Description: In stellar astronomy, the Algol paradox is an apparently paradoxical situation when elements of a binary star seem to evolve in discord with the established theories of stellar evolution. A fundamental feature of these theories is that the rate of evolution of stars depends on the mass of the star: The greater the mass of the star, the faster this evolution, and the more quickly it leaves the main-sequence, entering either a subgiant or giant phase. In the case of Algol and other binary stars we can observe something completely different: The less massive star is already a subgiant, and the star with much greater mass is still on the main-sequence. Initially, this seems paradoxical as the partner stars of the binary are thought to have formed at approximately the same time and so should have similar ages. Thus the more massive star, rather than the less massive one, should have left the main sequence. The paradox is resolved by the fact that in many binary stars, there can be a flow of material between the two stars, disturbing the normal process of stellar evolution. As the flow progresses, the evolutionary stage of the stars will advance, even as the relative masses change. Eventually, the originally more massive star will reach the next stage in its evolution despite having lost much of its mass to its companion.