Would you expect metal to be more abundant in old stars or in new stars?

Answer 1

This is an excellent question!

All the heavier elements past hydrogen and helium (and traces of lithium, beryllium, and boron) are NOT as old as the universe. Instead they have been synthesized inside stars; it is energy from nuclear fusion of light elements into heavier ones that powers stars. Stars on the main sequence, like the Sun, take hydrogen and fuze it into helium; once the Sun has sun out of hydrogen in its core it will begin turning helium into carbon, nitrogen,and oxygen, and become a red giant.

Our sun will not evolve past burning helium into carbon, but higher-mass stars will continue to synthesize heavier and heavier elements, and explode as supernovae once they have converted a mass of more than 1.4 times the mass of the Sun into iron. Pull out a Periodic Table; the elements up to iron are made in stars, and release energy when they are made. Everything past iron costs energy to make, and so is only produced in supernovae, and in the neutron-capture reactions in a special type of red giant star called an asymptotic giant branch (AGB) star.

When a high-mass star explodes as a supernova, it throws most of its mass back in to interstellar space. The same is true of AGB stars, which puff off their outer layers, forming so-called planetary nebulae. In either case, there are heavy elements (metals) being put in the interstellar medium. New stars are formed out of the gravitational collapse of cold gas and dust clouds which are just regions of denser interstellar medium. So, the heavy elements made in old stars get recycled into new stars! We know that the material that makes up the Earth was once inside a massive star; our solar system is 'recycled' from several previous generations of massive stars. Old stars formed much earlier in our galaxy's history are made of material that hasn't been recycled through as many generations of stars, so these stars contain less metals. We call these old stars 'Population II" stars, while metal-rich stars like our Sun are "Population I". There is active research trying to find primordial "Population III" stars, which are first-generation stars that contain no material that has been through a supernova.

By they way, note that this means YOU are made out of stars! They hydrogen in you is from the beginning of the universe ~13 billion years ago, while the carbon, oxygen, nitrogen, calcium, phosphorous, potassium, etc. are all from the interior of long-dead stars.

Answer 2

Following the Big Bang, all stars originally were composed only of hydrogen, and as nuclear fusion progresses, heavier elements are gradually cooked up. The longer this process continues, the more heavy elements there will be. Newer stars wind up having the most metal content, because they form out of older interstellar gas clouds, which have been enriched through the explosion of older stars. So, the older content, in the form of metals (and other heavier elements) is paradoxically found in the newer stars.

Answer 3

New stars