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Lesson Video: Stellar Evolution Physics

In this lesson, we will learn how to describe how stars of different masses develop over the course of billions of years.


Video Transcript

In this video, our topic is stellar evolution. We’ll be talking about the entire life cycle of a star from beginning to end. Over this time period, and it typically is a very long time period, stars go through significant changes. As we get started, let’s consider the star that’s more familiar to us than any other. And that is our sun. We’re all familiar with the sun because of the light and the heat that it sends our way. But where did the sun come from? And where is it going? To answer these questions we’ll need to know about stellar evolution.

Now, as far as any human being is concerned, the sun has been a constant in our world. And yet astronomers tell us that many years ago, the sun as we know it didn’t exist. What did exist were lots of particles of dust and lots of clouds of gas, which altogether is called a nebula. In a nebula, masses very spread out. It’s not concentrated enough to form a planet or a moon or start to form a star. But over time, thanks to the force of gravity, that can change.

We know that any object with mass, and that includes the dust and the gas in this nebula, will attract any other object with mass. So, thanks to this attractional force, the force of gravity, between the masses in a nebula, it’s possible for concentrated high-density spots to begin to develop. Over time, we can begin to recognize a portion of the nebula which is collapsing in on itself due to this gravitational force, all the mass is getting closer and closer together. As it does, it gathers into a roughly spherical shape.

The mass that’s collected in this shape is referred to as a protostar. This means it’s not yet a fully functioning star, but we expect it will become one. In order for that to happen, all of this mass just needs to continue falling in on itself and creating a more and more dense core. Now, the nebula, the clouds of gas and dust that we started out with, is made up mostly of hydrogen and helium, two of the simplest elements in the universe. As these elements are combined in a protostar, and they get closer and closer together, the pressure and temperature of the elements begins to rise.

This fact is important because for a protostar to become an actual star, there needs to be an energy producing reaction going on in the core of the star called nuclear fusion. Fusion occurs when smaller lighter atomic nuclei such, as hydrogen for example, fuse, or join together, to form larger nuclei. In the process, large amounts of energy are released. And it’s the release of this energy as light and heat that makes a star a star, as compared to, say, a planet, which doesn’t produce its own light. So, it’s necessary for the energy producing reaction of fusion to be going on at the center of this mass in order for the protostar to move into what’s called a main sequence star.

Once this stage of a star’s life has been achieved, that means the star is no longer collapsing in on itself. All that happened during the protostar phase in order to get the process of fusion running. But now, we could say that the engine of the star is running. Fusion is going on in its core, hydrogen joining with hydrogen to produce helium. The star is producing its own light and heat. And all the energy released from the fusion, going on in the core of the star, creates an outward pushing pressure which helps balance out the gravitational force pulling the parts of the star in on itself.

When a star is in its main sequence phase, which can last for billions of years, the outward pushing pressure from the core and the inward pushing pressure thanks to the force of gravity balance one another out. Now, our sun, the star at the center of our solar system, is a main sequence star. Fusion is taking place in the core of the sun that produces the light and the heat we all experience. And, as we said, this can go on for billions of years, so long as the star has enough fuel to run the engine.

Just like the engine of a car though, eventually a star will run out of fuel. For a star, this happens in stages. When all the hydrogen in the core of a star is used up, that particular fusion process comes to a halt. When this occurs, when a star is no longer fusing hydrogen in its core, two things take place. The first thing is that the core, or the center portion of the star, contracts down. But then, the outer portions of the star do the opposite; they expand outward. When this takes place, overall the star begins to cool down.

As it does this, with its outer layers expanding, it tends to take on a red shade. This is to reflect the fact that the star is no longer as hot as it used to be. It’s cooling. Since fusion is no longer happening in the core of the star, we can no longer call it a main sequence star. Instead, at this phase of a star’s life, it’s referred to as a red giant. This name is due to the fact that the star turns this color, and it also gets bigger compared to its main sequence phase.

