Most scientists believe that the universe was created by a 'big bang' about: 13 billion years ago.
At first the universe was a hot glowing ball of radiation, and in the first few minutes the nuclei of the lightest elements formed.
As it expanded (over millions of years) its temperature fell, uncharged atoms were formed.
The distance between neighbouring stars = millions of times greater than the distance between planets in our Solar system.
The distance between neighbouring galaxies = millions of times greater than the distance between stars within a galaxy
This means the universe is mostly empty space.
Eventually dust and gas were pulled together by gravitational attraction to form stars.
The resulting intense heat started off nuclear fusion reactions in the stars, so they began to emit visible light and other radiation.
Gravitational force attracts more dust and matter around stars and this may have formed planets.
Very large groups of stars are called galaxies. Our sun is one of the many billions of stars in the Milky Way galaxy.
The universe contains billions of galaxies.
The life history of a star
Stars form from clouds of dust and gas.
Gravitational forces make the clouds become increasingly dense, forming a 'protostar'.
As a protostar becomes denser, it gets hotter. If it becomes hot enough, the nuclei of hydrogen atoms and other light elements start to fuse together. Energy is released in the process so the core gets hotter and brighter and the star begins to shine.
Stars radiate energy because of the hydrogen fusion in the core. This is the main stage in the life of a star. It can continue for millions of years until the star runs out of hydrogen nuclei to fuse together.
During the 'main sequence star' period, the inward force of gravity is balanced by the outward force of radiation, so the star is stable. This lasts for billions of years.
The life history of a star cont.
Eventually the star runs out of hydrogen nuclei. The star then swells into a 'red giant'. It is red because the surface has cooled. The star then contracts into a 'white dwarf'
What happens after this stage depends on the size/mass of the star.
Low mass > 'Black (cold) dwarf 'no more light is emitted
High mass > continue to collapse, then explode into a 'supernova' the outer layers are thrown out into space, the core is left a extremely dense 'neutron star'
If the star is massive enough it becomes a black hole. The gravitational field of a black hole is so strong not even light can escape it.
Star life stages
Low mass star:
protostar > main sequence star > red giant > white dwarf > cold (black) dwarf
High mass star (after the white dwarf stage):
white dwarf > supernova > neutron star > black hole (if enough mass)
How chemical elements are formed
Chemical elements are formed by fusion processes in stars. The nuclei of lighter elements fuse to form the nuclei of heavier elements. The process gives out large amounts of heat and light.
Elements as hevay as iron are formed inside the star as a result of nuclear fusion.
Elements heavier than iron are formed in the final stages of a star's life in a supernova explosion as there must be an input of energy.
The presence of the heavier elements in the sun and inner planets is evidence that they where formed from debris scattered by a supernova.