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The Question

(Submitted October 28, 2001)

Is hydrogen necessary for the creation of stars. Can stars be born out of the fusion of heavier elements? What happens when molecular hydrogen in the Universe is exhausted or get to be very rare ?

The Answer

Thank you for your question. In principle, since any element lighter than iron will produce energy through fusion one could make stars out of helium, or carbon, etc. However, the heavier the element (hydrogen being the lightest) the higher the central temperature -- and therefore the higher the total stellar mass -- required for fusion to proceed. This means that the lower limit for an object to be a star would increase greatly from its present value of about one-tenth the mass of out Sun.

There is a certain amount of recycling that occurs in that stars will shed some fraction of their hydrogen during their lifetimes, and this material can eventually form new stars. But this recycling is not 100% efficient, so that in the far future the available hydrogen will indeed run out and star formation will essentially cease to occur.

-- Michael Loewenstein and Amy Fredericks for "Ask an Astrophysicist"

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