Over 90% of stars in the Universe are on or near the main sequence, like the Sun. Some are slightly more massive and some slightly less massive but looking at the Sun from the Earth is like looking at any typical star from the astronomically near distance of about 149 million kilometres (1 AU). Stars are long lasting sources of energy, often of great strength. They emit electromagnetic radiation over the full spectrum of frequencies from high energy X-rays and 7-rays through to very low energy radio waves. In fact, the spectrum bears the imprint of conditions deep inside which are very close to equilibrium. It must be remembered that recognising a state of thermodynamic equilibrium requires the thermodynamic variables to be measured so that the various critical relationships can be found and checked. All experimental measurements involve errors of some kind, and however small these may be, the equilibrium in a particular case can only be established within a certain error limit. A system with an error within an acceptably small limit must be accepted as being in thermal equilibrium because there is no way of establishing that it is not.1 Here we investigate the essential properties of stars — both those on the main sequence, like our Sun, and those composed of very dense matter, that is white dwarf and neutron stars. We use the methods of similitude.
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