As we will soon see, not all stars are fortunate enough to die a peaceful
death as a white dwarf. We all know about supernovae which is known
to be caused when a star dies. Most supernovae don't occur within
stars that become white dwarves but rather on larger stars which, after
the supernova, leave behind something even smaller than a white dwarf,
a neutron star.
Large stars about 20 times the mass of our sun, have a hotter and larger
core and as a result, is capable of carrying on where smaller stars have
failed, the fusion of carbon nuclei. The temperature inside the core
reaches about 600 million K ( about the same temperature in Celsius as
well since -273 degrees Celsius is 0 K ) and the carbon nuclei move so
fast that even their like positive charges cannot hold them apart and they
crash into each other fusing to form neon.
The reaction releases even more energy and the neon, at 1billion K, combines
with helium to form magnesium.
At 1.5 billion K, the oxygen
which was another bi-product of the helium and hydrogen reactions before
it, also starts to 'burn' producing more energy from the reaction and even
even heavier nuclei: sulphur,
silicon and phosphorus.
At 3 billion K the silicon begins to fuse with other nuclei and form a
variety of other elements. This process continues with more and more
nuclei until the core produces iron.
The core is now composed solely of iron nuclei which cannot be broken further
by fusion and so the star expands to create heat from its outer layers,
which are still composed of the lighter nuclei, now that the iron cannot
be fused. The star now becomes a red
super giant. This 'new' star is so huge
that if one were to be placed in the middle of our solar system, it would
engulf all the surrounding planets up to Pluto.
Since the star's core is now incapable of using the iron to convert its
energy of mass into energy
of motion, the star's gravitation begins to contract the core.
The core, being colder than usual since the iron can no longer be fused,
is almost powerless to this force. It becomes denser and the electrons
around it become degenerate.
While all this happens, even more iron is produced on the outer layers
and joins the core making it even denser. The mass of the core exceeds
the Chandrasekhar limit and the density
of the core reaches a billion g/cm³. At this density the matter
in the core collapses and in a tenth of a second, the temperature soars
to 5 billion K. The photons produced by the burning of the matter
flood out with so much energy that they explode the iron nuclei reducing
them to helium nuclei. This process is called photodisintegration.
Photodisintegration is the opposite of the fusion reaction which forms
heavier nuclei. The photodisintegration process breaks up nuclei
and absorbs energy. The energy created by the photons brings even
higher temperatures that break up even the helium into protons, neutrons,
and electrons. At such a temperature the electrons are moving so
fast that they combine with the protons to form neutrons in a burst of
neutrinos.
A neutrino is an particle which can go long distances without being stopped
or deviated from its course. Some neutrinos however are still absorbed
by the star whereas the rest escape at the speed of light. Neutrons
can be packed so they practically tuch each other making the star extremely
dense. The remaining core is much like one massive neutron having
eliminated all the empty space from it since it is packed so tightly.
The non-neutronised surface of the star is also affected by the gravity
and flies toward the core at speeds of 40,000 km/s. But when it tries
to join the core, it is blocked by the large wall of neutrons that the
core is made of, sending the non-neutronised particles rebounding as a
shock wave.
The shock wave reaches the surface
of the star after a couple of days and carries with it such an enormous
amount of energy that it literally blows away so much of the star that
if the star were to weigh 25 planetary masses before it was affected by
the shock wave, it would be left with
1 solar mass worth of matter. This is what we call a supernova.
All the elements - including iron, silicon, neon, and magnesium - created
by the star over its lifetime is carried to distant galaxies by the shock
wave.
We have now examined the fates of stars whose solar mass is greater than
1.4. All neutron stars have a solar mass ranging from 1.4 to 3.
Therefore, the formation of black holes must be caused by cores which have
a solar mass greater than 3. On to 'Black
Holes.'
