How did scientists determine sm0313 is the oldest star ever discovered?

i think it has to do with how old it looks, because a star starts off as a baby little star, a different color and size. its also in a nursery with a lot of other stars, which is called a nebula. as the stars grow older and larger, they change color and drift apart. the scientists must've seen that the star has a totally different color from the baby ones, way brighter and much larger, also seeing that stars grow apart, it has not really many stars living around it for miles and miles. they must've guessed that it was probably really old.

b

explanation:

thinking it would be d not too sure but i think it's right