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The life and death of stars

The following tables shows a series of important events in the lifetime of a star such as our Sun.
Larger stars may end their lives as a nova or supernova.


10 Important events in the life of the star with a mass like our Sun
1 From enormous clouds of very low-density hydrogen gas molecules begin to coalesce to form tiny particles of matter. The density of this cloud is about 106 hydrogen atoms per cubic metre, the same as a good laboratory vacuum. This interstellar matter in the cloud consists of:
(a) neutral and ionised hydrogen
(b) hydrogen gas
(c) interstellar dust
2 Tiny particles of the gas cloud move together to form larger lumps and the gravitational attraction between these lumps forms larger lumps.
3 Friction between these particles produces heat as the "protostar" grows.
4 In the centre of the protostar gravitational forces eventually build up an enormous pressure and hydrogen fusion occurs the star is born.
5 The star shines in this state for many millions of years. A star like out Sun will fuse hydrogen for some 1010 years.
6 The central core "runs out" of hydrogen and begins to cool. This means that the radiation from it is reduced, the radiation pressure is less and so gravity takes over. The core begins to collapse.
7 The layers of the star outside the core are still fusing hydrogen. They now fall inwards and actually heat up. The rate of fusion in this shell increases and the radiation emitted by this shell increases.
8 The layer of the star outside this is therefore heated up by this increased radiation. It expands.
9 This rate of expansion continues and the star expands to produce a Red Giant the size of the Earth's orbit.
10 This expansion is so rapid that the outer layers will cool, their temperature falling from the 6000o of its main lifetime to some 3000o in the Red Giant phase.
11 The core is still hot and continuing to collapse. It heats up further, the hydrogen runs out and the fusion of helium begins. Known as the triple alpha process helium is fused to give beryllium and finally carbon. For stars with a mass less than 3 solar masses this occurs rapidly and is called a helium flash.
12 The layers of the star will be heated an explosions will take place, blowing away an outer shell of gas. In stars like the Sun between 25 and 60% of the mass is blown away to form a planetary nebula.
13 The process repeats and the star flashes as layers of its matter are puffed away into space.
14 As time goes by the radiation from the fusion in the core gets less. The star shrinks and cools a little, although it is still hot enough to continue fusing hydrogen and becomes a small hot star a white dwarf. The core has the size of the Earth and therefore the density of a white dwarf is enormous 108 to 109 kgm-3. A teaspoon of this matter would have a mass of several tons!
15 Eventually all fusion ceases and a black dwarf is formed, a dark, cold ball of oxygen and carbon the star has died
 

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© Keith Gibbs 2020