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The birth of stars Big Bang 4 ext
When the Universe was about 100 million years old, hydrogen, deuterium and helium began to cluster together in large clouds. The gases were cool enough to allow gravity to draw the particles inwards to form stars – a process that is still going on today.
The first steps
As they lose gravitational potential energy, the hydrogen and helium atoms gain kinetic energy. The electrons are knocked off the atoms, forming a plasma of nuclei and electrons. The temperature of this plasma rises as its particles speed up. It forms a protostar and begins to glow. However, it is not yet a proper star.
A star is born
The positive nuclei are kept apart from each other by electrostatic repulsion. However, when the temperature reaches about 1,000,000 oC, they are travelling so fast that the hydrogen and deuterium nuclei start to fuse together. The fusion of two hydrogen nuclei releases an enormous amount of energy – about a million times more than burning hydrogen in oxygen. The star starts to shine.
And dies again
The early stars were very big. They reached extremely high temperatures and used up their nuclear fuel of hydrogen quickly – within about a billion years. When a star runs out of hydrogen in its core, it starts to collapse under gravity. Once again the temperature rises and helium will start to fuse. The fusion of helium produces heavier elements such as oxygen, nitrogen, carbon and magnesium. Each new type of fusion reaction uses up its fuel more quickly than the last.

Once the core has made iron, it begins to use up rather than give out energy. The core of the star shrinks rapidly and explodes in a huge explosion called a supernova. This scatters all of its material into space and creates elements heavier than iron.

Life cycle
The dust and gas cloud from a dead star is called a nebula. Some nebulae are so large that light takes thousands of years to travel from one side of them to the other. Eventually the material within a nebula will start to form into clumps and be drawn together under the force of gravity. A new generation of stars will form in this giant nursery in the sky.
The story so far
Stars are born and die a few billion years later
Large stars die in a huge explosion called a supernova
It is in the cores of dying stars that the elements with atomic number greater than 2 were made
Apart from hydrogen, all the atoms in our bodies have come from the core of a dying star
Question S5

a) A protostar isn't a proper star. What is the difference between a protostar and a proper star?


b) What is a light year?

c) What is a nebula?

The story so far
  • The fusion of hydrogen nuclei forms helium nuclei and releases an immense amount of energy
  • The Universe was about 100 million years old when stars began forming
  • Stars cluster together in galaxies
  • It takes light 100,000 years to travel across our galaxy