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The Question
(Submitted October 20, 1997)
Does the chemical composition of a star have
an effect on its luminosity and its life span? If so, why?
The Answer
The chemical composition of a star affects its evolution in
ways that most high-energy astronomers rarely worry about,
although low-energy astronomers who work with ordinary stars
have more experience with this.
Metals, (elements heavier than Helium), tend to be more opaque
than hydrogen and helium. This is because, being more complex,
they have more absorption lines in their spectrum, and these
lines represent opportunities for an atom to stop photons.
(This is extremely oversimplified.)
This makes high-metallicity atmospheres more opaque, so the
photons are trapped longer in the interior of the star, increasing
the temperature of the core and fluffing the star up to a larger size
for any given total energy production.
One example of this is that Cepheid variables have a different
period-luminosity relationship for Population I (Sunlike metallicity)
and Population II (lower metallicity) stars. This was the
cause of one of the sudden changes in our understanding
of the size of the Universe, when it was realized that the Magellanic
Clouds (mostly Pop II) were at different distance than
the calculated values based on Cepheids in our neighborhood (Pop I).
Another possible example is that Supernova 1987A in the
Large Magellanic Cloud was a blue supergiant before it went
off. It may be that if the 1987A precursor were Pop I instead
of Pop II, it would have been a red supergiant at the end of
its life. Red supergiant supernovae tend to be brighter than
blue supergiant supernovae, due to their greater initial size (and thus
radiating area). Since the red ones are so much more visible than the
blue ones, most people thought that this type of supernova
(Type II) occurred only in red supergiants. When
I was a lad (~1982), I learned that Type II supernovae were mostly
found among Pop I stars--an observational bias that was only
generally recognized after 1987A.
David Palmer
for Ask an Astrophysicist
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