Spectroscopy: determining the composition of stars

The spectrum of a star

A star such as the Sun initially produces a continuous spectrum (as its surface is very hot and dense), but this radiation then passes through the cooler, less dense layers of its atmosphere. The atoms present in this atmosphere absorb specific wavelengths: the spectrum observed from Earth is therefore a spectrum of absorption lines superimposed on a continuous background.

Identifying chemical elements

By comparing the positions of the dark lines observed in a star’s spectrum with the known lines of chemical elements (measured in laboratories on Earth), astronomers can identify which atoms are present in the star’s atmosphere: hydrogen, helium, sodium, calcium, iron, etc. This is how helium was discovered in the Sun’s spectrum in 1868, even before it was detected on Earth.

Other information derived from the spectrum

A star’s spectrum also provides information on:

• its surface temperature (the dominant colour and the shape of the continuous spectrum),
• its speed relative to Earth (due to the shift in the spectral lines, the Doppler effect: lines shifted towards the red if the star is moving away, towards the blue if it is approaching).

Practical example

The Sun’s absorption spectrum shows dark lines known as Fraunhofer lines, named after the German physicist who catalogued them as early as 1814, long before their physical origin was understood.

Common pitfall

A spectral line spectrum says nothing about the total amount of matter in a star; it mainly provides information about the nature of the elements present in the outer layers through which the light has travelled, not about the composition of the star’s core.