Blackholes are exotic objects
that exist in Nature. Astronomers have found and measured the masses of many of
them in our Galaxy and beyond. A blackhole is essentially and literarily a hole
in space. Blackholes come in 3 broad classes: 1) Planck-size blackholes with a
mass of 21 milligrams, and these may form in the Large Hadron Collider (LHC),
2) stellar-mass blackholes with masses of a few times the mass our Sun, and 3)
supermassive blackholes with masses greater than millions of times the mass of
the Sun. We are going to discuss here the birth of stellar-mass blackholes. To
do that we will need to understand something about the evolution of stars.
All stars, our Sun included, generate their light through
nuclear reactions in their cores. The nuclear reactions fuse 4 hydrogen atoms
to create a single helium atom. The process creates high-energy radiation. This
radiation is transformed to optical light as it traverses the body a star. The
radiation also provides supports star against collapsing on itself due to its
own gravity. One thing to keep in mind here is that nuclear reactions in the
core of a star keeping a star from imploding on itself. The nuclear reactions
proceed in the core as long as there is hydrogen. As the star ages, its
hydrogen supply in the core dries up. The core contracts to a point where it
starts fusing helium to make carbon. The cycle continues with the fusion of
further elements, and if the star is massive enough (greater than 3 times the mass
of the Sun) the star keeps fusing elements until it reaches a core of pure
iron. Iron is different than the other elements in that the energy one gets by
fusing iron is less than the energy one needs to put to make the iron atoms
fuse. Therefore, nuclear reactions involving iron do not generate energy. The
star is then left without any power source to support it against collapse.
The core and the rest of the
star, the envelope, will collapse leading to a rapid increase in the density of
the iron core. However, the increase in the density cannot go on forever. There
will come a time when the density of the iron core becomes comparable to that
of atomic nuclei. At this stage, the core cannot contract any longer. The
envelope, however, keeps falling and hits the now stationary core like a
speeding truck hitting a wall of rocks. The result is one of the biggest
explosions in the Universe, a supernova. The explosion may result in a bare
dense core made up of neutrons known as a neutron star. However, if the
leftover core is massive enough so that not even the highest densities can stop
its collapse then it will keep on collapsing to form a stellar-mass blackhole
with a mass greater than double that of the Sun. Basically, the core will tear
a hole in space.
The birth of a stellar-mass
blackhole is one of the most fantastic events in the Universe. It involves
nuclear reactions that dwarf all nuclear bombs and reactors here on Earth. It
also involves the most spectacular of explosions of all: a supernova explosion
in which the equivalent of the light output of an entire galaxy is given off by
a single star. The supernova explosion scatters the created elements into space
enriching clouds that later collapse and form stars and planets and, possibly,
life. The formation of a stellar-mass blackhole is a story of death that
signals a new birth.
I will discuss in a future blog the birth of supermassive blackholes.
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