By what different methods (there are at least 2) do we infer that globular clusters...?

Question

By what different methods (there are at least 2) do we infer that globular clusters are the oldest things in our galaxy? How do we infer that objects in the spiral arms are the youngest members of the galaxy?

Answer 1

Two things that tell us the globulars in our galaxy are very old are the main sequence cutoff and their low metallicity. Conversely, high metallicity and the presence of type O and B stars, along with the presence of T Taurii stars, protostars and such, tell us that stars are still being born in the spiral arms.

Answer 2

1. Most stars in globular clusters are population II type stars (..low in metalicity..)

2. Little if any new-star formation is observed in globular clusters, meaning that all of the ingredients for new-star formation have already been used up.

There are many Population I stars (..young, blue stars..) in spiral arms, plus there are lots of nebulae where new-star birth can and is occurring.

P.S.
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Answer 3

Hi. By the star types observed in them. Answer 2, By the star types observed in them.