The talk this month looks
at the, sometimes violent, process that created the universe
and the elements that enabled life to form on the ball of
Iron, rock and water we call Earth.
THE BIG BANG
To look at the
process we have to start at the beginning and ask where did
it all come from. In later half of the last century, there
were two theories describing how the universe formed. The
first was the Steady State Theory and the second the Big Bang
Theory. Both these theories had their followers and at that
time both were regarded with equal respect.
The Steady State Theory advocated that there was a central
region in the centre of the universe where matter was continually
being created from energy. As this matter was created it was
pushed out and formed into the stars and galaxies we see today.
With the advances in technology and as our understanding of
the motion and nature of the universe increased the Big Bang
Theory gradually developed into the accepted mechanism for
the creation of the universe. There is however a small number
of scientists who still subscribe to the Steady State Theory
but they are a very small minority these days.
The Big Bang Theory states that all the matter and energy
in our universe was created in an instant at a single subatomic
sized point known as a singularity. At the very instant of
its creation the whole universe occupied an infinitely small
space and was infinitely dense and infinitely hot. At this
instant there was no matter, only energy which erupted very
rapidly into a huge fireball. This fireball of pure energy
inflated at an enormous rate for about 100,000 years, gradually
cooling as it expanded. After this time, it had cooled enough
for sub atomic particles to form and then formed atoms. As
the atoms began to form, the universe started to become transparent
for the first time and light began to be emitted.
The Big Bang was the biggest explosion ever and was the event
that created everything we know and was the moment when everything
was brought into existence. Even time began for us at the
moment of the Big Bang because there was nothing before it.
We must now take up the story from the point where atoms had
formed and the universe, as we know it, came into existence.
In the very beginning the only matter in the universe was
in the form of the gas Hydrogen with a small amount of Helium
which is also a gas and a very tiny amount of the lightest
of all metals, Lithium. There was also a huge amount of energy
left over in the form of radiation. This radiation was in
form of the most energetic form of all radiation, Gamma Rays.
Over the enormous period of time since the Big Bang these
very high energy and extremely short wavelength Gamma Rays
have lengthened and can now be detected as longer wavelength
Micro wave radiation. These types of radiation were predicted
as part of the Big Bang Theory and when they were eventually
detected, established the theory as the front runner of the
two contenders.
The Cosmic Microwave Background CMB
GIANT EXPLODING STARS
The largest stars that can
be seen today, rarely exceed 50 times the Mass of our Sun.
The biggest star ever found, the Pistol Star, near the centre
of our Galaxy is thought to be close to 100 times the mass
of our Sun. These massive stars are very powerful and can
produce millions of times more energy than stars like our
Sun. Because large stars produce so much energy they do not
last very long, perhaps just a few million years, before they
tear themselves apart in huge explosions called a Supernova.
Smaller stars, like our Sun, will typically last for about
10 thousand million years.
ETA CARINAE - One of the largest stars known today
Theorists believe that any star forming from the materials
in the present galaxies cannot be greater than 100 solar masses
because it would be very unstable. Increases in the mass of
forming stars leads to a disproportional increase in temperature.
The radiation energy pushing out from the centre must equalise
the force of gravity pushing in. As the mass increases the
radiation pressure begins to exceed the gravitational pressure
and the star becomes unstable and starts to pulsate. The giant
star will then try to reduce its mass by throwing off huge
shells of hot gas but eventually looses the fight and collapses.
Scientists at the University of California at Santa Cruz have
used computer models to simulate the formation of giant stars
in the early universe and have now suggested that stars could
then have formed with masses up to 300 times that of our Sun.
The difference is due to the very early universe being comprised
almost entirely of hydrogen and helium. All heavier elements
were manufactured by dying stars to contaminate the material
forming into new stars. These heavier elements, Oxygen, Nitrogen,
Iron etc that we see all around us prevent the radiation produced
in modern stars from escaping as efficiently as it did in
the ancient pure Hydrogen stars. As a consequence modern stars
overheat and become unstable at a smaller size, thus reducing
the maximum size to between 50 and 100 solar masses.
