In the planetarium’s new Winter Night Sky show, we wrap up with a close look at the red, supergiant star Betelgeuse (pronounced: Beetlejuice). More specifically, we wrap up with a close look at the inevitable supernova explosion of Betelgeuse. A supernova, the spectacular final act of a supergiant star, is one of the universe’s most energetic events, shining as brightly as an entire galaxy for a few weeks or months. And while they are not particularly rare, there hasn’t been one in our neck of the Milky Way for a while. No human has seen one with their own two eyes since the year 1604.
But we might be in luck! Astronomers tell us that a Betelgeuse supernova is imminent. Unfortunately, “imminent” in astronomical terms means that that we might see it in the sky tonight…or one hundred thousand years from now. Nevertheless, on the chance that you might be the first generation in centuries to witness a supernova, I want you to be prepared.
So, let’s get you up to speed on supergiants and supernovas, shall we?
Let’s start with a look at stars, to get a sense of just how super supergiants really are.
Stars are big balls of gas – mostly hydrogen and helium. And when I say big, I mean, really, really big. For context, Earth is about 8,000 miles in diameter. The Sun is a little more than 850,000 miles in diameter. Betelgeuse? Around 765 million miles in diameter. Or, to think of it another way, you could fit 1.3 million Earths inside our Sun. You could fit around 700 million Suns inside Betelgeuse.
Along with their great size, stars have great mass. Think of mass as more like a measure of weight than a measure of size. (That’s not quite right, but it’ll do for the sake of the point I’m trying to make here.) Earth weighs 13,170,000,000,000,000,000,000,000 pounds. So I don’t have to write all the zeros, let’s just say that the Sun is 333,000 times more massive than Earth. Betelgeuse is 5 million times more massive than Earth, and a good 15 times more massive than the Sun.
And with great mass comes great gravity; the more massive an object, the more gravitational force it has. We’re familiar with the idea that stars have the gravitational power to keep their planets, which are millions or billions of miles away, in orbit. While gravity can act at great distances, the closer you are to a star the more powerful its gravitational force. So, all the gas that’s actually part of a star is really feeling it, as it is being pulled, gravitationally and inexorably, toward the center of the star.
It’s hard to even describe how much pressure the outer layers of a star put on the gas sitting at the center of the star. But let’s try: Here on Earth, you are sitting at the bottom of an atmosphere that is being pulled down toward the center of the planet by gravity. The resulting air pressure is pushing down on you at about 15 pounds per square inch. What would the pressure of the gas pushing down on you be if you were at the center of the Sun? Just a little more than 4 billion pounds per square inch. And if you were at the center of Betelgeuse? Something like 60 billion pounds per square inch.
Physics tells us that when pressure compresses something, like the gas at the center of a star, it heats up. So, what kind of temperatures do you get when you compress gas at the center of our Sun to 4 billion pounds per square inch? About 27 million degrees Fahrenheit. At Betelgeuse’s center, temperatures reach 5.4 billion degrees Fahrenheit.
Okay, it took a while to get all of that out, but we must go to extremes to fully appreciate the ridiculous size, mass, and temperature that we’re dealing with when we’re talking about red supergiant stars like Betelgeuse.
So, what happens at the center of supergiant stars, where the pressures and temperatures are so extreme?
Simply put, nuclear fusion is the process of packing protons together into the nucleus of an atom.
And while that sounds simple, it’s anything but. Protons have a positive charge and, just like when you try to put the positive sides of two magnets together, they repel each other. In other words, protons actively resist being packed close to each other in the nuclei of atoms. It requires extreme pressures and temperatures to force protons together – pressures and temperatures found only at the center of stars.
Once the protons are forced close enough together, they are roped in and held in place by a force stronger than the electromagnetic repulsion that is trying to push them apart. (This strong force is called “the strong force,” because the physicists who discovered it couldn’t be bothered with coming up with a catchier name.) We call this locking together of protons by the strong force fusion.
One result of fusion is the release of energy – this is what makes stars shine! Another result of changing the number of protons in an atom’s nucleus is real-world alchemy; it turns one element into another, different element. Have a look at the Periodic Table of the Elements, and you’ll notice that each element has an Atomic Number. The Atomic Number is simply the number of protons in a particular element’s nucleus; the higher the number, the “heavier” the element. All the elements heavier than Hydrogen and Helium are created, by fusion, in the cores of stars
Little stars, like our Sun, are good at fusing Hydrogen into Helium and maybe squeezing out a little Carbon. To get the pressures and temperatures needed to fuse elements heavier than Carbon, you need stars at least 8 times more massive than our Sun. You need supergiants, and they are pretty rare – less than 1% of stars are supergiants.
Inside those big, beautiful, rare supergiants you’ll find newly fused elements nesting in onion-like layers. Carbon is fused into Nitrogen and Oxygen and Magnesium. Then the Oxygen is fused into Silicon and Phosphorous and Sulfur. Then the Silicon is fused into Iron and Nickel.
Then…it all stops. No star, not even a behemoth red supergiant, is massive enough to create the pressures and temperatures needed to fuse Iron.
The cessation of fusion in the core of a supergiant star has explosive consequences. Throughout their lives, stars pull off a delicate, carefully calibrated balancing act between gravity and energy. Gravity is doing its level best to collapse all the gas in the star into a much smaller, denser sphere. But the tremendous amount of energy created by fusion in the core pushes back against the gravity, holding it at bay. And when fusion shuts down, there’s nothing left to stop gravity from doing its thing.
The star collapses, but this is no ordinary collapse. The gas from Betelgeuse’s outer layers will fall toward the star’s center at 100 million miles per hour, rapidly* (*an understatement) pushing all the gas beneath it into the core. When the collapsing material hits a critical density, a powerful shockwave will bounce back out from the center, blowing the star apart – the supernova explosion. All the new elements created inside the star will be blown out into space, where gravity can pull it all back together again to form new stars, planets, moons, and maybe even life.
Betelgeuse is pretty far from us, about 600 light-years (or 36 quadrillion miles) away, so when it blows, we’ll be in no danger. And don’t worry, Betelgeuse’s supernova won’t be a “blink and you missed it” event: we’ll see something as big and bright as a full moon in our night (and day!) time sky for about a month.
But given the 100,000-year “imminent” window for Betelgeuse to pop, I don’t want to get your hopes up and then have you never see a supernova. If you want to play it safe, come see our new Winter Night Sky program in the planetarium, where you are guaranteed a front row seat to Betelgeuse’s demise, one of the greatest shows the Universe has to offer.