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The well-known star Algol in Perseus is an eclipsing binary composed of a
main-sequence star and a sub-giant. It was naturally assumed that
the sub-giant, being the more evolved star, is also the more massive.
It was found, however, that it is actually the less massive star.
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This cannot be explained in terms of single star evolution, but finds
a natural explanation in terms of mass transfer: the sub-giant was
indeed originally the more massive star, which expanded to fill its
Roche lobe and then transferred mass until it became the less massive
star.
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Algol is still semi-detached, as shown in the animation below and in
this animation
by
J. Blondin (NCSU), with the
sub-giant filling its Roche lobe and transferring mass at a modest rate
to its more massive companion.
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©
L. Molnar (Calvin College)
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