For the most part, stars in binary systems proceed from birth to death as if they were alone. The exceptions are close binary stars. Algol, the “demon star” in the constellation Perseus, consists of two stars that orbit each other closely: a 3.7Msun main-sequence star and a 0.8Msun subgiant. A moment’s thought reveals that something quite strange is going on. The stars of a binary system are born at the same time and therefore must both be the same age. We know that more massive stars live shorter lives, and therefore the more massive star must exhaust its core hydrogen and become a subgiant before the less massive star does. How, then, can Algol’s less massive star be a subgiant while the more massive star is still burning hydrogen as a main-sequence star?
This Algol paradox reveals some of the complications in ordinary stellar life cycles that can arise in close binary systems. The two stars in a close binary systems are near enough to exert significant tidal forces on each other. The gravity of each star attracts the near side of the other star more strongly than it attracts the far side. The stars therefore stretch into football-like shapes rather than remaining spherical. In addition, the stars become tidally locked so that they always show the same face to each other, much as the Moon always shows the same face to Earth.
During the time that both stars are main-sequence stars, the tidal forces have little effect on their lives. However, when the more massive star (which exhausts its core hydrogen sooner) begins to expand into a red giant, gas from its outer layers can spill over onto its companion. This mass exchange occurs when the giant grows so large that its tidally distorted outer layers succumb to the gravitational attraction of the smaller companion star. The companion then begins to gain mass at the expense of the giant.
The solution to the Algol paradox should now be clear (Figure 12.23 above). The 0.8Msun subgiant used to be much more massive. As the more massive star, it was the first to begin expanding into a red giant. As it expanded, however, so much of its matter spilled over onto its companion that is now the less massive star.
The future may hold even more interesting events for Algol. The 3.7Msun star is still gaining mass from its subgiant companion. Thus, its life cycle is actually accelerating as its increasing gravity raises its core hydrogen fusion rate. Millions of years from now, it will exhaust its hydrogen and begin to expand into a red giant itself. At that point, it can begin to transfer mass back to its companion. Even more amazing things can happen in other mass exchange systems, particularly when one of the stars is a white dwarf or a neutron star. But that topic will come later on :)
-The Essential Cosmic Perspective (5th Ed.)