Spectroscopy

Once we can split light up into its component wavelengths we can analyse those wavelengths in order to extract information abut the source of light and the space the light has travelled through.

The most familiar form of spectrum is the continuous spectrum. This is produced by hot, condensed matter such as the walls of a furnace or the surface of a star. All the colours of the rainbow can be present, but in differing amounts, depending on the temperature of the source. The point at which the spectrum emits the greatest amount of radiation allows us to determine the temperature of the source.

Cool stars produce most of their light in the infra-red with some in the red end of the visible spectrum and little towards the blue end. As a result these stars appear red to the eye. Examples are Aldebaran (a red giant in the constellation of Taurus) and Barnard's Star (a red dwarf, actually the second closest star to the Sun but so dim that it is not visible to the naked eye).

The hottest stars produce most of their light in the ultra violet and far more in the blue end of the visible spectrum than the red end. As a result they appear blue to the naked eye. An example is Rigel (a blue super giant, 150,000 times greater than the Sun, the brightest star in the constellation of Orion).

The Sun itself has a spectrum that peaks in the green part of the visible spectrum. In spite of this the additional light from the red and blue ends of the spectrum makes the Sun appear yellow.

The following spreadsheet allows students to investigate the effect on the continuous spectrum as the temperature of the star is changed. Students enter the temperatures the wish to investigate and press F9 in order to recalculate the sheet. Peak wavelengths and power output per square metre are also calculated.

Black Body Radiation Curves

When matter is less compressed, as in a gas, the individual atoms can stamp their identities on the spectrum. In a hot gas the individual electrons in atoms can gain energy from collisions and then release it again as light. As the electrons can only occupy very specific energy levels the light that they can emit is equally limited. This gives rise to an emission spectrum; a series of bright lines set against a dark background.

If the gas is cool and light with a continuous spectrum is shone through it, then the electrons will pick out light with the energies that they can absorb. This gives rise to an absorption spectrum, with dark lines set against a bright, continuous spectrum. With this we can extract information about the material that the light has passed through, rather than the source of the light itself.

The following simulation program allows students to study both absorption and emission spectra in gasses and so carry out a chemical analysis of the gas. The early samles are simple, single elements. The later ones are more complicated as they have up to three different elements.

Outside of astronomy, few people realise that the temperature of the gas also affects the appearance of the spectral lines. If the gas is hotter then electrons will be sitting in different energy levels and so will emit and absorb different wavelengths. If the gas is sufficiently hot then it can become ionised, losing one or more electrons. This will change the energy levels themselves and so produce a totally different spectrum. Close by a black hole the accretion disc (where material is spiralling in to the black hole) can get so hot that iron can easily lose 25 of its 26 electrons. The spectrum of this iron will look nothing like the iron spectrum that we are familiar with.

In addition to temperature and chemical composition the spectrum of a body can also be used to determine gas pressure, magnetic field strength and of course velocity through the Doppler Effect.

 

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