About 4.6 billlions years ego, a great swirl of gas and dust some 24 billions kilometers across accumulated in space where we are now and began to aggregate. Virtually all of it - 99.99 % of the mass of the mass of the solar system- went to make the Sun. Out of the floating material that was left over, two microscopic grains floated close enough together to be joined by electrostatic forces. This was the moment of conception of our planet. All over the inchoate solar system , the same was happening. Colliding dust grains formed larger and larger clumps. Eventually the clumps grew larger enough to be called planetesimals. As these endlessly bumped and collided, they fractured or split or recombined in endless random permutations, but in every encounter there was a winner, and some of the winners grew big enough to dominate the orbit around which they travelled.

It all happened remarkably quickly. To grow from a tiny cluster of grains to a baby planet some hundreds of kilometers across is thought to have taken only few tens of thousands of years. In just 2 million years, possibly less, the Earth was essentially formed, though still molten and subject to constant bombardment from all debris that remained floating about.

At this point , about 4.4 billion years ago, an object the size of the Mars crashed into the Earth, blowing out enough material to form a companion sphere, the Moon. Within weeks, it is thought, the flung material had resembled itself into a single clump, and within a year, it had formed into the spherical rocks that companions us yet.
Most of the lunar material , it is thought , came from the Earth's crust, not its core, which is why Moon has so little irons while we have a lot.

When the Earth was only about a third of its eventual size, it was probaby already beginning to form an atmosphere , mostly of carbon dioxide, nitrogen, methane and sulphar. Hardly the sort of the stuff that we would associate with life, and yet from this noxious stew life formed. Carbon dioxide is a powerful greenhouse gas. This was a good thng, because the Sun was significantly dimmer back then. Had we not had the benefit of a greenhouse effect, the earth might well have frozen over permanently, and life might never have got toehold. But somehow life did.

For the next 500 million years the young Earth continued to be pelted relentlessly by comets, meteroits and other galactic debris, which brought water to fill the ocean and the components necessary for the succesful formation of life. It was singularly hostile enviornment, and yet some how life got going. Some tiny bag of chemicals twitched and became animate. We are on our way.

Four billion years later, people began to wonder how it all happened.

The core of a neutron star is so dense that a single spoonful of matter from it would weigh 90 billion kilograms. A spoonful! A supernova occurs when a giant star, one much larger that our own Sun, collapses and then spectaculary explodes, releasing an instant energy of a hundre billion suns, buring for a time more brighter than all other stars in the galaxy. It is like a trillion hydrogen bombs going off at once.

Space is enormous. The average distance between stars out there is over 30 million million kilometers. Nobody knows howmany stars are there in the Milky Way- estimates range from a 100 billion to perhaps 400 billion- and the milky way is just one of a hundred and forty billio or so other galaxies, many of them even larger than ours.