Astronomy

Astronomie Stars and their spectra

The Sun: some facts

The Sun is a fairly average Star, in size, age and composition. Its mass is 1.99×10³⁰ kg. The Sun loses about 4×10⁹ kg/s by radiation and about 1×10⁹ kg/s by solar wind. The total electromagnetic radiation (called the solar luminosity) is 3.85×10²⁶ W. This equates to a mean 1370 W/m² impinging on the Earth.

The radius of the Sun is 6.96×10⁸ m and is increasing by about 24 mm/year. The mean density of the Sun is 1.41 kg/m³, or about 700 times less than water. It consists of 72% H, 27% He and 1% of other elements.

The Sun's brightness has an absolute magnitude of 4.74. Magnitude is a logarithmic scale where an increase of 5 corresponds to 100 fold decrease in luminosity. The eye can see stars with apparent magnitudes between 0 and 6, with 0 being the brightest. Apparent magnitudes of stars are converted to absolute magnitudes by correcting for the distance of the star from the Earth.

Luminosity is proportional to the area of the solar disc and the fourth power of the effective surface temperature (Stefan-Boltzmann equation). Since we know the luminosity and area of the Sun we can calculate its effective surface temperature as 5777 K (i.e. 5505°C).

The Sun: its spectrum

Most of the Sun's radiant energy is in the visible and near infra-red. Over this region it closely resembles the radiation from a black-body with a temperature of 5777 K, peaking at 480 nm (blue-green) but with many superimposed absorption lines, including H at 656.3 nm (red). The ultraviolet spectrum is dominated by absorption lines down to 150 nm, known as Fraunhofer lines and include H, Na, Fe, Mg, Ca and Al. Below 150 nm the spectrum is dominated by emission lines, especially from H at 121.6 nm.The Sun appears yellow because our eyes have a peak sensitivity near that of the Sun's peak intensity. The Sun is redder at its outer limb than in its centre because the temperature is lower there. This variation provides information on variations in the interior atmosphere of the Sun.

The visible spectrum comes from the photosphere, the apparent outer layer of the Sun. Here free electrons from ionized metal atoms collide with H to form H^-, but the H ion is short lived as it then collides with high energy photons from the interior of the Sun. The result is a nearly continuous spectrum.

Outside the photosphere is the chromosphere and then the corona, each regions of increasingly rarefied gases. It is in the beginnings of the chromosphere where much of the absorption occurs giving rise to the absorption lines in the observed spectrum from stars. The existence of these absorption lines in the spectra from many stars suggests that they too have a photosphere.

[Eclipse] During a total eclipse of the Sun, as the photosphere disappears for a few seconds, the observed absorption lines change momentarily into bright emission lines from the chromosphere.

For references and further reading, see: M Stix, The Sun, Springer-Verlag, Berlin (1989), pp 1-12; C Emiliani, The Scientific Companion, John Wiley & Sons, New York (1988), pp 41-51; A D Thackeray, Astronomical Spectroscopy, Eyre & Spottiswoode, London (1961).

First published on the web: 15 December 1999.

Author: Richard Payling

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Spectra of the Stars

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 clusters] Stars 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 "http://www.hme.co.uk/samples/"

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

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