24.3. The Hertzsprung-Russell Diagram http://www.ck12.org
Most of the stars occupy a region in the diagram along a line called the Main Sequence. During that stage, stars are
fusing hydrogen into helium in their cores. The position of the Sun in the main sequence is shown in the diagram.
You should note that the axial scales for this diagram are not linear. The vertical scale is logarithmic, each line is
100 times greater than the previous line. On the horizontal axis, as we move to the right, the temperature reduces by
between 1,000 and 10,000 degrees K between each line.
If all other factors were the same, the highest temperature stars would also be the most luminous (the brightest).
In the main sequence of stars, we see that as the temperature increases to the left, the luminosity also increases,
demonstrating that the hottest stars in this grouping are also the brightest. There are stars, however, that are less
bright than their temperature would predict. This group of stars is called white dwarfs. These stars are less bright
than expected because of their very small size. These dwarf stars are only one one-thousandth the size of stars in the
main sequence. There are also stars that are much brighter than their temperature would predict. This group of stars
are called red giants. They are brighter than their temperature would predict because they are much larger than stars
in the main sequence. These stars have expanded to several thousand times the size of stars in the main sequence.
Stars that are reddish in color are cooler than other stars while stars that are bluish in color are hotter than other stars.
A white dwarf is a stellar remnant that is very dense. A white dwarf’s mass is comparable to the Sun and its volume
is comparable to that of Earth. The very low brightness of a white dwarf comes from the emission of stored heat
energy.
White dwarfs are thought to be the final evolutionary state of any star whose mass is not great enough to become a
neutron star. Approximately 97% of the stars in our galaxy will become neutron stars. After the hydrogen–fusing
lifetime of a main-sequence star of low or medium mass ends, it will expand to a red giant which fuses helium to
carbon and oxygen in its core. If a red giant has insufficient mass to generate the core temperatures required to fuse
carbon, around 1 billion K, an inert mass of carbon and oxygen will build up at its center. After blowing off its outer
layers to form a planetary nebula, the core will be left behind to form the remnant white dwarf. White dwarfs are
composed of carbon and oxygen.
A white dwarf is very hot when it is formed, but since it has no source of energy (no further fusion reactions), it will
gradually radiate away its energy and cool down. Over a very long time, a white dwarf will cool to temperatures at
which it will no longer emit significant light, and it will become a coldblack dwarf.
A red giant star is a star with a mass like the Sun that is in the last phase of its life, when Hydrogen fusion reactions