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In
our preceeding article, we told about the white dwarf stage that
is caused by rough electron pressure which comes out when the
Sun or a similar type of star gets collapsed. We have also told
that the sun would preserve this white dwarf stage.
Now, let us investigate the black dwarf stage which is the end
of the white dwarf stage and it is formed
by the collapsing stars who have outlived their lives and
who have a much bigger mass than the Sun.
White
dwarves turn their total fuel into iron element within
hundred or two
hundred million years.
And the stars, having these conditions, are not able to
burn the iron element they have anymore, because of insufficient
energy. They turn into black dwarves after fading away in the
cold and dark space. However, rough electron pressure does not
have an infinite power and
there is an upper
limit for balancing the matter weight.
This important limit named as
the chandrasekhar limit is 1,4 times the Sun mass.
Therefore, the mass of the white dwarves has to be less than
this limit.
Stars
with big masses burn hydrogen and helium at their centres just
like the ones with small masses when they come to red giant
stage. The difference is that the first group can start
thermonuclear reactions at high levels because of their big
masses. That is to say, the carbon and oxygen fuel in the
nucleus can’t be fired because stars with small masses have
low heat. However, the stars with big masses have extreme
pressure, therefore they can raise the
temperature in the nucleus to seven hundred million
degrees and carbon starts to burn. Later on,
when the temperature
rises to one billion degrees, oxygen is fired and the
reactions last till carbon and oxygen
in the nucleus finish totally. At that moment the
reactions stop for an instant and the nucleus starts to pucker
under the attraction effect. After a while, the heat gets so
high that, this time, the same reactions appear in a thin layer
around the centre.
Silicone
is the residue of the oxygen combustion and the oxygen reactions
leave this silicone as a thin layer while spreading outwards
from the centre of
the star. When
there is more pressure , it raises
the temperature in the nucleus to
billion degrees and
the silicone begins
to burn. The residue of the silicone combustion is the Iron
element. However,
the iron doesn’t go under the thermonuclear reactions
whatever the heat and the pressure in the centre is.
For this reason, towards the end of its life such a star
with a big mass has a nucleus rich in iron and silicone, oxygen,
carbon, helium in the layers around it which go outwards from
the center. At the most outer level the
thermonuclear reactions of hydrogen take place.
In such a star, the nuclei and the electrons of the iron atoms
are totally separated from each other in the centre.
Because, no atom can survive under such a heat and a pressure.
Therefore, the inside of the
star completely consists of iron nuclei swimming in a sea of
electrons.
In
the end, the dead
area in the centre is not able to carry the star’s pressure
anymore. And because of this extreme pressure, electrons are
pushed towards the nucleus of the iron atom, this causes each electron to join a proton and
they get transformed into a neutron and a neutrino. In other
words, opposite electrical charges, equal in numbers, destroy
each other, and transform the star to a big atom nucleus made of
single and neutral neutrons. Because
neutrons cover much more less volume than the protons and
the electrons from which they are made of,
the star collapses violently and this collapsing is so
sudden. Fired energy is equal to the total energy that the star
has spread from its birth until that moment. For billions of
years, slowly spreading energy, as the star’s light, fills
inside the star in a short time like a few hours. The star
breaks completely while shock waves extending from the collapsed
nucleus spread towards to the surface. This great explosion is
called Super Nova explosion. (Nova explosions also take place
between two separate star systems, one of them
at the red giant stage and the other one
at the white dwarf stage. The hydrogen gas, on the surface
of the red giant has
not taken place in the reaction.
Therefore, due to the force of attraction it gets
accumulated on the
surface of the white dwarf . The explosion occurs when
brightness increases by ten thousand times as a result of
the increasing heat and pressure by
that strong attraction.
This
explosion is the most magnificent and dramatic scenery in the
space. At this moment, the star can put the galaxy’s shine in
the shade by radiating millions times more than its normal
light. Right after the explosion, the inside centre, that is
left from the star, collapses towards to its inner side and is
not able to radiate anymore. Because the star has only neutron
nucleuses instead of hydrogen, helium and iron elements. In
other words, the star becomes an empty ball of neutrons and it
has an extremely intense
attraction field because of the over attraction of neutrons. But
this strong pressure is stopped by the neutrons according to the
Pauli Exclusion Principle ( two neutrons cannot be at the same
place in the quantum
state of ) just
like the electrons of white dwarves. Such stars at this stage
are called Neutron Star.
As
the star becomes smaller it rotates faster. Thus, the star,
whose diameter is about thirty to thirty five kilometres and
one spoon full of its
material weighs forty billion tons, makes one to ten rotations
in a second and spreads radio waves. Because, if a star, having
a weak magnetic field, collapses into small dimensions, its
magnetic field increases very much. The reason for this, the
area, which is spread over to millions and billions of square
kilometres before,
is squeezed in a very small surface after the collapsing. In the
end, neutron stars have a magnetic field which is more than
thousand billion times stronger than that of
the Sun. When
electrons in the star’s surface interact with the magnetic
field in the north and the poles, they accelerate and spread
radio waves. (Magnetic fields are not in the same direction with
the rotation axis, they take place with a certain angle and
behave like a rotating magnet.) Thus such neutron stars are
called Pulsars meaning that they are spreading periodic and
regular radio waves. And these are the stations on the road that
is going to the black hole stage.
A
star, whose mass is
forty to fifty
times greater than the mass of
the Sun’s, can’t repulse a major
part of its gases with the supernova explosion.
Therefore, at the
end of its life, it is transformed into a remnant star whose mass is two
and a half times bigger than the Sun’s.
As
this mass can’t be balanced by the rough electron and neutron
pressure therefore, the star material that is burned and keeps
getting smaller and lose its total existence at a single point.
( In the nature, there isn’t any force to balance the pressure
of the masses that are two and a half times bigger than the
Sun’s mass.) During
the collapse, the
attraction field around the star is so strong that it folds and
rips up the tissue of the space and the time on itself and
disappears in the Universe.
The
remainder is just a “Black Hole”.
To
be continued...
Istanbul-
November
02th 2000
http://afyuksel.com
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