  • Define hydrostatic equilibrium
  • Explain how temperature and density affect a star’s stability
  • Summarize how a star’s energy is produced

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