The formation of a star and its changes processes
Birth Stage of Stars
Star is formed by interstellar materials. These
materials are mixtures of gas and dust. About 96 to
99% of interstellar gas is made of hydrogen and helium
and the rest are carbon, oxygen and nitrogen. Dust
particles bearing an average diameter of 10-³ cm are
made from ice, frozen ammonia, graphite and silicates.
After a while, interstellar materials are contracted.
These are called “Protostar”. By
continuation of contraction, the central temperature
of Protostar increases up to the commencement of
nuclear reactions’ point. In these conditions,
Protostar converts to a real star. This star is
called “Main-sequence star”.
Main-Sequence Stage
This stage commenced with nuclear reactions of
conversion of hydrogen to helium. It continues till
the end of this conversion. As this reaction is
associated with mass reduction, energy is produced and
this energy acts as fossil fuel. At present, the Sun
is at this stage and remains in this condition for
about 5 billion years.
Giant Stage
When central hydrogen converted to helium, hydrogen
nuclear reaction commences in outer parts of star. As
in the star’s core, the temperature is low,
nuclear reactions of helium burning will not start.
Therefore, the star’s core shrinks. Parts of
released energy warms core and other parts radiate
outwards from the star’s surface. As a
consequence star expands. On the other hand, when
central heat reaches 100 million degrees, the nuclear
reaction of helium to carbon conversion commences in
star’s core. At this stage, star changes to a
“Red Giant”. The Sun has a radius of
70million kilometers and luminosity of 1000 times more
than its current luminosity at giant stage.
Supergiant Stage
Helium is run out in the star’s core. Then,
carbon core shrinks. As a result of this shrinkage,
outer layers inflate and star converts into a
supergiant.
White Dwarf Stage
When a low-mass star exhausts the nuclear fuel in its
core, White Dwarf Stage begins. It is the last stage
of star’s life. White Dwarfs are small stars and
have low mass, and their radii is about 0.02 of the
Sun radius and their mass is equal to the Sun mass.
With regard to the size and mass of these stars, it is
realized that their medium density is very high and
about 106 gr/cm3. The reason for this high density of
White Dwarfs is as follows;
After running out of nuclear energy sources, star
shrinks until gas pressure becomes equal to
gravitational force.
Black Dwarf Stage
After stopping the shrinkage in a star, it gradually
becomes cooler. Finally, Back Dwarf appears. Then,
it will not be visible in the sky moving forward in
the galaxy.
