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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,

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 an Astrophysicist"

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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