And then, over a long time, again, many, many years, the very dense core of this red giant continues to compress in on itself, while the outer layers, the expanding cooler parts, separate off completely. Once the outer layers are gone, and all that’s left is the core, the star has moved to another phase where it’s called a white dwarf. This is then the hottest part of what remains of the red giant. So, its color is white. But it’s much smaller than the star was before, so it’s called a dwarf.

Even though the star is still hot enough to be white, there’s no heat generation process going on in the star. This means that the natural path for the star to follow at this phase is to cool down over many, many years. As this happens, the color of the star changes to reflect its relatively cooler temperatures. And eventually it’s been predicted that when the star cools enough, it will move into a new phase known as being a black dwarf. The transition from going to a white dwarf to a black dwarf is expected to take so long that it’s longer than the age of the universe.

Perhaps for that reason, no black dwarf stars have been experimentally observed. We predict that they will exist, but the universe just hasn’t been running long enough for us to find any. So, this is the overall progression that we expect our sun to follow. And as we mentioned, the phase it’s currently in is the main sequence phase which can last, and is expected to last, billions of years more.

But it turns out that this progression isn’t followed by every single star. Rather, these are the steps followed by a star whose mass is relatively small. And, believe it or not, on astronomical scales, the mass of our sun is relatively small. So, what happens to stars that are bigger? What happens when much more mass starts to coalesce together in a nebula? Well, in that case, a much larger protostar forms. And then, when the star moves to its main sequence phase, it’s also much larger in mass. To reflect this fact, sometimes a star this large when it’s in the main sequence phase is referred to as a massive star.

Just like any star though, no matter how big it is, eventually this star will run out of fuel to run the fusion process. And when it does, it too will start to cool down. But it doesn’t form what’s called a red giant, but instead, thanks to its size, forms what is known as a red supergiant. Despite the different name though, the same physical processes as going on. The core of the star’s collapsing in on itself, and the outer layers are expanding.

Now, we’ve seen that, for smaller size stars, the next phase would be what’s called a white dwarf. But for a red supergiant, something different goes on. When the mass of a star is about five times the mass of our sun or greater, then the compression of the core of the star that goes on under the influence of the force of gravity creates such massive crushing pressure that it sends a shockwave out to the outer layers. The effect of this shockwave is a gigantic massive explosion. It’s called a supernova, which simply means that the star is blowing up.

Supernova explosions are the most powerful kind that happen anywhere in the universe. And we might think that after an explosion like this, there’s absolutely nothing left of the star. But in fact, there is something left behind. That’s the incredibly dense core that led to the explosion in the first place. Now, it’s at this point that one of two things can happen, and it depends again on the mass of our star. If the star has between five and 10 times the mass of our sun, then that core will collapse into what’s known as a neutron star.

This is a star made largely but not entirely out of neutrons. And since it no longer generates heat itself through the process of fusion, this star cools down over time. In that way, a neutron star and a white dwarf star are similar. But the formation of a neutron star isn’t the only possible outcome of a supernova. For stars that are about 10 times or more massive than our sun, when they experience a supernova, the core collapses in on itself so far that it effectively disappears. It turns into what has been called a black hole.

A black hole is a very special astronomical object. To get a sense for how black holes work, let’s think about how gravity operates on a large scale. Let’s say that over here, completely separated from our black hole, we have a large massive object. And then, let’s say that another massive object begins to move past this first one. We know that, thanks to the force of gravity between these two masses, the mass in motion will be pulled towards the larger one. That pull might not be strong enough to change the path of this object very much, or it’s possible that it could be. And the smaller moving object will be inescapably attracted to the larger one.

Okay, let’s consider another case. This time our smaller object is moving in the same direction, but at a much greater speed. Because of that increased speed, it will be comparatively harder for our large massive object to pull the smaller one in. Basically, the smaller object, thanks to its higher speed, has a greater chance of escaping the gravitational pull of the large object. But of course, one way to make it less likely for the smaller objects to escape is to increase the mass of our already large object. In that case, the pull of gravity on the small moving object will be even stronger.

