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
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
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.
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