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Spectra of the Stars

[Stars in Space]

The most noticeable feature of the Sun and Stars is that they radiate energy into space. The steady output from many stars over long periods indicates that the energy is being supplied from within. For the Sun, the fusion of H into He is providing most of its radiated energy.

Four important properties for understanding a star are its mass, radius, composition and age. Most of our information about stars comes from their radiation, though the rotation of binary stars can be used to determine their masses.

The total light emitted by a star can be used to estimate its size and surface temperature. 'Normal' stars have surface temperatures between 2000 K and 40 000 K.

The presence of characteristic emission and absorption lines indicates the presence of these elements in the outer layers of the star. Helium, for example, was discovered in the Sun's spectrum before it was even discovered on Earth

[Galaxies]Most of the radiation from stars, and hence most of the information we receive, comes from the outer layers of the stars. For most stars the composition of the outer layers is similar to their composition at birth. Heavier elements are thought to be formed during the life of the star from successive nuclear reactions building heavier and heavier elements, though they may have been formed much earlier in the life of the Galaxy.

Most stars are about 90% H, and other than He, have similar contents of heavier elements, notably Fe, Mg, Ca, C, etc.

Varied star clustersStars are given a Harvard classification according to the dominant element found in their spectra and the classes are ordered according to their surface temperature. The dominant element in the spectrum need not be the dominant element present in the star because of differences in the strengths of emissions lines and emission temperatures.

There are seven classes:

O B A F G K M

often remembered as: "Oh Be A Fine Girl/Guy Kiss Me". Each class is divided into 10 subclasses, labelled 0-9.

The first class is type O with a surface temperature of about 40 000 K and ionized helium. The hottest known classified star is an O4 star at 40 000 K. The coldest is an M8 star at 2500 K. The Sun is a type G2 star, typically with surface temperatures of 6000 K and ionised calcium.

There are hotter stars, e.g. 'white dwarfs' with temperatures around 100 000 K, but their high pressure means few lines are detectable. And there are colder stars, called 'brown dwarfs', but these have properties somewhere between planets and true stars.

Between the stars in our galaxy, and largely confined to the galactic plane, are mixtures of atomic and molecular clouds and intercloud medium. The clouds have temperatures of 15 K to 100 K, while the less dense intercloud temperature is about 8000 K.

Doppler broadening of emission and absorption lines (caused by atoms moving relative to the observer) provides a means for studying the thermal motions of atoms in stars. Doppler shifts of the lines can be used to determine stellar velocities. Their velocity in a line with the Earth is called their 'radial velocity'. Pressure broadening of lines gives a measure of surface gravity and Zeeman splitting of lines provides information on magnetic fields in stars.

For a description of Galaxies and a fine picture of the Sun, check at Research Institute of sustainable Energy "http://www.rise.org.au/info/Res/sun/index.html/"

For references and further reading, see:

  • R J Tayler, The Stars: their structure and evolution, Cambridge University Press, Cambridge (1994), pp 8-46;
  • D Emerson, Interpreting Astronomical Spectra, John Wiley & Sons, Chichester (1997), pp 139-186, 253, 274;
  • C R Kitchin, Optical Astronomical Gordon and Beach, New York (1970).

First published on the web: 15 December 1999.

Author: Richard Payling