Red Giant Stars - Introduction
Since a Red Giant is the dying stage of a star, there is not a lifecycle descrption
here. These objects are huge compared to our Sun, with diameters from 100,000,000
to 1,000,000,000 km. They have cool surface temperatures in the range of 2500
- 3500K, and thus are red or orange in color. Stars like Antares in Scorpio,
Betelgeuse in Orion (image above and right), Arcturus in Bootes (image above
and left), and Capella in Auriga are great examples of Red Giants that are eaaily
visible to the naked eye of an astronomer in the northern hemisphere. While
they may be cool in temperature, they are so large that their total luminosity
is very great. Thus, they are so readily visible even from great distances.
The internal structure of these objects varies. The simple Red Giant stage of
a star like our Sun that would have a core of helium burning into carbon, a
shell of hydrogen burning into helium, and an outer layer of various gases of
which hydrogen is dominant. The more complicated Red Giant stage of a very high
mass star takes on a structure like an onion. The core is made of layers of
different stages of nuclear fusion of elements on the Periodic Table from Hydrogen
to Iron. These objects live extremely short lives in comparison to other stars,
and some even appear to throb in space as the outer envelope is so distant from
the center of gravity that it is hard for the star to hold itself together.
Starting Mass - 1 - 60 solar masses
General Length of Life - stars spend a few thousand to 1 billion years as
Red Giants, depending on whether the mass is very high or relatively small.
General Lifecycle - The Red Giant stage is the dying stage of Main Sequence
stars. Below are the final stages of 2 Red Giant stars. The image below and
left depicts the interior of a Sun-class Dwarf that has evolved into a Red Giant
and is now in the AGB stage. Below and left is a High-mass star that has well
over 10-20 solar masses. Note the different reactions inside the cores of these
During the latter parts of the Red Giant stage of a star, the
star begins to throb and pulsate. The helium-burning shell collapses into the
core when its contents are fused into carbon. There is a brief shut-down of
one form of nuclear fusion and the star shrinks slightly. Then a new shell of
helium ignites and blows the star outward. This shrinking and expanding is called
the Asymptotic Giant Branch lifestage of a star, and during this time, the star
sheds much of its outer material into space in huge rings of gas and dust. The
of the AGB stage might blow out several hundred-thousandths of a solar mass
each year. The number seems plenty small, but in terms of star lifetimes, this
is a rapid riddance of stellar matter. The expanding shells create spectacular
planetary nebulae (clouds that look like
"planets" in a telescope). Three planetary nebulae are seen below.
Above are two images of a planetary nebula in Monoceros, taken
less than one year apart. In the center of the gas cloud is a Red Giant star
that is blowing its outer layers into space.
Final Life Stage - these objects either shed so much mass that their cores
die off as White Dwarfs, or else there
is so much mass to begin with that they will go supernova.
To learn more about these relatively mundane objects, go to the Red
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