The Types of Stars
The
main factor that determines the life, death, and eventual size of a star is the
amount of matter that it started off with.
Medium Sized Stars:
For the first few billions of years (talk about staying power), the new star
continues to shine as more hydrogen is changed into helium through nuclear
fusion within the star's core. Eventually, the star's original supply of
hydrogen begins to run out causing the core to be made up of helium. This
heavier helium core begins to shrink because of greater gravitational forces.
This results in the core heating up. This heat now begins to heat up the
surrounding hydrogen shell, causing it to expand in size. As this outer shell
expands, it begins to cool off and its color turns red. This star has now become
a Red Giant.
As this red giant ages, it continues to burn the hydrogen gas in its shell.
The temperature increases until it reaches 200 000 000 C. At this point, the
helium atoms within the core begin to fuse together to form carbon atoms. At the
same time, the last of the surrounding hydrogen gas begins to drift away to form
a ring around the central core of the original star. This ring is called the Planetary
Nebula.
Once the last of the helium atoms in the core is fused into carbon atoms, the
star begins to die. Without nuclear fusion taking place in its core, the star
begins to cool and fade. Finally, gravity causes the last of the star's matter
to collapse inward so tightly that it forms a White Dwarf.
The matter that is squeezed into a white dwarf is extremely dense. A single
teaspoon of this matter is several tons. The white dwarf will continue to shine
with a cooler white light until the last of its energy is eventually gone. It
then becomes a Dead Star.
The Super Stars:
These massive stars have masses that are at least six times that of our sun.
They usually start out their lives like that of a medium sized star and continue
their life cycle to that of a red giant. From there, their lives take on a
different path.
In a massive star, the gravity continues to pull together the carbon atoms in
the core and squeeze them so tightly that the temperature increases to above 600
000 000 C. The carbon atoms now begin to fuse together to form new elements such
as oxygen, and nitrogen. The core of the star becomes so hot that fusion
continues and forms the heavier element iron. After this point, the temperature
is not high enough to continue the fusion process from iron atoms to other
heavier elements. From this stage, this star enters the stage of a Super Nova.
Supernovas:
By the time the nuclear fusion in the massive star stops, the central core,
made up of mainly iron, begins to absorb energy rather than release it. At this
point, the star begins to break apart in a tremendous explosion known as a Supernova.
A supernova can light up the night sky for several weeks and appear as bright as
a million stars.
During the explosion, temperatures can reach as high as 100 000 000 000 C. At
this temperature, the iron is now able to fuse together to form new elements.
These new elements and resulting gases and dust now become part of space forming
the ingredients necessary for the formation of a new nebula.
One of the more famous supernovas was recorded by the Chinese astronomers in
1054. This supernova whose remains is now known as the Crab Nebula, lit up the
day sky for 23 days and could be seen at night for over 600 days.
Neutron Stars:
The fate of the core remains of a star that has undergone the stage of a
supernova depends again on the starting mass of the original star. A star that
began as 6 to 30 times the mass of our Sun will most likely end up as a Neutron
star after a supernova. A Neutron Star has about the same mass as our Sun but is
only about 16 km in diameter. This means that this star has become very dense. A
teaspoon of this star's matter would have a mass of about 100 million tons.
Neutron Stars spin very rapidly and gives off energy in the form of Radio
Waves. This Radio waves show up as pulses of energy. As a result, Neutron Stars
that have this ability are also called Pulsars. Astronomers have
discovered a pulsar in the center of the Crab Nebula that pulses at a rate of 30
times per second.
Black Holes:
A star that starts off with a mass of 30 times or more that of our Sun will
also undergo the stage of a Supernova, but will not form a Neutron Star. The
remaining parts of this star's core will become so unbelievably dense that it
will begin to swallow itself up because of its intense gravitational force. This
force is so great that not even light can escape it.
Black Holes have often been described as "Cosmic Vacuum Cleaners"
because they swallow any nearby matter of energy.
Black Holes can be detected by the X-Rays given off when matter falls into
them.
Quasars:
It is believed that the Universe is expanding. This being the case, one can
argue that the objects near the edge of the universe are probably the oldest
objects in the universe. The most distant known objects from Earth are about 12
billions light years away. Some of these objects are called Quasars.
Quasars are among the most studied, yet most mysterious objects in space.
They give off mainly radio waves and X-rays. The energy they give off is immense.
Some of them give off more energy than 100 or more galaxies combined, yet they
are found to be smaller than these galaxies.
Scientists believe that quasars are representative of the earliest stages of
the formation of a galaxy. When scientists are researching these objects, they
are probably looking, not only at the edge of the universe, but to the very
beginning of it.
Hertzsprung-Russell Diagram
Basics of the HR diagram
In a Hertzsprung-Russell diagram, each star is represented by a dot. One uses
data from lots of stars, so there are lots of dots. The position of each dot on
the diagram corresponds to the star's luminosity and its temperature.
