the algol paradox back to top

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.
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.
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.
© L. Molnar (Calvin College)