Spectra of the Stars
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
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
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
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.
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.
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