Due to the distances involved, it is not possible to directly sample stars in order to find out their composition. However, scientist can use another method to identify the elements that make up a star, even millions of Light-years away, by using thelLight they give out. By observing the light spectrum of the star, every element present can be found. There are two main types of spectra:-
1. Continuous Spectra - a 'Rainbow' (light of all Frequencies).
2. Line Spectra - a 'Rainbow' cut down to several lines of colour (light of specific Frequencies).
Rainbows and Continuous Spectra
When a ray of white light passes through a Triangular Prism, the ray is refracted twice. The overall effect of these refractions is to cause the white light to be dispersed. This causes a spectrum to be observed:-
This is the same process of how a rainbow is seen on a rainy day:-
When we observe the light dispersed by a rain drop we see a Rainbow, we see a (approximate) continuous spectrum of light from red to violet. The diagram below shows a true continuous spectrum in visible light:-
In the above image there are no gaps in this spectrum, it runs continuously from Violet to Red.
Note - a Rainbow is not a true continuous spectrum, but our eyes cannot resolve the gaps as they are so small, the light forms an absorption line spectrum (see below).
The light emitted from the Sun starts out as a continuous spectrum, but when it passes through the Solar Photosphere (the outer layer of the Sun's atmosphere), some of the light is absorbed by the gases present. This means that the spectrum that reaches the Earth is incomplete, missing certain Frequencies. The diagram below shows the absorption line spectrum of the Sun:-
As can be seen above, the light from the Sun is littered with these gaps, and it is these lines of absorption that allow scientists to identify the elements present in the Sun.
By observing the spectra given off by a particular element in vapour-form, it can be shown that each individual element has a unique set of emission lines, which match exactly the gaps in the Solar Spectrum. The diagram below shows the emission line spectra for several elements:-
By observing a star's spectrum, and by matching the absorption lines to the known pattern for each element, a star's composition can be found.