Other than the earth itself, the sun is the most vital cosmological object for life, without
it, there would be no life on Earth.
The sun is our closest star, and stars shine thanks to nuclear fusion taking place inside
their core.
Stars represent a celestial battle of two opposing forces, gravity trying to collapse
the matter to near nothingness, and an opposing radiation pressure trying to explode the star.
This tug of war goes on throughout the life of a star.
But it comes to an end close to the time that it runs out of fuel, at which point it becomes
unable to continue the fusion process, and so the outward radiation pressure decreases.
Gravity then inevitably becomes overwhelming, and the star undergoes collapse.
When a star collapses, many things can happen depending on the size and properties of the
star.
Large stars collapse in a colossal explosion called a supernova.
But if the star is extremely large, at least 30 times as massive as the sun, it undergoes
an even more massive explosion so enormous that its power is second only to the Big Bang
itself.
This is called a hypernova.
Such an explosion can be detected because it sends out an enormous, long lasting gamma
ray burst.
Gamma rays are the most powerful and energetic form of electromagnetic radiation known.
And such long lasting bursts are a telltale sign of the most powerful explosions in the
universe, which are often hypernovae.
These bursts are like a laser beam on steroids.
They are so powerful that they’ll fry anything in their path, including the earth.
If such a hypernova burst occurs even 200 light years from us, it would fry our atmosphere,
and end all life on earth.
And the scariest part is that we would not see it coming because the gamma ray beam would
be traveling at the speed of light.
The moment we saw it hit us, it would kill us.
Is this a real existential danger to us here on earth?
How do these hypernova occur, how do these gamma bursts form, and why are they so deadly?
That’s coming up right now…
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Stars begin their lives as clouds of gas and dust in space, known as molecular clouds in
stellar nurseries.
All the stars you see in the sky, including the sun started out in such gases.
Gravitational forces cause these clouds to condense and contract, increasing the density
and temperature at their cores.
As the cloud continues to contract, it forms a dense, hot core called a protostar.
This core continues to accrete more and more matter from the surrounding cloud.
During this phase, the protostar is shrouded in a cocoon of gas and dust.
When the temperature and pressure at the core of the protostar reachs a critical point (around
10 million degrees Celsius for most stars), nuclear fusion reactions begin.
These reactions primarily involve the conversion of hydrogen into helium through a process
similar to what occurs in the Sun's core.
The energy generated by these reactions provides the heat and light that make the star shine,
as well as the radiation pressure to hold off gravitational collapse.
The balance between the inward force of gravity and the outward pressure generated by the
fusion reactions keeps the star stable and in a state of equilibrium.
Larger and more massive stars tend to be hotter and brighter, but also have shorter lifespans
compared to smaller stars.
For example, the lifespan of our sun, a medium sized star is about 10 billion years.
It's about half way through its life right now
But stars that are about 30 times more massive than the sun, last only a few million years.
Eventually the core of the star runs out of hydrogen.
When that happens, the star can no longer hold up against gravity.
Its inner layers start to collapse, which squishes the core, increasing the pressure
and temperature in the core of the star.
While the core collapses, the outer layers of material in the star start to expand outward.
The star expands to larger than it has ever been – a few hundred times bigger!
At this point the star is called a red giant.
What happens next depends on the mass of the star.
For stars like our sun, the radius at this stage may reach out to the orbit of the earth
and swallow it.
But its core will still have enough heat and pressure to cause helium to fuse into carbon,
keeping the core from collapsing further.
But once the helium runs out, the sun will shed most of its mass, forming a cloud of
material called a planetary nebula.
Its core however will cool and shrink, leaving behind a small, hot ball called a white dwarf.
A white dwarf doesn't collapse against gravity because of the pressure of electrons repelling
each other in its core.
But for massive stars greater than about 8 times the mass of our sun, their fate is much
more spectacular.
These high-mass stars go through some of the same steps as the medium-mass stars like our
sun, with the exception that after helium runs out, their mass provides enough gravitational
pressure to continue fusion reactions.
So after the supply of helium runs out, it is hot and dense enough to fuse carbon into
neon.
fusion reactions occur, until the core is filled with iron atoms.
Up to this point, the fusion reactions put out energy, allowing the star to fight gravity.
But fusing iron takes more energy than it produces.
So the star eventually loses the battle against gravity.
The core temperature rises to over 100 billion degrees as the iron atoms are crushed together,
until the repulsive force between the positively-charged nuclei overcomes the force of gravity, and
the core recoils out from the heart of the star in an explosive shock wave.
This shock can propel the material away from the star at close to the speed of light in
a tremendous explosion called a supernova.
About 75% of the mass of the star is ejected into space in the supernova.
So if the star prior to the explosion was 20 times the mass of the sun, after losing
75%, its core will be about 5 times the mass of the sun.
