A star and an onion might not seem to have much in common. But as the heaviest stars near the ends of their lives, they’re put together a lot like an onion.
All stars begin their lives by “fusing” together atoms of hydrogen in their cores to make helium. When they’ve used up all the hydrogen they can, gravity squeezes their cores more tightly, making them hotter, so they fuse the helium to make carbon and oxygen.
For stars like the Sun, that’s as far as it goes — they can’t get hot enough to make heavier elements. But stars that are at least 8 to 10 times the mass of the Sun can.
Gravity squeezes them much more tightly, allowing the carbon to fuse to make neon and other elements, which takes a few thousand years. It then takes just a few years for the neon to make oxygen, and a few months for the oxygen to make silicon. Each of these steps leaves a layer of the earlier elements, creating the onion-like structure.
The final step takes just a few days. The silicon fuses to make iron. The iron can’t fuse to make heavier elements, so the reactions stop. With no energy to resist the pull of gravity, the core collapses, creating a shock wave that blasts away the onion-like layers around the dead core as a supernova.
One star that will undergo that fate is Antares, which is to the lower right of the Moon as night falls. It’s probably fusing helium in its core right now, so the clock is ticking toward its demise — sometime in the next million years.
Script by Damond Benningfield, Copyright 2012
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