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The evolution of stars has many stages. Stars are formed, change into other forms, and in the final stage they are destroyed. The first two stages in the evolution of stars are the same regardless of whether the star is the same size as our Sun or larger than it.
The first stage in the evolution of all stars involves the formation of a nebula. A nebula is a cloud of dust and gas which has been collected together by gravity in a process known as accretion. These clouds collect together to form a protostar.
These clouds are so large that they condense down under their own weight. The clouds initially contain large amounts of hydrogen atoms. Under the high pressure and high temperatures ranging from 2,000 to 30,000, these hydrogen atoms undergo nuclear fusion to produce helium. A chain reaction begins where more and more of the hydrogen nuclei fuse forming helium. These nebulae are known as the birthplace of stars and the huge heat and pressure found here is the reason why stars are formed.
As the nuclear fusion reactions continue, large amounts of heat and light energy are produced. This allows the process of nuclear fusion to continue. The outward pressure caused by the nuclear fusion and the force of gravity keeping the star together are balanced. During this stage of evolution, the star is stable and is known as a main sequence star. This stage of the evolution can last for several billions of years, and our Sun is thought to be in the middle of this stage.
The stages of evolution that follow on from here are different depending upon the mass of the star in comparison to the mass of our Sun.
Evolution of stars with a similar mass to the Sun
For stars that have a mass similar to that of our Sun, the next stage of evolution is the formation of a red giant.
Once the hydrogen nuclei have been used up, fusion of helium and other elements occurs. Heavier elements up to iron are formed. The nuclear fusion reactions do not produce as much energy and the star becomes much cooler with its surface temperature decreasing to 2,000 to 3,000. This cooling causes the star to change colour, becoming red. The star also grows in size with its diameter increasing to up to 1,000 times that of the Sun. The star is now known as a red giant. The nuclear fusion reactions can only continue as long are there are nuclei to fuse. Once the nuclear fusion reactions have stopped, the red giant becomes so unstable that it explodes, shedding its outer layers of dust and gas which can then form a giant ‘planetary nebula’. This stage of the evolution takes approximately ten thousand years, but this is a relatively short period in the whole stellar life cycle.
Once the outer layers of a red giant have been shed in a planetary nebula, a hot, dense solid core remains. This core is known as a white dwarf. White dwarfs have a mass similar to the Sun but are only around 1% of the size, meaning they have greater density. White dwarfs emit large amounts of heat and light energy. The amount of energy emitted decreases as the white dwarf cools. Eventually the amount of energy emitted becomes so low that the star becomes a black dwarf and can no longer be seen.
Evolution of stars with a mass larger than that of the Sun
For stars that have a larger mass than our Sun, the next stage of evolution is the formation of a red super giant. In a red super giant, a much higher number of nuclear fusion reactions occur, producing greater amounts of energy much faster. This causes rapid contraction and expansion of the red super giant which becomes so unstable that a large explosion known as a supernova occurs. The energy produced during this explosion forms elements even heavier than iron. The elements are emitted from the supernova with great energy into the universe where they can form new planets or stars.
The supernova leaves behind a very dense core known as a neutron star. If the mass of the original star is great enough, the force of gravity will be so great that the material left behind will be so dense that a black hole will be produced. A black hole is a collection of material so dense that not even light can escape from it.