![]() Compared to the intimidating power of nuclear weaponry, the energy generated by dropping a few rocks doesn't seem very impressive. This is because we tend to underestimate gravity. At the height of the flash, the Sun's core will very briefly equal the combined luminosity of all the stars in the Milky Way galaxy! One might imagine that a conflagration of this magnitude would have a dramatic impact on the red giant – and it does, in a way, but not nearly so suddenly or violently as you might think. ![]() In roughly the time it takes to toast a bagel, the flash releases as much energy as our current Sun generates in 200 million years. (This corresponds to burning roughly ten Earth masses of helium per second, if you are keeping score.) For obvious reasons, astronomers call this the helium flash. About 6% of the electron-degenerate helium core, which by now weighs in at about 40% of a solar mass, is fused into carbon within a few minutes. In short, the center of the helium core explodes. If you add heat to a white dwarf, it just gets hotter.Īs it happens, the triple-alpha process is exceptionally highly temperature dependent: doubling the temperature of the reaction causes it to run roughly a trillion times faster! So, as the fusing helium heats the core, which cannot expand to cool down, the increased temperature causes the helium fusion to suddenly proceed millions of times faster, which very rapidly heats the core even more, which in turn causes the helium to fuse way, way faster. In other words, the self-regulating mechanism that keeps main-sequence stars so stable (hydrostatic equilibrium) is turned off in electron-degenerate matter. As I noted when I was discussing quantum mechanics, electron-degenerate matter behaves more like a liquid than a gas when you heat it: its temperature swiftly rises, but it doesn't expand. However, unlike when the Sun was young and its core contained normal matter, adding more heat to the electron-degenerate helium does not cause it to expand and cool. When the temperature in the core reaches about 100 million degrees, the helium will begin to fuse into carbon by a reaction known as the triple-alpha process, because it converts three helium nuclei into one carbon atom. And roughly 1.2 billion years after it leaves the main sequence, at the height of its glory as a red giant, the center of the helium core of the Sun will become sufficiently massive, dense, and hot that something amazing will happen: within a matter of minutes, it will ignite and burn. The temperature and pressure in the Sun's core will soar to 10 times their current values. It is an odd paradox: even as the outer layers of a red giant star are expanding into a huge but tenuous cloud, its inner core is contracting down to form a buried white dwarf. As the helium "ashes" continue to pile up at its center, a higher fraction of them turn electron-degenerate. The beginning of the end for a red giant the mass of our Sun occurs very suddenly.
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