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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 a High-Energy Astronomer"
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