So what were these Super Stars like? Firstly while they were
shining they were very bright indeed, as much as 10 million
times brighter than our Sun with a surface temperature up
to 100,000°K. These giant stars could only exist for a
very short time probably less than a million years but while
they did exist they were dazzlingly bright. This brightness
during their life was nothing compared to their spectacular
death. It has been calculated that these giant stars finished
their lives in two ways depending on their mass. Under 270
solar masses the star explodes in a hyper novae, shining for
a month as bright as 100 of today's galaxies and leaving almost
nothing behind.
Stars over 270 solar masses explode in an even more violent
way, briefly producing as much energy as 10 billion modern
galaxies. However the mass would have been so great that a
30 solar mass black hole would be left behind. Because of
the very fast spin, material formed into a very bright accretion
disc much like that seen in Quasars. Quasars consume about
10 solar masses per year but these black holes would suck
in 100 solar masses in just ten seconds producing a flash
brighter than the whole of today's universe.
GAMMA RAY BURSTERS
During the Cold War the USSR
and the USA were worried that either side may contravene the
nuclear test ban treaty so the Americans positioned satellites
in space to detect Gamma Rays from the detonation of nuclear
weapons. So after these satellites were commissioned they
started to detect massive Gamma Ray flashes lasting a few
seconds. These flashes were so powerful that they sent the
detectors right off the scales.
After some rapid but thorough investigations it was established
that these flashes or Gamma Ray Bursters did not originate
on the surface of Earth but appeared to be coming from space.
Further investigation showed that these bursts which occurred
about once a day were originating randomly from all directions.
This came as a great surprise because it meant that they came
from outside our galaxy and therefore originated at vast distances
from us. If their origin was that far away and yet they are
still so powerful the event that created them must produce
unbelievable amounts of energy. Gamma Ray Bursters appeared
to produce more energy than could be produced if all the mass
of a giant star was converted into energy. In fact it would
take the combined mass of many giant stars to produce enough
energy.
The remnant of a Gamma Ray Burster
Over the years much research has been carried out and the
latest satellites can detect Gamma Ray Bursts and tell the
worlds largest telescopes within a few minutes. The telescopes
can then search out the remnant and analyse the light and
establish what caused it. The latest theory is that Gamma
Ray Bursts are caused when two Neutron Stars or Black Holes
combine. The two objects are probably a binary system orbiting
each other and gradually get closer and closer. As the get
closer they orbit ever faster until eventually their gravity
tears them apart and they combine in a blinding flash of Gamma
Rays. Because the two objects were spinning round each other
the spin is retained and the Gamma Rays are concentrated and
directed out of the axis of spin in a powerful but parallel
beam, rather like a laser beam.
SMALLER BANGS
CLOSER TO HOME
We have looked
at some of the biggest bangs in the universe so now let's
consider some of the smaller bangs which can occur much closer
to us. These events, although much smaller, can be devastating
because of their close proximity to us.
Super Novae (exploding stars) still occur and can occur in
our galaxy close to us. It is calculated that a super nova
should occur in our galaxy about once every 300 years. We
haven't recorded one for more than 300 years so one is due
at any time. What is the danger to us ? If a Super Nova occurs
within a distance of about 1000 light years from us then the
bad news is life on Earth could be threatened. The good news
is there are no Super Nova candidates within that distance
that are likely to blow within the next 10 million years.
Rigel in Orion is a potential Super Nova at a distance of
900 light years but it appears quite stable at the moment.
The biggest bang that ever effected Earth occurred when the
Moon was created. Evidence suggests that in the early solar
system there were many more planets than the nine we have
today. There may have been more than 30 and as many as 100
when the Solar System formed. Many of these fledgling planets
had erratic orbits and close encounters and collisions occurred.
Close encounters could throw planets out of their orbits and
send them hurtling out into space or crashing into the Sun.
It is thought that Earth suffered a collision with another
planet very soon after they formed. A huge mass of molten
rock was thrown up and went into orbit around what was left
of the planets. Eventually this debris cooled and reformed
into the Moon we see today.
Objects still hit the Earth today but fortunately not so large
as that which formed the Moon. Every few million years we
can expect that a large lump of rock or ice may hit the Earth
and cause an explosion that could threaten life. The last
really large event was about 60 million years ago when an
asteroid hit, off the coast of what is now Mexico and wiped
out the dinosaurs and many other species.