Let us begin with a simple description of spectra. Light is radiated in waves of many different lengths. The eye rather crudely distinguishes the different wavelengths as colours - long waves are red, and short waves are blue or violet. When we look at a luminous body, the eye receives a beam of composite light - many different colours, mixed in different proportions. However, if the beam of light passes through a glass prism, or other suitable device, the individual rays are bent in different degrees, depending on the wave-length, and the colours are spread out in an ordered sequence called a spectrum. The rainbow is the familiar example.
The sequence never varies. From the long waves of the red, the wave-lengths steadily diminish to the short waves of the violet. The spectrum may be long or short, depending on the apparatus, but the relative positions in the sequence remain unchanged. Position in the spectrum indicates the wave-length of the particular light in question; relative brightness at the position indicates the relative abundance of the particular wave-lengths in the composite radiation. Therefore, a spectrum furnishes valuable information concerning a distant light-source because it indicates the particular colours that are radiated, and their relative abundance.
For instance, an incandescent solid, such as an electric light filament, radiates all possible colours; the spectrum is continuous from red to violet, and beyond in either direction. The relative abundance of the various colours measures the temperature of the light-source.
Again, an incandescent gas, such as a neon sign, radiates only certain particular colours. The spectrum, known as an emission spectrum, is a pattern of isolated colours separated by dark gaps. The pattern is characteristic of the particular gas involved, hence an emission spectrum serves to identify the chemical composition of a distant, inaccessible light-source.
There is still a third type of spectrum, and it is of especial interest for our immediate purpose. When an incandescent solid, or equivalent source, giving a continuous spectrum, is surrounded - by a cooler gas - for instance, a star with its surrounding atmosphere - then the gas absorbs from the continuous spectrum of the background just those colours which it would emit if it were itself incandescent and isolated.
The result is called an absorption spectrum. It presents a fairly continuous background, interrupted by dark gaps or lines which represent the colours absorbed by the superimposed gas. The distribution of intensities in the continuous spectrum indicates the temperature of the star, and the pattern of dark absorption lines identifies the gases in the stellar atmosphere.
Absorption lines in the spectrum of the sun indicate the presence of many or most of the elements known in the laboratory. Lines due to metal vapours are predominant, the most conspicuous being a close pair of calcium lines in the violet region, known as the H and K lines. Other conspicuous lines are due to iron and to hydrogen.
Now the spectra of nebulae are very similar to the spectrum of the sun., Stellar systems, it appears, are dominated by yellow dwarf stars like the sun. The nebulae in general are so faint that their light can be spread only over very short spectra. Nevertheless, even on the small scales, the H and K lines of calcium are strong and unmistakable, and the more conspicuous lines of iron and hydrogen can also be identified.