In the photograph above we are looking through a telescope at another galaxy far away behind the bright dots and discs which are nearby stars in our own galaxy . A large spiral galaxy like ours contains about a hundred billion (10¹¹) stars, each like the Sun, some much larger and some smaller. The visible universe which we can reach with our telescopes contains about a hundred billion galaxies and they contain a total of about 10²² stars. 10²² means one followed by twenty-two zeroes. That's a heck of a lot of stars.

Our Sun is located in one of the outer spiral arms of the Milky Way galaxy. This is fortunate because if we were in the bright central part of the galaxy millions of bright stars and dark dust clouds would prevent us from seeing other galaxies, we would only be able to see nearby parts of the Milky Way. As it is, we can see the great empty void of outer space in most directions. There is however a faint band of light across the sky that blocks our view in that direction, it is the rest of the Milky Way galaxy. It is as if we were near the edge of a plate and the plate blocked our view in some directions. If you can get away from the city lights and look up into a black cloudless night sky you can clearly see this hazy band of light across the sky from horizon to horizon. A modest telescope will show you that it is made up of millions of stars. Within the galaxy new stars are being born and old ones are dying. Big stars are very bright but their lives are much shorter than those of smaller stars. The lives of stars range from millions to billions of years. Our Sun is about half way through a life of ten billion years.

When the Universe was born, after the "Big Bang", about fifteen billion years ago, it consisted of a thin cloud of gas, three quarters hydrogen and one quarter helium. It was unstable, in the sense that where the gas was a little bit more dense its gravitational pull attracted more gas which made it denser still. It tended to form huge clouds of gas a million light years across which separated from other similar huge clouds. These clouds, pulled together by their own mutual gravitational attraction, became maybe a thousand times denser, the material of our present galaxies. Within the galaxy some parts, especially near the center, grew very dense and these dense irregularities formed stars. As the hydrogen and helium atoms fell towards these centres of gravitational attraction they moved faster and faster like falling stones. When they collided with each other the collisions became more and more violent, we say the gas was heating up, gravitational energy was being converted into heat. The temperature at the centre rose rapidly, first into thousands of degrees then into millions of degrees. At these temperatures the hydrogen atoms often combined to make larger helium atoms creating additional heat from the nuclear reactions. It was like having a continuous hydrogen bomb. It is also the way that the Sun generates its heat today.

Some of these stars were larger than others, they were much hotter at the centre, they were much brighter and they burnt up their hydrogen much faster than smaller stars, often in only a few million years. When the hydrogen was finished they shrunk a bit which made them hotter still, they were then able to start converting their helium into carbon, oxygen and nitrogen and continue generating heat. But in another million years all the helium had been burnt as well. There was then no energy left to create the pressure pushing outwards to balance the weight of outer material pressing inwards. Within a few days the star collapsed in a spectacular explosion. It became a supernova. For a few days it produced more light than all the other stars in the galaxy put together, then, over a few weeks, it faded back to normal. Astronomers have seen supernovas in other galaxies through their telescopes and this is the pattern that they all follow. There is a supernova in our galaxy every 300 years on the average. They have been reported by those who could write in days long before there were any telescopes.

Without supernova explosions we wouldn't be here, because that is the only way that heavier elements like iron, gold and uranium are made. As the star collapses, gravitational energy is converted into heat and the temperatures at the centre of the star soar to hundreds of millions of degrees and the pressure is enormous. These conditions provide the energy required to make the heavier elements. Then while the central core shrinks to become a neutron star only a few kilometers in diameter the outer shell of the star is blown away into space, as a gigantic expanding cloud of dust.

About five billion years ago a smaller cloud of hydrogen and helium gas contracted, heated and started a nuclear reaction. It became our Sun. There was dust from earlier supernovas mixed in with the gas which became, not only our Sun, but also our solar system. A lot of the incoming gas and dust was not headed directly towards the centre of the sun but came in at an angle and formed a rotating disc of gas and dust with the newly formed Sun at the centre. Not all the material was rotating in the same direction and so there many violent collisions which heated the material up and sometimes scattered it. Gradually, after a few million years there were eight huge lumps made of rocks and dust which had welded together. These lumps were the future planets. If we follow the fortunes of the third lump from the Sun we will see how the Earth was made.

This lump eventually became a sphere of molten rock about eight thousand miles in diameter. It was much smaller than the Sun, which was a million miles in diameter and which contained a million times as much material. It was only one tenth the diameter and one thousandth the mass of Jupiter the largest lump. It had enough gravitational attraction to mop up most of the remaining gas, dust and rocks near its orbit around the Sun. The heavier elements like iron and nickel sank to the centre of the Earth and the lighter ones like aluminum and silicon moved up and away from the centre. Then a terrible thing happened; a really huge lump, perhaps one tenth the size of the Earth collided with the newly formed Earth at high speed. It ploughed deeply into the planet but didn't penetrate the iron and nickel core. It threw out a huge plume of dust and molten rock which went into orbit around the planet. In time, through collisions between the dust and rocks in its orbit, the moon was formed. There were other large lumps that continued to bombard the surface of the Earth and the Moon, making craters which can still be seen on the face of the Moon, but nothing quite so catastrophic as the moon making lump ever occurred again.

