Ask A Scientist© Astronomy Archive

Question:  I don't know how a red giant star comes along. Could you please mail
the answer to mrabkin@eis.calstate.edu
smarty

Answer:  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 hot-
ter than ever. This heat is conveyed to the outer layers, which speeds up
the hydrogen-to-helium reactions in the outer layers. This causes 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.
Rcwinther