Why Do Stars Explode?

ERIC: Stars burn brightly for millions or billions of years.

And then sometimes they blow up.

I'm your host, Eric, and today on Pulsar we're answering some of the most common questions we get asked at the Museum of Science about why stars explode and what happens when they do.

Joining me once again is Caity from our Charles Hayden Planetarium. Caity, welcome back to the podcast.

CAITY: Hi, everyone. Thanks, Eric.

ERIC: So we're talking about stars exploding today and I thought a good place to start would be why don't stars explode? Can you tell us about how a star can be stable while generating so much heat and light and energy?

CAITY: Stars are made of hydrogen and helium mostly. And in the core of a star there are really, really high pressures and temperatures which can cause hydrogen atoms, for example, to smash together.

And if they smash together, they can form a helium atom. The process is a little more difficult than that or a little more complex than that. But essentially, it's hydrogen fusing to form helium and also releasing energy at the same time. And that energy that comes from this fusion process is creating this outward force of pressure.

Stars over the course of their lives are in something called hydrostatic equilibrium. So that means that the reactions that are happening in their core are producing enough energy to keep the star puffed up and to kind of balance out the gravity from all of the material that the star is made of, wanting to kind of collapse it down.

ERIC: So gravity pulling in, the core generating energy and pushing out, those two immense forces in perfect balance to give a star it's nice, round shape.

CAITY: Exactly. Yeah, those two forces are kind of in a giant battle.

ERIC: Now we get a lot of people asking if the sun will explode. Thankfully, we get to tell them that it won't. It has a much more gentle end of the road.

CAITY: Yes. Exactly. So our sun is actually an average star, so it's not particularly huge or very small at all. So average stars don't have enough mass to explode at the end of their lives. And our sun still has another 4 and 1/2 to 5 billion years before it starts to run out of fuel anyway.

ERIC: So no explosion for the sun. What will happen to it billions of years from now?

CAITY: So our sun is going to expand for a while. It will turn into a red giant, a much larger star that takes up a lot more space. And it will be slightly cooler on the surface, and it will actually get so big that it will engulf the orbit of the Earth. So it'll expand and then eventually kind of shed its outer layers of gas and form a beautiful nebula.

ERIC: Well, it sounds like we've got a few billion years to figure out a new place to live because Earth won't be looking too good.

CAITY: Yeah. Definitely.

ERIC: Now, not all stars end this way and the most massive ones have a finale you can see clear across the universe. So give us the details on what causes a star to explode.

CAITY: So if a star is at least eight times the mass of our sun, it will collapse in on itself once it starts to run out of fuel. So once that fusion process gets to iron, when a star is trying to fuse iron there's not enough energy for it to do that, so gravity wins the battle in this case.

It can't actually stay puffed up anymore. So gravity causes the star to collapse in on itself and then it kind of rebounds in this really massive shockwave that we call a supernova.

ERIC: The entire core of a star collapsing from its own gravity, how long does that take?

CAITY: That's a great question, because there are a couple of different steps. So the actual collapsing of the star, specifically the core, is very, very quick. It happens in a fraction of a second.

This material is moving at a quarter of the speed of light, and then the actual shockwave or that rebound takes a couple of hours to get from the core to the surface of the star because there's just so much material in the way.

But that takes a couple of hours and then the material that just is expelled into space can move very quickly as well at a fraction of the speed of light.

ERIC: So that's got to be a ton of energy released all at once.

CAITY: That is one of my favorite things about exploding stars is that there's so much energy involved. They're some of the most violent and energetic events in the universe.

And this energy from the explosion can fuse iron and elements that are even heavier than that, so we get gold and silver all the way up to uranium. And what's really cool about that is that there's so many of those elements in our bodies that literally came from exploding stars.

ERIC: That's so cool that we are made of star explosion debris. We also get a lot of questions about what is left over after a star explodes. So what happens to the parts that aren't blown away?

CAITY: So the core of a star collapses when the rest of it is exploding outward. So that core will continue to collapse under its own gravity and it can form one of two objects. It can become something called a neutron star or it can form into a black hole.

ERIC: OK. Neutron star sounds like kind of ridiculously awesome, like it's from a bad science fiction movie. Where does it get its name from?

CAITY: So a neutron star is called a neutron star because of their density. So you have all of this material that is squished into the size of really a small city.

You have so much mass squished in there that protons and electrons that make up atoms actually end up squishing together because of how high the density is. And so you no longer have positively or negatively charged particles. You now just have neutrons. So it's just a giant ball of neutrons, essentially.

ERIC: So a huge amount of mass in a tiny, tiny space.

CAITY: Yeah, exactly. It would be like trying to squish two of the sun into the size of Boston.

ERIC: That's intense. The other outcome you mentioned is a black hole and we get so many questions about those that we'll have to spend an entire episode at least talking about them. But can you tell us what is a black hole and why do we call it that?

CAITY: Yeah. So black holes are even denser than neutron stars. A black hole happens when all of that material that makes up the core of an exploded star collapses into an infinitely small point. So you can think of taking 10 times the mass of our sun and squishing it into a period at the end of a sentence, but even smaller than that.

And so these objects, these black holes, have so much mass in such a small amount of space that their gravity is immense. If even light gets too close to a black hole, it can't escape.

In order to escape a black hole you'd need to be moving faster than the speed of light. And we don't know of anything that can do that. So there's no light that's actually being reflected out of a black hole. That's why we call them black.

ERIC: So even though their gravity is incredibly strong, they're not actually sucking things up.

CAITY: That's right. Yeah, they're not a giant space vacuums.

ERIC: We got a question on one of our Ask A Scientist live streams from a hero about the Crab Nebula. It's a really beautiful deep sky object and Mahiro wanted to know why does it look like an explosion?

CAITY: The Crab Nebula is one of my favorites because it's the remains of a supernova that humans observed about 1,000 years ago. During that time it was recorded that it looked like a new star in the sky, but when we look now with telescopes we see all of this material that is left over from an exploded star.

ERIC: To finish, I thought we could talk about the stars that we could see in our night sky. We often get asked if any of those could explode any time soon. And if they did, what would it look like from Earth and would it destroy us?

CAITY: That's a good question. And I get this one a lot, especially in the planetarium. But yeah, I mean there are a few stars that we can see with our eyes that are nearing the end of their lives and that are massive enough to go supernova.

A couple that come to mind our Betelgeuse in the constellation of Orion, or Antares in the constellation of Scorpius.

And both of these stars are red super giants that are nearing the end of their lives, but it's hard to tell exactly when they are going to go supernova. When astronomers say soon they generally mean within 100,000 years or so.

And so it's hard to predict exactly when that will happen, but they're at a distance where it won't really cause any alarm. It'll just look like a really bright star for maybe a couple of weeks. You'll be able to see it during the day, too. So it'll be very bright and then eventually it will fade out.

ERIC: Well, if it's just a great show without vaporizing the earth, then hopefully we get to see a good supernova within our lifetime.

CAITY: That would be the best.

ERIC: Well, Caity, thanks so much for telling us all about stars that explode.

CAITY: Yeah, thanks, Eric. Thanks so much for having me.

ERIC: For more on things in the night sky that could blow up at any time, tune in to our virtual planetarium shows at mos.org/mosathome.

Until next time, keep asking questions.

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