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The Question
(Submitted January 01, 1998)
At a recent visit to the Pacific Science Center in Seattle Washington, I
attended a Planetarium presentation in which we learned about the life
cycle of a star. The guide explained how a star will expand as the gases
burn up and change (I might not be describing this very well). I asked
whether the weight of the star changes as it expands. The guide did not
answer my question -- I don't think he knew. Also: does the sun get
lighter (in weight) as it burns up?
The Answer
What you're describing is the change of a star from the 'main sequence' to
the 'red giant' stage. (See, for example,
http://imagine.gsfc.nasa.gov/docs/teachers/lifecycles/LC_title.html
for further explanation.)
A star will lose mass in two different ways:
(1) Nuclear fusion --- when 4 hydrogen nuclei are combined to form a
single He nucleus, about 0.3% of the original mass is converted into
energy. However, this is an extremely slow process and much less important
than:
(2) A star expels matter in the form of a "stellar wind". Although it
happens to all stars to some extent (including our Sun), and can be
spectacular for some stars at certain stages of their life cycle,
it's not particularly strong for a star changing from a main sequence
star to become a red giant.
Best wishes,
Koji Mukai
for "Ask a High-Energy Astronomer"
p.s. We use the term mass to describe what I think you're asking about.
You weigh much less if you are standing on the Moon instead of here
on Earth, but your mass would be the same --- so the mass is a much
more fundamental quantity than the weight.
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