Early galaxies experienced "growth spurt"

One question I've always had was how do astronomers know how quickly a star is forming by just looking at a specific star for as an example let's say 2 months,.. a star takes millions of years to form and they can look at it for only a couple of months and be able to say that it is forming quickly. I understand how they could tell what stage it is in,but how did they figure out how quickly the star reached this stage. Because if you were to stare at a star for a thousand years from a human timescale nothing about it will change. How do they see millions of years of changes in 2 months. I'm not... [More]

By looking at it composition and knowing what different stars are made of and what stars live longer than others so in the same way they can tell how rapid its spending its life, Regards; TFF

One question I've always had was how do astronomers know how quickly a star is forming by just looking at a specific star for as an example let's say 2 months,.. a star takes millions of years to form and they can look at it for only a couple of months and be able to say that it is forming quickly. I understand how they could tell what stage it is in,but how did they figure out how quickly the star reached this stage. Because if you were to stare at a star for a thousand years from a human timescale nothing about it will change. How do they see millions of years of changes in 2 months. I'm not... [More]