The life of a star

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THE BIRTH OF A STAR

(1) Large clouds of dust and hydrogen gas are called nebulae. There are many nebulae that we can observe from earth.

(2) The gravitational force in the nebula slowly pulls all the particles and gas together.This is the beginning of the star.

(3) As the star gathers more material, it becomes more compact.The force of gravity compresses the star.The temperature in the middle of the star rises.

(4) Eventually, when all the gas is fully compressed, we have a star in its steady state.

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THE LIFE OF A STAR

(4) Once the inside of the star gets sufficiently hot the hydrogen atoms get squeezed together to make helium. This process of nuclear fusion releases a huge amount of energy. All of the energy that the star radiates away as heat and light is created by nuclear fusion.

(5) The star is now stable. The inwards force of gravity is balanced by the outwards pressure inside the star due to the incredibly high temperatures

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THE LATER LIFE OF A SMALL STAR

(7a) A small star is stable for around 10,000 million years. After this time all of the hydrogen in the core has been converted into helium and the star becomes unstable.

(8a) The star cools down and expands to form a red giant. When this happens to the sun, it will engulf the inner planets of Mercury, Venus and Earth. All life on Earth would be destroyed.

(9a) The star then collapses over time into a white dwarf.

(10a) The white dwarf slowly cools first to a red dwarf, then finally to a black dwarf (like a cold burnt log in a fire place)

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THE LATER LIFE OF A LARGE STAR

(7b) The large star cools down and expands to form a red supergiant star.

(8b) The supergiant then becomes unstable. It starts to convert helium into other heavier elements including carbon and iron.

(9b) The supergiant then contracts very quickly and explodes in a dramatic supernova. Gas and particles from the outer layers are thrown into space and form a nebula.

(10b) The object that remains after the supernova is incredibly dense. All the protons and electrons are squeezed together and form neutrons. This kind of star is known as a neutron star. If the sun collapsed into a neutron star it would only be a few kilometers in diameter. In some cases, when the initial star is even more massive a black hole will form. The gravitational force around a black hole is so strong that not even light can escape.

(11b) The story does not end here. The dust and gas thrown off by the supernova can form a nebula. Out of this cloud, a new star can start to form. Scientists believe that our sun was formed out of such a nebula.

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LIFE CYCLE OF A STAR

A simplified version:

  • The material in a nebula collects together to form a star.
  • The temperature inside the star increases and nuclear fusion begins.
  • When the core's hydrogen has been used up the star becomes unstable.
  • The star cools and expands to form a red supergiant.
  • The star then collapses violently and explodes into a supernova.
  • The core turns into a neutron star and the outer layers are thrown off.
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SUMMARY - LIFE OF A STAR

  • Stars form when the gravitational force in large clouds of dust and gas pulls these particles together.
  • The temperature in the middle of the star rises as it becomes more compact.
  • When the inside of the star is hot enough, nuclear fusion occurs. This releases a huge amount of energy.
  • When the star is stable, the inward force due to gravity is balanced by the outward pressure inside the star due to the high temperature.
  • When a star has exhausted the hydrogen in its core, its surface cools down and it expands to form a red giant. A small star will then collapse to form a white dwarf. If the star is large enough it will contract very quickly and then explode, throwing off gas and dust. A very dense neutron star is all that remains.
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