Introduction to SpectroscopySpectroscopy is the study of the interaction between matter and electromagnetic radiation. In simple terms this means the effect on light (or any form of radiation from X-rays to radio waves) as it passes though or reflects off gases or reflects off solid objects.
Studies of these effects can give information on the nature or chemical composition of the matter involved, and its temperature.
This is how astronomers can tell what stars are made of and what chemicals exist in the space, gas and dust clouds that exist between the stars. It can also tell us what the clouds of planet such as Jupiter and Saturn are composed of and what gases are given off by comets.
The principle of spectroscopy is to see what 'colours' make up the light from objects in the same way that a prism shows us the colours that make up white light - its spectrum. Most amateur astronomical spectroscopy studies the visible range of light from red to blue.
Hot solids emit a continuous spectrum in visible light, called a continuum. The intensity distribution across the spectrum depends on the temperature of the object.
In astronomy we typically see three main types of spectra from objects using visible light:
These are typical of most stars. The spectrum shows a continuum with dark lines at specific colours.
The lines are caused by chemicals absorbing light as it passes though. The absorption occurs at specific wavelengths which are characteristic of that chemical and can be used like fingerprints. The spectrum is, however, complicated by the fact that it will show the superimposed fingerprints of all the chemicals involved.
These are usually obtained from nebulae, which are clouds of gas at low pressure but high temperature, and of certain types of very hot stars. The spectra show bright lines with little or no continuum.
The lines are caused by the hot gas emitting light from its constituent chemicals.
As the name implies these are of objects reflecting light from other sources. These are typically taken of planets with cloud covered surfaces.
They consist of the original light from the source modified by any absorption caused by the nature of the reflecting chemicals. This typically causes dark bands rather than sharp lines.
The above spectra are from different equipment and the wavelength scales are different.