From White Dwarfs to Supernovas: When Stars Collapse, Part 2

When stars die, they don't go quietly. They can shrink, expand, explode, and everything in between. How does a star know what to do when it's done? We're covering all that in the conclusion of our two-part series.

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Transcript

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Last week we explained black holes with a big assist from Canadian rock band Rush. Now, collapsing into a black hole is just one of the many things a star can do at the end of its life. A lot of pretty smart people figured out that you can predict how a star will die before it dies. On this week's show, that's what we're talking about. We choose to go to the moon. The Rem Tumtagga is a curious cat. Hundred billion other galaxies. Right now, don't like Welcome to Five Minute Trivia, where we're making the world smarter five minutes at a time. I'm your host, R. M. Zuberi. In our last show, we went on a journey that started from an idea in a Swiss patent office to the Cygnus constellation, where the first black hole was discovered. But not every star collapses into a black hole when it dies. Some stars get bigger. Our own Sun is one of those. So who is calling the shots on this? Well, the stars themselves. Einstein's special relativity gave us the equation E equals MC squared, probably the most famous equation ever that nobody really understands. E stands for energy, m for mass, and C is the symbol for the speed of light. Among other things, that equation explains how stars generate all that heat. The tremendous gravity and pressure inside stars squeezes every two hydrogen atoms together into one helium. Now one helium atom is less massive than two hydrogens, so the remaining mass, that's the M, is converted and expelled as energy, which is the E. That explosive outward energy balances out the star's internal gravity and keeps it from collapsing inwards. When stars run out of hydrogen fuel, they can't carry out those fusion reactions and stop exploding. So their own internal gravity takes over and forces them to collapse. Or not. So for stars, like many things in life, size matters. And stars come in different sizes. In our last show, we mentioned a physicist named Subramanian Chandrasekhar. He worked out which stars do what when they die based on their mass. Let's start with the sun. Our sun is a yellow dwarf. Nobody calls me yellow. Yeah, sorry, son, it's uh nothing personal. Actually, it's a good thing because yellow dwarf stars are ideal for having life-sustaining planets around them. They burn bright, but not too bright. They're hot, but not too hot. When stars like our sun exhaust all their hydrogen, they can't collapse into black holes like Cygnus X1. They're just too small. Who are you calling small? When the Sun exhausts its supply of hydrogen, it starts to collapse and its helium atoms get smushed together into carbon. Now, massive stars with lots of internal gravity will keep collapsing that carbon into heavier and heavier elements all the way into iron. But for lighter stars like the sun, carbon is as far as it can go. It isn't massive enough to form heavier elements, but there's enough fusion energy from that collapse to heat and expand the outer layers. As they expand, they cool and look more red. That giant red star is called a red giant star. In becoming a red giant, the sun expands so much that it will eventually engulf Mercury and Venus and uh uh you know Earth. But that's not going to happen for a while, so if you're listening to this, you're safe. So the outer layers of the sun expand while the core is collapsing. It's a bit like a skinny person wearing a fat suit. Who are you calling fat? You, Sun, we're calling you fat. The sun will be a red giant for another billion years or so until one last burst of fusion energy expels all those outer layers and becomes a shimmering, beautiful nebula, which would be fascinating to look at if everyone who could look at it wasn't dead. What's left of the core becomes a white dwarf star, which is about the size of Earth. You know, the planet that got eaten, the closest orbiting planet would now be Mars. Now, massive stars with much more gravity don't stop at carbon. When they collapse, they can squeeze their hydrogen and helium into heavier and heavier elements. When they do that, it releases fusion energy like it did before until the core gets compressed into iron. That's where all this fusion ends, and there's suddenly no more outward energy. The core then collapses all at once, at speeds up to a quarter the speed of light, and then bounces off that iron core into a massive explosion called a supernova. What happens next depends on how massive the star's core is. Subermanian worked out that a stellar core between 1.4 and 3 times the size of our Sun will compress their leftover subatomic particles, protons and electrons, into neutrons. The entire leftover star just becomes an incredibly dense ball of neutrons that packs more mass than the sun into a space that's about the size of a small city. They're called neutron stars. They have immense gravity and density, like one teaspoon weighs about 10 million tons. Stars with cores more massive than three times the sun will turn into black holes. Now, we already covered black holes last week, but we did just want to clear up a bit of misinformation provided by the grunge band Soundgarden. Black hole, Sun, won't you come? Wash away the rain. Once again, our sun will not become a black hole. We really don't want it to come anytime soon, and there would be no rain to wash away because everything would be solid ice. That's our show this week. Join us next time, and thanks for listening.