Spectral Lines

We are all familiar with the colours of a rainbow, a phenomenon seen when the sun shines during a shower storm. Shining through water droplets the light is split into the colours of the spectrum. It so happens that chemical elements also produce colours. Although the mechanism is different, the principal is the same.

Spectral  lines are produced by the transition of electrons within atoms or ions. As electrons move close to or father from the nucleus of an atom, energy in the form of light is emitted or absorbed.

The direct spectrum of light radiating from a body is called an “emission spectrum”. It is made up of bright lines. The positions of these lines depend on their frequency/wavelength. When electrons move closer to their nucleus by jumping from one orbit to the next inner orbit this is accompanied by radiation of a discrete amount of energy at a specific frequency. Seen in a spectroscope these lines occupy a specific position on the spectrum and display a specific colour.Spectral Lines of Elements

There is also an indirect spectrum of light, shown by the absence rather than the presence of a colour. In this case we talk of an “absorption spectrum”. When a gas absorbs radiant energy its electrons jump from inner to outer orbits and with each jump a discrete amount of energy is absorbed creating dark lines, again at specific positions, within the spectrum. These dark lines are called “absorption lines” and can be interpreted to show the chemical composition of a star.

To give a brief example, the absorption spectrum of hydrogen, formed when light passes through cold hydrogen gas, is identical to that of the emission spectrum of burning hydrogen. The electron changes are the same in both cases, except that they occur in different directions.

Taken together the emission and absorption features of a spectrum can be analysed and interpreted chemically. Each element has a unique spectrum, different from those of all other elements. It is like a signature. Read the signatures and you know the chief chemical components of a given star.

In addition to their chemical signature the lines you see in a given spectrum also indicate a star’s temperature.

As a footnote you may come across the term, “continuous spectrum”. This simply refers to the whole spectrum, unbroken colour from infra-red to ultraviolet within the optical region, or an unbroken emission band in the radio frequency region. Continuous spectra occur in stars in two ways:

  • Through the capture of free electrons by ionised atoms, emitting kinetic energy as radiation; or,
  • Through the release of a portion of kinetic energy (photon) by one atom passing another. This so-called free-free transition is the emission or absorption of radiation by an electron that is unbound to an atom before and after the event.

A continuous spectrum results since the kinetic energy of electrons may assume all values.

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By Nigel Benetton, science fiction author of Red Moon Burning and The Wild Sands of Rotar.

Last updated: Wednesday, 3rd March 2021