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The Question
(Submitted November 12, 1996)
I was looking on the Internet for information on the composition of
stars and was not finding any information. I looked at a site you
put together and thought perhaps you might know where I can find some
good information. If you have the opportunity I would appreciate
any help you might have.
The Answer
The area you are interested in, the composition of stars, is a very
important and interesting field for astronomers. It touches on many related
questions, such as how do stars form? How do stars shine? Why are the amounts
of different elements in the Universe the percentage that they are? It is not
easy to find WWW references simply on "chemical composition of stars", but if
you pick one of the related areas of interest to you then it is possible to find
some information about this on the WWW.
Stars begin their life when an ordinary dense cloud of interstellar matter
becomes unstable and begins to collapse. The composition of such a cloud of
matter determines the composition of the star which results from the collapse.
Let's start by looking at the chemical composition of the star we know the most
about, our Sun.
Astronomers study the spectrum of the Sun to determine it's chemical
composition. In the visible region alone, from 4000 to 7000 Angstroms
(10-10 meters),
there are thousands of absorption lines in the solar spectrum. These lines
have been cataloged, and tell us that there are 67 chemical elements identified
in the Sun. There are probably even more elements in the Sun that are present
in such a small amount that our instruments can't detect them. Here is a table
of the 10 most common elements in the Sun:
| Element | Abundance (% of total | | Abundance |
|---|
| number of atoms) | | (% of total mass) |
| Hydrogen | 91.2 | | 71.0 |
| Helium | 8.7 | | 27.1 |
| Oxygen | 0.078 | | 0.97 |
| Carbon | 0.043 | | 0.40 |
| Nitrogen | 0.0088 | | 0.096 |
| Silicon | 0.0045 | | 0.099 |
| Magnesium | 0.0038 | | 0.076 |
| Neon | 0.0035 | | 0.058 |
| Iron | 0.030 | | 0.014 |
| Sulfur | 0.015 | | 0.040 |
You see that hydrogen is by far the most abundant element in the Sun,
followed by helium. Those two together make up 99.9 percent of the
total atoms in the Sun! This is also what we find in the composition
of the Universe as a whole.
When other stars are studied spectroscopically it is found that most stars
are composed of around 70 percent hydrogen and 28 percent helium, very
similar to what we see in the Sun. The fraction of all other elements,
the "heavier" elements, is small and varies considerably from 2 or 3 percent
in Sun-like stars to 0.1 to 0.01 percent in stars found in globular clusters.
We call those stars with very little heavy elements "population II stars" and
those with Sun-like heavy element abundances "population I stars". Theories
of stellar evolution state that the population I stars are a later generation
of stars, that formed after some enrichment of gas clouds between stars had
already taken place. That is because stars "burn" lighter elements into heavier ones during their lives. Right now, the Sun is burning hydrogen into helium
at it's center, or "core". This is the chain of nuclear fusion that powers the
Sun. The net effect is that four hydrogen nuclei combine to create one helium
nucleus, some gamma-ray radiation and two neutrinos. The gamma-ray photons
slowly lose energy as they pass through the solar interior, and the energy
eventually escapes in the form of visible light. The neutrinos escape
unhindered into space at the speed of light, and the helium stays in the core. Other stars, which have used up all the hydrogen fuel in their cores, burn
helium into beryllium and carbon. Massive stars that evolve beyond this point
then burn carbon into heavier elements, and so on.
During the later parts of their lives, stars can shed material into the
surrounding space, depositing heavy elements. The most dramatic way this is
done is through a supernova explosion. In fact, since the earliest moments of
the Universe, during the Big Bang, heavy elements have only been produced as a
by-product of stellar evolution! That's what astronomers mean when they say "we are all star-stuff."
Why is there so much hydrogen and some helium to begin with? This is
tied to our theories of the Big Bang. If the Universe started in conditions of
extremely high temperature, then the matter would organize in a way that there
were the most particles, or more simple elements like hydrogen and helium.
So, now after my long-winded introduction, here are the WWW sites I can
recommend:
- "Big Bang Nucleosynthesis Homepage" (http://www-physics.mps.ohio-state.edu/~bbn/research/bbn.html) (http://www-physics.mps.ohio-state.edu/%7Ebbn/research/bbn.html) - college level introduction
with links to research papers and new results.
- "Stars" (http://math.ucr.edu/home/baez/stars.html) (http://math.ucr.edu/home/baez/stars.html) - a fun
page put together by a student learning about stars, in diary form.
- "Astronomy HyperText Book" (http://zebu.uoregon.edu/text.html) (http://zebu.uoregon.edu/text.html)
- an online textbook with a number of topics in astronomy, including
stellar evolution and nucleosynthesis in stars, about high school level
I hope this helps in your search for information on the chemical
composition of stars using the WWW.
Regards,
Padi Boyd and the Ask a High-Energy Astronomer team
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