runs out of hydrogen; most of the mass of the star, in fact, is hydrogen! But fusion only occurs in the core, where the pressure and temperature are highest. In the outer layers of a star it is much cooler (tens of thousands of degrees as opposed to millions), so fusion cannot take place. This gas isn’t available to the core anyway, so it can’t be fused. It’s like having a gas can in the backseat of your car. It’s there, but it doesn’t do much good while you’re driving.
But in the core, eventually, the available hydrogen runs low. As the process of converting hydrogen into helium goes on, the helium nuclei build up in the very center of the star. Because helium has two protons, its nuclei resist coming together even more than hydrogen nuclei do, so it takes higher temperatures and pressures to fuse them. For stars with half the mass of the Sun or less, these conditions are never met. Eventually the star runs out of available fuel, and energy generation ceases.
But for more massive stars the helium “ash” can continue to build up. The core gets more massive, its own gravity crushes it more and more, and eventually the conditions for helium fusion are met. In a flash, helium nuclei smash together to form both carbon and oxygen nuclei. This process releases even more energy than hydrogen fusion, so the star becomes more luminous—it literally gets brighter. All the extra heat from the core is dumped into the surrounding envelope of hydrogen. This throws off the balance of pressure outward versus gravity inward, so the star responds as any gas does when heated: it expands. The star swells in size to epic proportions.
Ironically, though, the outer layers of the star cool off! While the total energy emitted by the surface of the star increases, the surface area increases even more. Each square inch of star emits less energy; it’s just that there are a whole lot more square inches than before. Even though the star gets more luminous, it cools off, becoming red. Because of its color and size, the star is called a red giant.
This is the eventual fate of the Sun. Eventually carbon and oxygen will build up in its core, and just as before, it takes more heat and pressure to fuse them than helium. The Sun doesn’t have what it takes to fuse carbon or oxygen, and the process ends there. 21
Just before a massive star explodes as a supernova, elements are piled up in its core like the layers of an onion. Iron sits at the very center, surrounded by shells of silicon, oxygen, neon, carbon, helium, and hydrogen. When a star gets to this stage, it doesn’t have very long to live.
AURORE SIMONNET AND THE SONOMA STATE UNIVERSITY EDUCATION AND PUBLIC OUTREACH GROUP
Stars with more than about twice the Sun’s mass do have what it takes to get to this third round of nuclear fusion. In their cores carbon can fuse into neon, releasing even more energy. But it takes even more massive stars to get neon to fuse into magnesium and oxygen, and more massive stars yet to get oxygen to fuse to silicon. 22
Silicon will fuse into iron, but it takes a vast amount of pressure and heat, and that can only come from stars with a mass more than twenty times that of the Sun. All of those steps get their turn in such a star, one after another. Each of the steps in the chain, though, takes less and less time, since the temperatures and therefore the fusion reaction rates increase hugely with each process. A 20-solar-mass star will fuse hydrogen for many millions of years, helium for one million years, carbon for a millennium, and neon for just one short year (those steps happen even more rapidly for stars with more mass).
Eventually, the massive star’s core is layered like an onion: hydrogen lies in a shell on the outside, surrounding a shell of helium, surrounding a shell of carbon, then neon, then oxygen, then silicon. Finally, at the deepest part of the core is a sphere of white-hot iron. To be sure, there is some mixing going on,
But in the core, eventually, the available hydrogen runs low. As the process of converting hydrogen into helium goes on, the helium nuclei build up in the very center of the star. Because helium has two protons, its nuclei resist coming together even more than hydrogen nuclei do, so it takes higher temperatures and pressures to fuse them. For stars with half the mass of the Sun or less, these conditions are never met. Eventually the star runs out of available fuel, and energy generation ceases.
But for more massive stars the helium “ash” can continue to build up. The core gets more massive, its own gravity crushes it more and more, and eventually the conditions for helium fusion are met. In a flash, helium nuclei smash together to form both carbon and oxygen nuclei. This process releases even more energy than hydrogen fusion, so the star becomes more luminous—it literally gets brighter. All the extra heat from the core is dumped into the surrounding envelope of hydrogen. This throws off the balance of pressure outward versus gravity inward, so the star responds as any gas does when heated: it expands. The star swells in size to epic proportions.
Ironically, though, the outer layers of the star cool off! While the total energy emitted by the surface of the star increases, the surface area increases even more. Each square inch of star emits less energy; it’s just that there are a whole lot more square inches than before. Even though the star gets more luminous, it cools off, becoming red. Because of its color and size, the star is called a red giant.
This is the eventual fate of the Sun. Eventually carbon and oxygen will build up in its core, and just as before, it takes more heat and pressure to fuse them than helium. The Sun doesn’t have what it takes to fuse carbon or oxygen, and the process ends there. 21
Just before a massive star explodes as a supernova, elements are piled up in its core like the layers of an onion. Iron sits at the very center, surrounded by shells of silicon, oxygen, neon, carbon, helium, and hydrogen. When a star gets to this stage, it doesn’t have very long to live.
AURORE SIMONNET AND THE SONOMA STATE UNIVERSITY EDUCATION AND PUBLIC OUTREACH GROUP
Stars with more than about twice the Sun’s mass do have what it takes to get to this third round of nuclear fusion. In their cores carbon can fuse into neon, releasing even more energy. But it takes even more massive stars to get neon to fuse into magnesium and oxygen, and more massive stars yet to get oxygen to fuse to silicon. 22
Silicon will fuse into iron, but it takes a vast amount of pressure and heat, and that can only come from stars with a mass more than twenty times that of the Sun. All of those steps get their turn in such a star, one after another. Each of the steps in the chain, though, takes less and less time, since the temperatures and therefore the fusion reaction rates increase hugely with each process. A 20-solar-mass star will fuse hydrogen for many millions of years, helium for one million years, carbon for a millennium, and neon for just one short year (those steps happen even more rapidly for stars with more mass).
Eventually, the massive star’s core is layered like an onion: hydrogen lies in a shell on the outside, surrounding a shell of helium, surrounding a shell of carbon, then neon, then oxygen, then silicon. Finally, at the deepest part of the core is a sphere of white-hot iron. To be sure, there is some mixing going on,
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