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Nuclear Fusion & Star Formation

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  • 0:01 Nuclear Fusion
  • 0:47 A Balance of Forces
  • 2:33 How Nuclear Fusion Begins
  • 4:24 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will explain how the processes of stellar formation leads to nuclear fusion. We'll also define nuclear fusion and hydrostatic equilibrium, and find out why temperature is so important to all of this.

Nuclear Fusion

During the summer, I enjoy making a fire outside and making some s'mores. I burn logs to get the fire nice and hot. After finishing eating the s'mores and earning a couple more cavities in my teeth, the fire dies down and ash is left over.

An adult star, like our sun, also burns something to generate energy. But it's not wooden logs, of course. And this process also produces a kind of ash as a result. The star uses a process called nuclear fusion to generate energy. Nuclear fusion is a process that combines nuclei in order to release energy. In very simple terms, the stars burn hydrogen, and the ash that's left over is helium.

A Balance of Forces

Of course, the details of nuclear fusion are far more complex, and another lesson gets into the nitty-gritty of it all. However, what this lesson will focus on now is how nuclear fusion arises in the first place as a star develops.

When a star begins to form, it's called a protostar. This protostar forms from dense molecular clouds in space. As it forms, the molecular cloud condenses and contracts thanks to the force of gravity, pulling matter inwards into the forming protostar.

So, the force of gravity can be seen as your hand pushing down against a big spring resting on the table. But the protostar doesn't condense into a tiny little point because of gravity. This is because even though a spring contracts as you push down on it, it will push back up to resist the force of contraction. Eventually, a point will be reached where you can't compress the spring any further, but neither can the spring expand any further due to a balance of forces.

In a star, the balancing forces are gravitational contraction, the inward pull of gravity, and outward pressure from very hot gases. Those gases are very hot and that pressure is very high, precisely because of the gravitational contraction.

Again, it's just like our spring. Your hand mimics gravitational contraction as it pushes down onto the spring. Your hand forces the spring to push back on you, just like gravity forces the gases to become hot and pressurized, which results in a push-back effect on gravitational contraction.

The outward pressure of hot gases balancing the inward pull of gravity is called hydrostatic equilibrium.

How Nuclear Fusion Begins

Eventually, as the star forms, nuclear fusion begins and the star 'turns on,' so to speak.

But when in this process does nuclear fusion finally begin to generate the energy in a star, and why at that moment? The answer is: when the temperature reaches about 10 million Kelvin. This is about 18 million degrees Fahrenheit. OK, well, fair enough. But why must the temperature be this hot for nuclear fusion to begin?

This is because the ignition for nuclear fusion depends on the collision of hydrogen nuclei, or hydrogen protons. Protons have a positive charge.

In love, we say that opposites attract. Well, protons aren't opposites. They're all positively charged. And even though they have a bubbly positive demeanor, they don't want to be anywhere near each other.

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