Finding various ageing stars from local galaxiesWatch
I have some knowledge but it feels quite half arsed and I'm interested in knowing more. I know young stars characteristically burn hydrogen. I know that the heaviest metal you can form in stars is iron (anything higher requires energy added in).
Why do some papers have various plots of hydrogen, magnesium and iron spectral lines against each other?
Why is just looking for hydrogen spectral lines not enough?
I understand metalicity changes as stars form but why do people need to "decouple metalicity from age"? How would you?
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the earliest division of stellar populations made by astronomers was into population I and population II :
pop I - high metalicity
pop II - low metalicity
the system has been expanded and refined since
confusingly we now think pop I is young stars and pop II is old stars (I think this is the opposite of what they had in mind when they came up with the naming convention)
Basic principle is that the amount of metal in the universe always increases over time because stars work by turning H into heavier elements which are then blown off them at the end of their life - therefore the more metal a star contains, the more recently it formed.
I'd advise you to search on stellar populations and metalicity e.g. http://abyss.uoregon.edu/~js/ast122/lectures/lec26.html