Algol consist of a 3.7 MSun main-sequence star and a 0.8 MSun subgiant. Why does this seem surprising, at least at first?
A) The two stars should be the same age, so we'd expect the subgiant to be more massive than the main-sequence star.
B) The two stars in a binary system should both be at the same stage of life; that is, they should either both be main sequence stars or both be subgiants.
C) It doesn't make sense to find a subgiant in a binary star system.
D) A star with a mass of 3.7 M Sᵤn is too big to be a main sequence star.

Question

Quizplus · Accepted Answer

In a binary system, both stars are formed at the same time. Normally, the more massive star evolves faster and becomes a subgiant first. Hence, it is surprising that the less massive star is the subgiant.

Algol Consist of a 3