Do binary or triple star systems form together?
Can binary or triple systems have stars with different ages?
Although it is possible that two stars which formed individually encounter one another in such a way that they form a binary, this scenario is highly unlikely. It is therefore accepted that stars in binary systems formed at roughly the same time. That is, they formed from the same collapsing cloud of cold gas (we think, in fact, that about half of all stars born are born in a binary in this manner). So it is safe to say that two stars that exist in a binary were formed at the same time.
However, this does not mean that stars in a binary are necessarily at the same stage in their evolution. The reason is that the lifetime of a star is governed almost entirely by its mass at birth: stars with higher masses age more rapidly than stars with lower masses. So, although both stars in a binary are born at the same time, they can be in very different stages of evolution (one may be burning hydrogen to helium, and the other one might already be a white dwarf!) when we observe them.
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