Helium is a noble gas and will not react chemically with anything. It also has a very low density. Therefore, it was not found combined with other elements either in the sea or in the earth's crust (as are other elements) and neither is it found in any appreciable amounts in the atmosphere - as, like hydrogen, the lightest gas of all, it will escape into space because the earth' gravity is not enough to hold it in the atmosphere. Much of the helium found on earth is obtained fro the decay of radioactive sunstances that give off alpha radiation - alpha particles are the nuclei of helium atoms. Therefore on earth helium is not particularly common. In the sun, however, helium is formed by the fusion of hydrogen atoms to form helium and great deals of energy during thermonuclear reactions that are happening on an enormous scale within the sun. The helium therefore is much more abundant in the sun. All elements, when heated sufficiently, give off light of a particular kind. As an example sodium produces a yellow light (as in sodium street lamps). The light from individual elements can be analysed and the various colours in the light (caused by differing intensities at different wavelengths) - known as the substance's spectrum - acts as a sort of 'fingerprint' for a particular element. When the light from the sun was analysed in this way, the spectrum for hydrogen was present (as the spectrum for hydrogen was well known as the hydrogen spectrum had already been analysed in the laboratory). However another sectrum was also present superimposed on the hydrogen - a spectrum that was unknown. Therefore it was assumed that this was due to an as yet undiscovered element - named 'helium' from the Greek for sun - helios. Only later on was this element, abudant in the sun, discovered on earth where it was much rarer.