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Date: Prior to 1993


Question:
I do not know how a red giant star comes along.



Replies:
The "Red Giant" phase is part of the "lifetime" of average stars like the Sun -- kind of like a star's "late middle age". All stars begin as being made up almost entirely of hydrogen. A star begins its "life" by converting its hydrogen into helium. Because the heat and pressure of a "young" star is greatest at its center, the conversion of hydrogen to helium is fastest in the deep interior. Eventually, all of the hydrogen in the star's core is converted to helium and nuclear reactions stop there, while outside the core the reactions continue. At this stage,there is no heat being generated in the core; because of this, the core starts to contract. The contraction supplies energy to the core, which then gets hotter than ever. This heat is conveyed to the outer layers, which speeds up the hydrogen-to- helium reactions in the outer layers. Thiscauses the outer layers to get hotter too, and to expand enormously. But the expansion causes the temperature of the outermost layers to drop, and they become "only" red-hot (just as white-hot metal cools to red-hot). This will happen to our Sun (several billion years from now); it will expand past Mercury's orbit, perhaps as far as Earth's orbit. Our oceans will boil away, and everything on Earth will be burned to a crisp. I know of three easy-to-see red giants that are visible in the northern hemisphere: Antares in the constellation Scorpio, Betelgeuse in the constellation Orion, and Aldebaran in the constellation Taurus. If you can view them in a clear, dark sky away from city lights, you will see that they truly look reddish -- a sort of rusty red, like Mars.

RC Winther


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