Around fifty thousand millions of years ago
the Solar System began to form.
Imagine an immense low-density cloud in the
outer arms of our galaxy. This huge cloud of gas (mostly hydrogen with a little helium) and
dust was originally so thin that it was virtually invisible. Millions of years passed, the cloud
contracted, cooled and began to spin. Gravitational forces in this contracting cloud pulled
particles towards the centre and the temperature there began to rise.
Eventually in
the middle of the sphere at the centre of the cloud the temperature and pressure were high
enough for nuclear fusion to begin. Energy was radiated as light and heat and a star shone.
This was the birth of our Sun.
The central part formed the Sun and this left the
remains of the low-density cloud of gas circling the Sun as it rotated. This rotating cloud
began to shrink as the particles in it were attracted to each other. Over millions of years
these particles formed into lumps, then bigger lumps and finally into the planets, asteroids
and comets as we know them today. About five thousand million years ago the planet Earth
was formed.