The Question

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

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 by number 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 by mass, 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 by mass in Sun-like stars to 0.1 to 0.01 percent by mass 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 (scientists call this process "nucleosynthesis"). 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. This process is called nucleosynthesis.

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.ohio-state.edu/~phillips/bang/bang.html) - college level introduction with links to research papers and new results.
• "Stars" (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) - an online textbook with a number of topics in astronomy, including stellar evolution and nucleosynthesis in stars, about high school level

Regards,
Padi Boyd and the Ask an Astrophysicist team

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