Hello.
The basic reason for this is that the sun's surface temperature is not
high enough to have strong helium lines. On the other hand, the sun's
surface temperature is sufficiently hot enough to produce strong
calcium lines.
source:
"...It turns out the temperature of the star's photosphere determines
what pattern of lines we will see. Because of this, we can determine
the temperature of a star from what pattern of absorption lines we see
and their strength.
.... Hydrogen lines will be strong for temperatures = 4,000 - 12,000
K. Helium atoms hang onto their electrons more strongly and,
therefore, require higher temperatures of 15,000 - 30,000 K to produce
absorption lines in the visible Calcium atoms have a looser hold on
their electrons so Calcium lines are strong for cooler temperatures of
3000 - 6000 K. The strengths of each element's absorption lines are
sensitive to temperature."
source: "Astronomy Notes: Types of Stars," hosted by University
College London:
http://www.star.ucl.ac.uk/~idh/STROBEL/starprop/strpropd.htm
As a "Class G" star with a surface temperature between 5000 K and 5999
K, the Sun is thus characterized by strong calcium lines and weak
helium lines.
Also see:
"Properties of the Principle Spectral Star Types"
http://astronomy.wakaf.net/galaxy/htm/stellar_classes.htm
"Stellar Spectra," hosted by Royal Astronomical Society of Canada
http://victoria.rasc.ca/articles/1996/art9611.html
search strategy:
"calcium lines are strong"
I hope this helps. |
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