Stellar Evolution
Stellar evolution refers to the life-cycle of a star. Ancient sky-watchers believed the heavens were perfect and unchanging, but in fact the stars are constantly undergoing change. This change is only noticeable over the course of a great deal of time. Since it is impossible to observe the entire life-cycle of a given star, astronomers have search the heavens for centuries for clues to understand what happens to stars as they age.
French astronomer Pierre Laplace devised a theory in 1796 to explain how the Sun and planets originated. He spoke of a great rotating nebula, or cloud of gas. The central portion condensed into the Sun, while the outer rings clumped together and became the planets. German philosopher Immanuel Kant (1724-1804) had devised a similar theory in 1755.
In 1938, German-American physicist Hans Bethe worked out the details of what powers the stars. He proposed that the hydrogen in stars was being converted into helium. Four atoms of hydrogen would fuse together to create one atom of helium plus a burst of energy.
At the same time Bethe was working on his theory, German astronomer Carl von Weizäcker (1912-) was developing an identical concept of stellar energy. He also enlarged on the Kant-Laplace theory of a rotating nebula by suggesting that it did not rotate as a single system, but had numerous currents at work within it. If planets could form around the Sun, they should be able to form around many other stars.
Russian-American physicist George Gamow took Bethe's work one step farther. Gamow calculated that as a star's hydrogen is used up, the star gets hotter. This went against the idea current thought that stars cooled off as they grew older.
Current understanding of stellar evolution begins with a large cloud of gas condensing into a star (or binary star or star cluster, depending on the size of the gas cloud). At some point the object gets dense enough, and the core gets hot enough, to begin fusing hydrogen into helium, and a star is born. Most stars last for billions of years, but eventually they use up their supply of hydrogen.
How a star dies depends on its mass. A star like the Sun expands to become a red giant. The core grows hot enough to start fusing the helium it created into carbon. When the last reserves of helium are exhausted, the star's core collpases and its outer layers are ejected. The expanding outer layers form a planetary nebula (which got its name from its resemblance to the disk of a planet seen through a telescope), while the collapsed core becomes a white dwarf star.
A more massive star continues the fusion process, fusing carbon into oxygen, then, then magnesium, and finally iron. No element heavier than iron can be synthesized by fusion and produce energy for the star to maintain itself against its own gravity, so the core of the star at this point collapses catastrophically and explodes as a supernova. The various elements the star synthesized are blasted into space. The collapsed cores of these massive stars become neutron stars or black holes.
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