If you’ve been reading along, and you’ve finished your PB and J, you will remember that when we last left our helium it was inside a star where it had just been born. But this newborn helium isn’t the last stop – far from it. Today we look at what happens when the helium’s hydrogen parent starts to run low.
When that occurs, the star starts to shrink. Why? Polyester.
No, that’s not it. When the hydrogen runs low, fewer gamma rays come flying out of fewer nuclear reactions. It’s the pressure created by those gamma rays that keeps the Sun from collapsing in on itself. Think how amazing that is! Imagine shining a flashlight at your chest. Just how much does the light push you? Not much, but essentially the same thing happening inside the Sun keeps the whole thing from imploding every moment.
OK, so now we have a shrinking star. As the star shrinks and compresses, the temperature inside goes up, just as a bicycle pump gets hot as it compresses air. This hotter helium starts to move faster and faster, and as it does so it slams into other helium nuclei. But nature has another trick in store.
Helium-4 contains two protons and two neutrons. Two helium-4 nuclei slammed together will give four protons and four neutrons. Try to find this nucleus on the periodic table. Go ahead, I’ll wait.
OK, you didn’t find it. That’s because any element with four protons is beryllium. Add in the four neutrons and you’ve got beryllium-8. But there’s no such thing as beryllium-8, because it falls apart as fast as it can come together. If we had to depend on helium changing into beryllium to keep stars burning, we’d soon have a very dark universe.
Since the universe isn’t dark, you’d probably guess there’s more to the story. And you’d be right. Because helium has another path, one that skips over the beryllium sinkhole. If three helium atoms come together at almost exactly the same time, they can stick. First two come together, and immediately start to move apart again. But before they can separate, a third helium nucleus comes barreling in, and a magical thing happens. Instead of three helium nuclei, we suddenly have something new, something that was never there before. With six protons and six neutrons, the new thing is called carbon-12.
If you’d like an example of carbon-12 in action, close your eyes. OK, now you can’t read, but when you open them again I’ll tell you that the insides of your eyelids are made of carbon-12. So are the outsides, but they’re a lot harder to see unless you’re really gross. Your arms, your legs, your fingers, toes, and belly button (whether you’re an innie or an outie) are all made of carbon-12. And every bit of it came from helium, cooked up inside a star experiencing a hydrogen shortage.
And yet there’s one more piece of the puzzle, a thought that takes my breath away. This process, called triple alpha (we’ll come back to that “alpha” word in a later blog entry), shouldn’t work very well at all. It should be almost impossible for three helium nuclei to hit at just the right time to form a carbon-12 nucleus. But it turns out that the nuclei don’t have to hit exactly, because of a hidden secret shared by helium and carbon. There is a radioactive form of carbon-12, called a resonance. The energy of that resonance of carbon-12 turns out to be almost exactly the same as the energy of the three helium nuclei that go into making it. And this resonance means that it’s much, much more likely that carbon-12 can form inside a star.
In fact, a scientist named Fred Hoyle predicted that the resonance of carbon-12 had to exist. How did he know? Because he knew that he was made of carb0n! The carbon had to come from the insides of stars, and the only way stars could make enough carbon to make fingers and legs and eyelids and belly buttons and brains that could think of the triple alpha process was if there was a resonance at carbon-12 with just the right energy. Scientists looked for the resonance, and there it was.
Quickly the radioactive carbon=12 fires off a gamma ray and settles down to ordinary carbon-12, and the star can live again. For a while.
Later on, when things get really bad for our helium-burning star, it will either blow off its outer layers (making what’s called a planetary nebula, here’s one now:)
or else the star will explode in a supernova, spilling its insides out all over the place. Even later, the carbon from either kind of star can drift about, settle into a collapsing cloud, and end up on a young planet, where billions of years later it might form the inside of your eyelids. All because helium got hot enough for a threesome! (sorry, couldn’t help myself)