Esther Sullivan and Sarah Howie, of Craighead Diocesan School, asks :-

How do scientists know that the Sun is made of hydrogen and helium when no-one has been close enough to get samples?

John Campbell, a physicist with an interest in colour and spectroscopy, responded.

Because the atoms themselves tell us.

If we excite atoms they radiate coloured light which is specific to the species of atoms. The yellow street lights are due to sodium atoms. The red light of neon advertising signs and helium-neon lasers is due to neon atoms. The blue light of advertising signs comes from argon atoms.

There are only 90 naturally ocurring types of atoms. Their chemistry is determined by the number and arrangement of electrons in orbit about the nucleus of the atom. After these electrons are excited into higher energy orbits they return to their original orbits and in doing so emit an amount of energy (in the form of light) which is specific to that atom. So all we have to do is to put the light from a distant source through a prism or diffraction grating to separate it into its component colours (frequencies or wavelengths) and we can tell what atoms are present in the source of light. More than that, we can tell the actual number of electrons around the atom as they can lose some of their electrons, (ie the state of the ionization of the atom), their temperature (by the width of the spectral lines), the speed they are travelling at and the direction they are travelling (the Doppler effect).

Humans are basically blind because our eyes respond to such a tiny portion of the whole electromagnetic spectrum. If we use detectors of all other waves, such as radio waves, microwaves, heat waves, ultraviolet light and X-rays, we can cover all atoms. For example, radio-astronomers often search for hydrogen by looking for microwaves of wavelength 21cm.

Although we haven't been to the Sun, particles from the Sun come to us. During violent explosions on the Sun, which we see as dark Sun-spots, atoms are blasted outwards and reach the Earth where they smash into atoms in the Earth's upper atmosphere causing auroral displays. Special collectors on satellites can gather these atoms.