The explosion of a very hot and dense object, in what is called the "Big Bang", produced the Universe that we see around us some 13 billion (13 000 000 000) years later. After the Big Bang, space itself expanded very rapidly at first, and later more slowly. This expansion let the object cool down. After 300 000 years it had cooled down enough, to a few thousand degrees, that atoms could form. Light from the hot soup of matter could pass between the atoms, and we can still see it today with radio telescopes, as a faint glow from across the sky, which we call the "cosmic microwave background".
Two sorts of atoms formed in that hot soup - the simplest, hydrogen (90% of the atoms) and the second simplest, helium (10% of the atoms). In clumps of gas stars formed over millions of years. Where lots of gas and stars were clumped together, their own gravity stopped them from flying apart and they formed huge groups of stars, called galaxies.
Inside these stars, hydrogen atoms fused to make more helium, some carbon, nitrogen, oxygen atoms and other more massive atoms. This process of "nuclear fusion" produces the light and heat that comes from stars. At the end of their lives, these stars exploded, blasting these heavy atoms into the gas clouds, where new generations of stars were born.
Some eight billion years after the Big Bang, in a large spiral galaxy containing about 100 billion stars, a very ordinary star formed. Thanks to the previous generations of stars, there were enough heavy atoms in the gas cloud from which it formed that solid planets could also form, orbiting this star. As the star heated up inside the gas cloud, it blew away the remaining gas, preventing any more planets from forming. Several solid, rocky planets remained, orbiting close to the star. Some large gaseous planets orbited at greater distances.
At first the planets were not in stable orbits around this star, and collisions occurred. In one collision, a small planet crashed into the third planet from the star. The debris from this collision produced a large moon orbiting the planet.
This planet was massive enough to keep an atmosphere of nitrogen, carbon dioxide and water. As it cooled down after the collision that formed its moon, seas of liquid water formed. Over hundreds of millions of years, complex chemical reactions occured that resulted in molecules based on carbon atoms that were able to replicate themselves. More and more complex versions formed, including a type that could produce oxygen from the carbon dioxide. After billions of years, the planet's atmosphere built up a significant amount of oxygen. New self-replicating molecule systems developed that could use the oxygen. Over many millions of years, new forms of these air-breathing "animals" arose. Some survived and others did not, as conditions changed. Over the last few million years, one type of animal learned how to walk on its two back legs, and use the five "fingers" on its front legs to make tools.
These animals discovered the power of giving names to what they could see, and called themselves "human beings". They named the planet they walked on the "Earth". The bright "stars" seen in the night sky were given various names by people in many civilizations. The names in general use today come from Egypt several thousand years ago. Examples are Sirius and Canopus, the two brightest stars as seen from Earth. Middle Eastern civilizations gave names to some patterns of bright stars in the sky, called "constellations" These bright stars are often not physically near each other at all, they simply lie in similar directions in space. Like a map of a continent divided into countries, eventually the whole sky was divided up into the constellations used today. Well-known constellations visible on a summer evening are Orion the hunter and Canis Major, the big dog. In winter Scorpius the scorpion and Crux the southern cross are easily found. Southern African peoples have their own names for some bright stars and constellations. For instance, in Sotho and Tswana, "dithutlwa" are two male and two female giraffes, being the pointers and two brighter stars of the southern cross.
One of the tools humans invented four hundred years ago consisted of two curved bits of a transparent material called "glass". These two "lenses" that focussed light to a point made an instrument that was able to make distant objects look much closer and reveal what was not visible to the naked eye. Called the "refracting telescope" by Galileo Galilei who had perfected it, this instrument started the modern age of the scientific exploration of what lay beyond the humans' planet. Shortly after, Isaac Newton found that a curved metal "mirror" that focussed light like a lens could be combined with a lens to make a new sort of "reflecting telescope".
Telescopes showed that the "wandering stars" that moved across the sky following the path of the Sun were not stars at all. They were seen to be small objects, much smaller than stars, and in fact like the Earth on which the telescope-makers lived. These planets were often found to have their own moons. The planets and moons were clearly cold objects reflecting the light of the central star, the Sun, around which they orbited. They did not make their own light, unlike the stars.
The stars themselves were found to be like the Sun, made much fainter only by their great distance. The most massive stars have masses up to 100 times that of the Sun. Their lifetime is measured in tens of millions of years, at the end of which they explode as "supernovae". They may leave behind collapsed remains, either neutron stars about 15 kilometres in diameter and visible as "pulsars", or black holes. The least massive stars have masses one tenth that of the Sun. They glow a dim red and will last many billions of years.
The Sun is a very average star, halfway through its 10 billion year lifespan as a normal star. Near the end of its life, as it uses up its hydrogen fuel, it will swell up to become a pulsating "red giant star", engulfing the nearest planets. Eventually it will throw off its outer layers, leaving behind a hot core known as a "white dwarf star" which will collapse to the size of the Earth, or about 10 000 kilometres diameter.
Telescopes showed that the "Milky Way" crossing the sky consisted of many millions of stars, together with dark clouds of gas and dust. These clouds were found to be made by exploding old stars. In the clouds new stars were seen to be forming, showing that this cycle of birth and death goes on continuously. The size of the Milky Way was measured - it turned out to be 100 million times larger than the solar system. The whole system of stars was seen to be rotating slowly about a centre 30 000 light years away.
Less than 100 years ago, as bigger telescopes were developed, Edwin Hubble showed that small faint patches of light scattered across the sky were galaxies in their own right, like the Milky Way. He also showed that the further away these galaxies were from the Milky Way, the faster they were moving away from us, as space continues to expand. Just thirty five years ago, Arno Penzias and Robert Wilson discovered the weak radio waves from the cosmic microwave background, produced just 300 000 years after the Big Bang. Only a few years ago the question of whether any other stars have planets orbiting them was answered. Some stars were found to be wobbling slightly, a motion that matched what would be caused by the gravity of a large orbiting planet. By 2010 about four hundred planets had been discovered in this way. We now understand a great deal about stars and planets, space and time, and we have moved a step closer to answering "Are we alone in the Universe?"
Stars and planets: How far? How big? How many?
For the latest news of the universe, Ned Wright's Cosmology Tutorial is the place to go.