When does a star become a main sequence star?

A) A star with a mass of 15 solar masses is too big to be a main-sequence star. B) The two stars should be the same age, so the more massive one should have become a giant first. C) The two stars in a binary system should both be at the same point in stellar evolution; that is, they should either both be main-sequence stars or both be giants. read more

Based on its main-sequence turnoff point, the age of this cluster is about 10 billion years. The following questions refer to the sketch below of an H-R diagram for a star cluster. read more

Eventually, a main sequence star burns through the hydrogen in its core, reaching the end of its life cycle. At this point, it leaves the main sequence. Stars smaller than a quarter the mass of the sun collapse directly into white dwarfs. read more