Over most of the next billion years the surface of the Earth cooled and the rocky material on the surface solidified to form a skin a few miles thick. At first the skin was often punctured by the impact of rocky masses still left in the Earth's orbit. As time passed fewer rocks were left to hit either the Earth or the Moon also the skin gradually grew thicker. Of course all the planets, including the Earth, were surrounded by a blanket of gas which included water vapour. The rocky surface continued to cool and at some point it dropped below the boiling point of water, The water vapour condensed and rain fell. Great floods of rain fell out of the sky often boiling off again when they hit the hot rock below. Over time the oceans filled with the water and the intensity of the rain abated.

About 3.8 billion years ago, before the planet was a billion years old we find the first signs of life in the form of bacteria whose activities have left clues in the oldest rocks we know of. The atmosphere of the planet consisted mostly of nitrogen and carbon-dioxide so we assume that these early anaerobic bacteria acquired the energy that they needed to live from the abundant inorganic chemicals. About a billion years later blue-green algae developed, they used sunlight to provide energy and in the process converted carbon-dioxide into free oxygen and organic chemicals that they needed to live. Over several hundred million years they converted the entire atmosphere to a combination of nitrogen and free oxygen plus a little water vapour and carbon-dioxide. This is similar to our atmosphere today.

Among the materials produced in the supernova that later got carried to the Earth were some very heavy radioactive elements like radium, thorium and uranium. These elements are unstable and ultimately break apart into lighter elements like helium and lead.When they break they give up some of the energy they gained in the supernova. This energy appears in the form of heat which helps to keep the Earth's core molten and very hot. Around the iron core is a 'mantle' of lighter rocks which are also molten or plastic which means that they can move, although they do so very slowly. As these rocks are heated at the core, they become a little lighter and move up towards the surface crust. The movement is very slow, it takes millions of years to move the two thousand mile distance from the core to the crust. When the rock reaches the surface it moves to one side to let other hotter rock from below take its place. After travelling several thousand miles near the surface it becomes cooler and, consequently, heavier. At some point it starts to drop down from the surface back towards the core where the process repeats itself. If we could cut the Earth in two we would see maybe a dozen of these great cyclic movements of rock ascending at one location, moving sideways and descending at another location. This motion of the rocks has two consequences. First it creates a magnetic field and turns the Earth into a great magnet. Second it carries the Earth's crust along with it when it moves sideways.

Under the centre of the Atlantic Ocean there is an upwelling of rock from the core. Some of the rock moves Eastward and some moves Westward. The Eastward movement carries Africa, Europe and Asia towards the Western Pacific Ocean. The Westward movement carries North and South America towards the Eastern Pacific Ocean. The Atlantic Ocean gets about an inch wider each year and the Pacific Ocean gets about an inch narrower. The continents and oceans float on top of the sphere of molten rock that is our planet. They move slowly but they are constantly changing their positions as shown below.

The maps above show where the continents were 180 million years ago, 70 million years ago and where they are today. The first map shows South America joined to Africa to make a super-continent which has been called Gondwana. A look at the last map or any atlas will show that the East coast of South America almost exactly fits the West coast of Africa. A look at the deeper (and therefore older) rocks of both continents shows that the pattern of rocks also matches across the divide. The sub-continent of India makes an interesting journey from near Antarctica heading North until it slams into Asia. What happens then? The moving molten rock below continues to carry India North and it slides under the Southern edge of Asia pushing Asia high up into the air creating the Himalayan mountain range and the Highlands of Tibet. These are the world's highest mountains and they are still growing higher.

These pieces of the Earth's skin that are being moved around in different directions have been mapped and are called plates. North and South America each form a plate as do the other continents but the seabed is also divided into plates which are moved in different directions by the rock below. We noted that North and South America are each being carried slowly further to the West. They are sliding over some of the plates in the Pacific Ocean and these plates push up the Western side of America from below. This has created the Andes in the South and the Rocky Mountains in the North.

This has been the story of our planet Earth. A cloud of hydrogen and helium gas mixed with a little dust contracted under the force of gravity to make our Sun and its Solar System. Some of the dust and gases remained swirling around the newborn Sun and formed nine planets, some thirty moons revolving around these planets and leaving a mess of rocks (asteroids), dust and huge dirty snowballs (comets) that wander around the system. The third planet from the Sun, our Earth became a ball of molten rock which it still is. A smaller planet hit it, joined it and splashed out a huge plume of rock that became the moon. As it cooled a hard skin formed on its surface which later cooled enough for water vapour to condense and form the oceans. Radioactivity still heats the centre and causes the rock to move around taking the continents with them. This is our Earth today.

Essay By: J. A Williamson

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