ABSOLUTE BEGINNERS

This article featured in the December 2002 Beginners Magazine

BIG BANGS / LITTLE BANGS

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.

Return to Absolute Beginners