As a side note, if this object on the right was a black hole, it would actually be very small compared to its mass. Black holes are extremely dense. But anyway, let’s think now about a limiting case. This object on the left can’t be moving faster than the speed of light. That’s the fastest anything can be moving. But what if even moving at a very, very high speed, the larger object was able to pull it in? In that case, we would say that the gravitational force of attraction towards this large object is so strong that not only can massive objects not escape its pull, but even something moving at the speed of light can’t escape it either. That also is pulled in.

Now, what moves at the speed of light is light. And so, if light itself is pulled in by a gravitational attraction, then that means that no light can escape that mass. And if no light can escape something, what would that something look like? Well, it would look black. There’s no light coming from it. And this is the idea of a black hole. Once objects get close enough to it, no matter how massive and no matter how fast they’re moving, they won’t escape the gravitational pull of this hole. They all get sucked in and nothing escapes.

Interestingly, this makes it very hard to actually see or detect a black hole. Since nothing escapes from it, we can’t see anything coming directly from the black hole. In spite of this limitation though, we still do have experimental evidence of the existence of black holes. We can tell they’re there by the way that matter behaves nearby them. That being said, not many black holes seem to be in existence, which, considering how powerful they are, may be a good thing.

Having talked about black dwarfs, neutron stars, and black holes, we’ve reached the end of the stellar life cycle. As far as we know, all stars begin as nebula, and then they end up in one of these three states. Now that we have a sense for how stars progress over time, let’s get a bit of practice through an example exercise.

What is the most common element in the universe?

When we talk about elements, we’re speaking of the basic building blocks of matter. All the different varieties of elements are listed out on the periodic table. There are over 100 of them. And this question asks, which element is most common in the universe? To help us answer this question, we can think about how stars form in the universe.

Stars begin as clouds of dust and gas begin to coalesce together under the influence of the force of gravity. These clouds, one of which is called a nebula, are made mostly of the elements of hydrogen and helium. Hydrogen is the simplest element of all because it only has one proton in its nucleus. And helium is the next most simple with two protons in its nucleus. These two elements, and in particular hydrogen, are the fuel for the nuclear process of fusion that helps stars to produce light and heat.

And this fuel, not coincidentally, is the most common element there is. The element that’s most commonly used in the core of stars to produce energy, and one which makes up a large part of nebulae, is hydrogen. This is both the simplest and the most common element in the universe.

Let’s summarize now what we’ve learned about stellar evolution. Starting off, we saw that all stars, including our sun, go through a life cycle and that particular cycle depends on how massive they are. All stars begin as nebula, which are clouds of dust and gas. Then, as gravity pulls the dust and gas together into more condensed masses, a protostar can begin to form. Depending on several factors, a protostar can be larger or smaller, less or more massive.

A smaller protostar eventually becomes what’s called a main sequence star. This is a star where fusion is taking place in the core and the star is producing its own light and heat. The sun in our solar system is a main sequence star. But for larger masses, fusion does start in the core, but the star is sometimes called a massive star. This typically signifies a star whose mass is at least five times greater than our sun.

Once a main sequence star runs out of fuel, it becomes what’s called a red giant, while a massive star transitions into what’s called a red supergiant. As the outer layers of a red giant disperse, its remaining inner core is called a white dwarf. A red supergiant, on the other hand, is massive enough that as its core compresses, this leads to a shockwave, creating an explosion of the outer layers of the star. This explosion is known as a supernova.

A supernova can lead to one of two outcomes, again, depending on the mass of the star. For a relatively smaller red supergiant, a supernova will create a neutron star. This is a small very dense star made largely of neutrons, where fusion is not ongoing. The destiny of a neutron star is to slowly cool down over time. On the other hand, the supernova of a red supergiant that is big enough creates what’s known as a black hole.

Black hole is the term we use to describe a mass which is so high density and so compressed that the force of gravity it exerts on surrounding objects is strong enough that it pulls in light as well as mass. These two possible end states for a star, becoming a black hole or a neutron star, apply to stars that are larger, while stars that are smaller and become white dwarfs are thought to eventually turn into what’s called a black dwarf.

So, then, all stars start out as nebula, and they end up as either a neutron star, a black hole, or a black dwarf. And finally, the sun in our solar system is a main sequence star, a star with fusion occurring in its core and predicted to have billions of years to go.

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