A star is a big ball of hot gas. But it's not the same all the way through. Instead, it forms a series of layers. How many layers, how thick they are, and what they're made of depends on the mass of the star and its stage in life.
One example is Antares, the brightest star of Scorpius, which keeps company with the Moon late tonight. They're in the southwest at first light, with Antares to the Moon's left or upper left.
Antares is far bigger and more massive than the Sun. Its great mass squeezes the star tightly, heating its core to hundreds of millions of degrees.
At those temperatures, the nuclear reactions that power the star happen at a staggering pace. Hydrogen atoms "fuse" together to make helium. When the hydrogen is used up, helium fuses to make carbon, and so on. Each of these elements eventually forms its own layer.
As a star like Antares reaches the end of its life, it consists of several distinct layers: hydrogen on the outside, with helium below that, then carbon and oxygen, and several more layers below that. A small iron core sits in the middle.
The iron atoms can't combine to form heavier elements, so the process stops. The core collapses to form a neutron star or black hole, and its outer layers are blasted into space -- seeding the universe with the raw ingredients for future stars and planets.
Antares probably isn't quite to that point yet. But it won't be long. It'll likely explode within the next million years.
Script by Damond Benningfield, Copyright 2009
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