For stars with a left-over core of anywhere from 1.4 to 5 times the mass of our Sun, it
will collapse into a neutron star.
But if the core is larger, it will collapse into a black hole.
So only stars with more than 20 times the mass of the Sun will become black holes.
The black hole forms when the remaining core of the supernova because of its huge mass,
is able to overcome quantum mechanical pressures which keep the core from collapsing in lower
mass stars.
The core continues to compress uncontrolled into theoretically an infinitesimally small
volume.
This means the entire mass of the core has compressed into zero volume.
This is called a singularity.
At least that’s what our equations say.
But infinities in our math equations usually indicate some kind of breakdown of the theory.
So a singularity is probably an undefined realm of spacetime which our current theories
can’t explain.
According to scientists, a hypernova occurs when even larger stars, more than 30 times
the mass of the Sun, quickly collapse into a massive black hole.
What distinguishes this explosion from other supernova explosions is that they are
10 to 100 times more powerful than a supernova, and that they result in a collimated beam
of gamma rays, or put more simply, a narrow beam of gamma rays, similar to a laser that
travels across the universe.
These gamma ray bursts can last anywhere from fractions of a second to hours.
They are highly energetic.
A typical burst releases as much energy in a few seconds as the Sun will release in its
entire 10-billion-year lifetime.
They are rare.
A few such bursts occur per galaxy every million years.
But since there are at least 200 billion galaxies in the universe, and these rays have infinite
reach, a few hundred thousand of these occur every year somewhere in the universe.
About one gamma ray burst is detected every day on earth . But so far, every burst we
have detected has occurred outside of our own galaxy.
I should mention that besides hypernovae, gamma rays can also be formed by very heavy
mergers, such as when two neutron stars merge, or when a neutron star merges with a black
hole.
The biggest question you may have is how does a narrow beam of gamma rays form?
Shouldn’t these photons be emitted spherically, just like the way that the matter is expelled
from a hypernova?
The interesting thing is that when scientists first discovered these gamma ray bursts, calculations
showed that the source of the explosion would have more energy than the big bang itself,
which was not possible.
But this calculation was based on a spherical photon burst.
When they discovered that the burst actually occurs in a narrow beam, the energy numbers
were more realistic.
The exact mechanism of the formation of this beam is not settled science, so I will tell
you what scientists speculate is going on.
What we do know is that it comes out in two jets in opposite directions.
It has to be two in opposite directions so the momentum of each cancels out.
The hypothetical mechanism is the following: The material ejected from the explosion is
a plasma, which results from gases heated to such extremes that they lose electrons
from their atoms.
In addition, the rapid rotation of the dying star twists up its magnetic field.
This means that the easiest path of escape for any of the charged particles being ejected
from the supernova is along a narrow beam at the poles of rotation, since the magnetic
field can’t get as tangled in that direction.
These particles being ejected are largely electrons.
The twisted magnetic field causes the electrons to travel in a kind of helix, like a DNA molecule.
If the electrons are moving fast enough, the rapid change of direction can produce high
energy photons, or gamma rays.
Gamma rays can also form if extremely fast electrons crash into photons.
The transfer of energy to the photon can cause it to gain energy and become a gamma ray.
The other question you might have is, can one of these things kill us?
First, let me point out as I said earlier, they are quite rare within our galaxy.
They might occur a couple of times every million years or so.
If the beam is not directly pointed at us, then we would be fine.
Since the gamma ray beam is collimated like a laser, it does not spread out much, like
the light from a lamp.
So there is a low probability that it would just happen to be directly pointed at us.
Also if the burst originated far enough away then we might be protected because space is
not completely empty, and the beam could collide with other celestial bodies between us and
the origin of the beam, diffusing the ray, and making it more harmless.
But if it is pointed directly at us within our galaxy at about 200 light year or closer,
it would cause a global extinction event.
It would burn our atmosphere and vaporize our oceans.
But even at greater distances, from within our own galaxy on the order of a thousand
or more light years away, it would still sterilize life on the side of earth pointed towards
the burst.
Such a burst is believed to have caused a mass extinction on earth about 443 million
years ago.
A hypernova is the most energetic event in the universe, exceeded only by the Big Bang.
Could it happen again?
Sure, it’s a possibility.
There are about 100,000 stars within a 200 light year radius of earth.
Although scientists haven’t discovered any stars in this vicinity that are large enough
to explode in a hypernova, you might take some comfort in knowing that if we do get
hit, we will not see it coming.
Since gamma rays are a form of light, they travel at the speed of light, so we would
have no warning.
In addition, they are completely invisible anyway because their frequencies are too high
for our eyes to detect, even higher than x-rays.
We and the earth would just fry with no warning whatsoever.
But for now, I’m pretty sure I’ll be around to you in the next video my friend!