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Next
to these facts it was also discovered that Sirius B had another
important trick up its sleeve: it was the first star of which the light
showed a gravitational red-shift, making a nice piece of evidence to
support Einstein's theory of relativity. Einstein had predicted that
photons (light particles) that meet a strong gravitational pull will
lose energy. Thus, the light's wavelength stretches so that their color
will shift toward the red spectrum. Until that moment (in 1924) it had
been very difficult to detect red-shifted light in low-mass stars such
as our sun. You're probably now saying, "Light particles and light
waves! Which is it!"? We will discuss this effect of light shifting
toward the red again when the black hole is being explained.
A big star dies
Contrary
to what you might think, a larger
star burns out more quickly than a small star like our sun. The moment
all of a star's fuel is consumed, the big star will shed most of its
mass into space - much like our own sun will do, but then with an
incredible force, a stellar explosion which astronomers call a
supernova. There are more spectacular explosions, called hypernovae,
but scientists are still in doubt as to their cause. What happens
before the bang of a supernova?
We are stardust
Massive
stars burn up hydrogen, which is
converted to helium. They do that at tremendous rates: a star, 25 times
the mass of our sun will live its life a thousand (!!) times faster. It
will also burn a 100,000 times brighter. Because a massive star has
more mass, gravity will build up pressure and temperature around the
core, which will help to fuse the fuel into elements of increasing
atomic weight. There are many of these processes going on in a star,
and depending on the distance from the core, we will see different
layers.
At the stars
surface we would see hydrogen being fused to helium,
somewhat deeper there would be a layer where helium was fused into
carbon and oxygen, carbon would be fused into neon and magnesium and so
on. At the stars deepest point, where it is really hot (8 billion
degrees Kelvin), iron is created by fusing silicon. The creation of
this iron core takes place in about a week.
Once the iron
core is formed it is no longer possible to produce more
energy just by compressing it to start a new fusion reaction. Gravity
is indifferent to this and will go on compressing the core, raising
temperatures to about 10 billion degrees Kelvin.
At this
temperature the photons split the iron nuclei into protons and
neutrons. They don't do that quietly: in a tenth of a second a 12,000
km iron core collapses into a neutron star of about 20 km in diameter.
The outer layers of the star are suddenly without support, and they now
collapse and bounce on the dense, incompressible neutron core,
resulting in the instant release of a huge amount of gravitational
potential energy. Boom!!
As you see,
during its lifetime and especially toward the end the sun
is the creator of all elements we find on earth and in ourselves. Truly
we are stardust, the remains of a dead star, which once burned brightly
in the heavens.
Neutron Star
A star that
exceeds 1.4 solar masses, and is limited to 3 solar masses,
after its supernova will collapse further than a white dwarf into a
very dense star called a neutron star.
A neutron star is
nothing more than an incredibly dense core made of
just neutrons. Its mass is packed in a volume roughly 10^14 times
smaller than our sun and has a mass density around 10^14 times higher
than the sun; it is so dense that a teaspoon would weigh 100 million
tons. A neutron star less than 3 solar masses will not contract
any further, because the neutrons will resist the inward push of
gravity, just like the white dwarf's electrons do.
This is now
called neutron degeneracy pressure. When the neutron
star's mass far exceeds 3 solar masses (no-one exactly knows the
precise critical point) there is a good chance that the process of
inward gravity exceeds that of the neutrons' resistance. The core of
the neutron star collapses further and then there's no more stopping
the ongoing process, the star infinitely collapsing; a black hole is
formed.
Black Hole: the making of:
What
exactly IS a black hole? A black hole is
a region in space-time that has a gravitational field so strong that
the escape velocity is faster than the speed of light.
This means
nothing can escape its clutches, not even light. When
the core of a massive neutron star collapses, the inward gravity
prevailing over the neutron degeneracy pressure, the process will go on
and on, until we reach a point in which all matter of the star if being
compressed into a point of infinite density.
The tale of the black hole has the following
chapters:
-A
singularity
-The
Schwarzschild radius
-The event
horizon
-The apparent
horizon
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