The key feature in the H-R diagram is the band of stars running from top left to bottom right (high luminosity-high temperature to low luminosity-low temperature) known as the main sequence. About 90% of all nearby stars lie on the main sequence. The Sun is a typical main sequence star. Once a star is formed and starts burning hydrogen in its core to produce helium it appears on the main sequence at a location dependent upon its luminosity and surface temperature. Typically, a star will remain on the main sequence until it has converted about 20% of its hydrogen into helium. It then undergoes a dramatic internal restructuring. The pull of gravity in the stellar core is balanced by the heat of the nuclear fusion causing the star to remain in a stable state.
Once a significant amount of the hydrogen is converted to helium (about 20%) the nuclear processes slow down and the the core of the star can no longer compete against its own gravitational pull and so begins to contract. To counterbalance this, the outer layers of the star start to expand and cool and the star turns into what is known as a red giant. The change in color of the star signifies the effects of the cooling as the star expands, while the large increase in size of the star increases its luminosity. The star therefore moves to a different part of the H-R diagram known as the giant branch. An examination of the H-R diagram shows that a significant population of these stars exists in our galaxy.
When the Sun has
burned up its hydrogen fuel it will leave the main sequence to become
one of these red giant stars with its outer shell expanding to engulf
the planet mercury.
The core continues to collapse under its own weight
until the density
and temperature are high enough that the nuclear fusion of helium
begins. Helium atoms will be cooked in the Sun's internal furnace to
create carbon. The pressure generated in this process will
temporarily halt the gravitational collapse of the core, and the star
will shine as a red gaint star with a luminosity some 1500 times that
of the Sun at present and with a diameter some 50 times bigger than
its current value.
In burning helium to form carbon the stellar core
becomes unstable and starts to pulsate. The pulsations generated
travel through the red giant star causing the outer layers to blow
off into interstellar space forming what is called a planetary
nebula. Essentially all that remains of what was once the Sun is the
hot helium burning core. For a star like the Sun, the core is as
dense as it can be and gravity no longer can cause it to contract
further. After about 75,000 years the core becomes what is
called a white dwarf star, composed mostly of carbon. Once the
remaining helium is used up the white dwarf cools to a black dwarf, a
dark remnant of what was once a brilliant Sun.