Video Transcript
Which of the following is the correct description of a black dwarf star? a) A black dwarf star is an object that has been left after a supernova in which all of the matter has been compressed into a single point. The gravitational field is so strong that not even light can escape. b) A black dwarf star is an extremely dense object that is made up only of neutrons. c) A black dwarf star is a type of star that only emits gamma radiation and does not emit any visible light. d) A black dwarf star is a white dwarf star that has cooled so that it no longer emits any light. e) A black dwarf star is a type of star that is not massive enough to trigger a nuclear fusion in its core. And so the only light it emits comes from its heat.
Before we start answering this question, let’s summarize all five of the answer options on this one screen. We’ve already got options d and e in front of us. So let’s recall what a, b, and c said. Option a said that black dwarf stars are stars that are compressed to a single point so that light can’t escape. Option b said that these stars are extremely dense. And they’re made only of neutrons. And option c said that black dwarf stars emit gamma radiation but no visible light.
To figure out which of these five answer options is correct, we can consider a little bit about how black dwarf stars form. We know that, in a star, for example, the Sun at the center of our solar system, fusion in the core of the star, that is, its center, makes light and heat. This fusion process creates pressure from the core, pointing outward. This pressure generated from the core helps to balance out the force of gravity, which is pulling all the mass within the star towards its center. So we could think of it as a balance of forces. The force of gravity pulling in and then pressure, created by the fusion going on in the core, pushing out. Over time, what can happen to a star is that it runs out of fuel to run the fusion process on. When this happens, the pressure from its core goes away. And there’s no longer a balance of the forces acting on the star.
Now, under the influence of gravity, the star begins to contract. It begins to collapse in on itself. So the star gets smaller and smaller. Now it’s not losing mass. But it’s just having that same amount of mass compressed into a smaller volume. All this compression could make the star heat up to very high temperatures. A star like this, a contracted version of a larger star that is run out of fuel, is called a white dwarf star. This kind of star is called white because it’s hot enough that it gives off radiation at all kinds of wavelengths, including all visible wavelengths. And it’s called a dwarf because originally the star was much, much larger. But under the influence of gravity has compressed down to a significantly smaller size.
To get a sense for this, imagine a star as big as the Sun being compressed down into the size of the Earth. Even though in the sketch, we see the sun being much, much bigger than the Earth. Even this sketch is not to scale. The Sun is actually bigger compared to the Earth than this shows. So if we took all that volume of the Sun and compressed it down into something even smaller than this dot. That provides a sense for why it is that this smaller version becomes so hot and is yet so small.
Now remember that once a star has become a white dwarf, there’s no fuel in the star for fusion. That is, once the star has become compressed down to its smallest size, there’s nothing in the star that would generate heat. So over a long amount of time, and we mean really long on the order of billions of years, this star cools off. As it releases its heat, the color of the star changes. In time, the star cools down enough so that it’s no longer radiating any kind of light to its surroundings. Since no light is coming from it, it looks black. Black is the absence of any light. It’s at that point that our star becomes what’s called a black dwarf star.
Now that we have a sense for the process of what a black dwarf star is and how it’s created. We can look through our answer options again and see which one matches that description. If we go alphabetically for our options, starting with option a. This says that a black dwarf star is compressed to a single point. And that light can’t escape. Well, it’s not true that a black dwarf star is compressed to a single point. It still have some nonzero volume. And it’s not that light can’t escape from the star. It’s just that the star doesn’t radiate any light because of its cold temperature. Option a actually sounds more like a black whole than like a black dwarf star. So we won’t choose that as our answer.
Option b says a black dwarf star is extremely dense and that it’s made only of neutrons. Well, it is true that a black dwarf star is quite dense. Like we said, it’s roughly equivalent to a mass, the size of our sun, being compressed into a volume, the size of the Earth. But this second part, that it’s made up only of neutrons, isn’t accurate. In fact, it doesn’t seem that any astronomical bodies, even neutron stars, are actually made up only of neutrons. So option b isn’t our choice either. Now option c, which says that black dwarf stars emit gamma radiation but no visible light. For a star to be hot enough to be able to emit high energy gamma rays, they would also have to be hot enough to be able to emit visible light. But a black dwarf star, we’ve seen, is so cold that it doesn’t emit any visible light. And that means it doesn’t emit any higher energy light, such as X-rays or gamma rays. Therefore, option c isn’t our choice either.
Option d says that a black dwarf star is a white dwarf star that has cooled so that it no longer emits any light. This description agrees with our understanding of how a black dwarf star is formed and what it is. Just to make sure that this is the best answer, let’s look at option e. This says that a black dwarf star is not massive enough to trigger nuclear fusion in its core. So the only light it emits comes from its heat. But as we’ve seen, black dwarf stars are actually very massive. But that’s not the thing that limits fusion from happening in its core. It’s actually the fact that the star has run out of fuel for fusion. And when option e says that the only light the star emits comes from its heat. That’s more descriptive of a white dwarf star, which, as it cools down, gives off light, which comes from its heat.
So option d then is indeed our best answer. A black dwarf star is a white dwarf star that has cooled so that it no longer emits any light.
