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.
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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.
