Bryony Hall, of Nayland College, asks :-

How do scientists know what are stars made of?

Saskia Besier, a research student in astronomy at the University of New South Wales in Australia, responded.

The answer in one word is spectroscopy, which I use regularly in my research. The technique is used extensively in many areas of science such as physics, chemistry, biology and medicine to determine what atoms are present in the object being studied. Spectroscopy basically involves breaking up light into its component wavelengths. A simple way to do this is just using a prism which will produce a nice rainbow from ordinary white light. (Pure white light is made by adding together all the colours of the visible spectrum.)

When an atom or molecule loses or gains energy it usually does so using radiation (which we call light when we can see it with our eyes, or things like microwaves, radiowaves, gamma-rays and so on, when we can't). An atom that gains energy will do so by absorbing radiation and one that loses energy will emit radiation (this is a nice law in physics, the Conservation of Energy). The joy of all this is that each leap of energy lost or gained by an atom or molecule has its own particular colour or wavelength of light associated with it. For example a sodium street lamp emits yellow light whereas a neon sign emits red light.

Absorption of light shows up in a spectrum as a dim or black area which we call an absorption line, whereas an emission of light is a brighter area and is called, funnily enough, an emission line. From looking at emission and absorption lines in spectra, we can not only tell what elements or molecules are present in the object but also their temperature, their state of ionisation (how many electrons the ion retains), how fast they are travelling and how much of a particular substance there actually is.

So in the case of stars we just point the telescope towards the star, select out the light from that star and then pass it through a spectrometer. The star itself then tells us what it is made of.