The stars and galaxies were formed due to gravitational attraction. Uncharged atoms don’t repel each other but they are able to attract one another. With no repulsive force at work, clumps of space matter appeared and as they became denser so their gravitational attraction increased, forming larger clumps. The formation of the stars and galaxies saw an end to the Dark Age.

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The life cycle of a star

The protostar

A star is created from a cloud of dust and gas called a nebula. Gravitational attraction joins this matter together and as different clouds merge the mass increases in density until a protostar is formed. The protostar continues to get denser and also hotter until a point is reached when the temperature is high enough for the nuclei of hydrogen and other light elements to fuse. The energy released makes the core get even hotter. It increases in size and brightness until it shines. A star has now been born

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The star

Stars radiate energy due to hydrogen fusion within their cores. As long as a star contains hydrogen nuclei it will continue in this state which can mean billions of years. Fusion is able to continue due to the fact that energy is released from fusion thereby ensuring that the core is kept hot enough. Radiation is released from the core and travels out in every direction. The pressure of the radiation balances out the force of gravity that causes the star to contract. Until the hydrogen nuclei starts to run low, these forces stay in a balanced state.

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The red giant

Once the hydrogen nuclei have run out, the star swells outwards. As it gets bigger it also becomes cooler and red in colour. It is now a red giant. The helium and other light elements fuse which creates denser and heavier elements. After all the light elements in the core have run out fusion and radiation release stops. The star collapses in on itself and is it does so it heats up, turning from red to yellow to white. It has now become a white dwarf.

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The white dwarf

A white dwarf is the smallest hottest and densest state of a star. Two things can now happen, depending on the size of the star.

• – Small stars, which have low mass, cool down until they no longer emit any light and become black dwarves.
• – Larger stars, which have high mass, continue to collapse until the process reverses and a cataclysmic explosion, called a supernova, occurs. This explosion is so powerful that the light it produces can be present in the galaxy for weeks after. The core of the star is compressed into a neutron star which is is very dense, and composed only of neutrons. If the neutron star is large enough it can then becomes a black hole. Black holes have a gravitational field that is so strong that even electromagnetic radiation can’t escape them.