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The Question

(Submitted April 16, 2010)

I have been watching 'The Universe' series from the History Channel. After watching the episode on stellar evolution, I began to wonder. How much on the hydrogen in our own sun has never been in a star before, and how much is 'recycled' hydrogen from stars that previously lost their hydrogen by a supernova explosion? Or, how much of the hydrogen in the universe has never been in a star?

The Answer

This is an interesting question. It is known that most of the hydrogen now in the galaxy is within stars. One estimate I found (Gene Smith's Astonomy Tutorial: http://casswww.ucsd.edu/public/tutorial/ISM.html is that only about 5% of hydrogen in the galaxy is in the form of interstellar gas. That's within a galaxy. Outside of a galaxy, but within a galaxy cluster, the density of hydrogen is about a thousand times less, but clusters are millions of times larger than galaxies, so there is likely much more hydrogen in intergalactic gas than there is in stars.

Now, knowing how much of this hydrogen has never been in stars is a tougher problem. Within a galaxy it is likely that most of the gas is "recycled." We can infer this from the abundance of heavy elements in interstellar gas. In intergalactic space, it was thought that the gas was largely primordial, but astronomers have detected signs of iron in intergalactic gas, telling them that at least some of the gas was ejected by supernovae.

Cheers,

Hans Krimm and Kevin Boyce
for Ask an Astrophysicist

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