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The Sun

some facts

[Sun]

The Sun is a fairly average Star, in size, age and composition. Its mass is 1.991030 kg. The Sun loses about 4109 kg/s by radiation and about 1109 kg/s by solar wind. The total electromagnetic radiation (called the solar luminosity) is 3.851026 W. This equates to a mean 1370 W/m2 impinging on the Earth.

The radius of the Sun is 6.96108 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. 5505C).

[Black-body radiation curves]

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:

  1. 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;
  2. A D Thackeray, Astronomical Spectroscopy, Eyre & Spottiswoode, London (1961).

First published on the web: 15 December 1999.

Author: Richard Payling