How does the distribution of elements change over time?

[BWV]

Been reading basic stuff about the astrophysics behind the prevalence of various elements, curious at what rate this changes over time. The Milky Way is something like 14 billion years old. At first it was all H and He then heavier elements were created and distributed through supernovae.

Questions
a) How long did it take before the prevalence of the elements looked close to its current state? there was about 10B years between the formation of the galaxy and our solar system. Was the galaxy significantly different, say, 8B years ago? Is this maybe a potential answer to the Fermi Paradox? How about 10B years from now?

b) I had thought is was exclusively supernovae that created heavier elements, but the chart below on the wikipedia entry lists merging neutron stars as the most common source of heavier elements, how does that work? would have guessed that merging neutron stars, depending on the mass, either created a bigger neutron star or a black hole

 

[phyzguy]

On your question a), this is an active area of research. This paper is a recent study that goes back about 8 billion years. But since the rate of star formation was much higher in the past, I think even 8 billion years ago there were plenty of heavy elements about. The graph below (from the paper above) shows that some galaxies had metallicities( the measure astronomers use to measure elements heavier then He) that were as high as the Milky Way is today even 8 billion years ago.

On your question b), it's true that merging neutron stars generate either a heavier neutron star or, more usually, a black hole. However, when the two neutron stars merge, some of the neutron star material (a few percent) get ejected into space. This neutron rich material, relieved from the huge pressure it was under when part of the neutron star, evolves into a whole host of heavy elements. This was confirmed in spectacular fashion by the merging neutron star pair GW170817, which was detected in gravitational waves, optical, radio, X-rays, ... So the presence of ejected material was confirmed and this material was characterized in great detail. While it isn't certain, most astronomers believe that the heaviest elements are produced primarily in these neutron star mergers. This paper is a recent reference.

 

[BillTre]

a) How long did it take before the prevalence of the elements looked close to its current state? there was about 10B years between the formation of the galaxy and our solar system. Was the galaxy significantly different, say, 8B years ago? Is this maybe a potential answer to the Fermi Paradox? How about 10B years from now?

I like this question for the same reason.
The underlying question is when was the elemental diversity of the universe sufficient to support the chemical evolution of life. Then (from that time), start thinking of Fermi's paradox.
Beyond H, common life associated elements (based on current understanding (life on earth)) would include: C, N, O, P, S, along with things like Fe for catalyst components of enzymes.

 

[stefan r]

I like this question for the same reason.
The underlying question is when was the elemental diversity of the universe sufficient to support the chemical evolution of life. Then (from that time), start thinking of Fermi's paradox.
Beyond H, common life associated elements (based on current understanding (life on earth)) would include: C, N, O, P, S, along with things like Fe for catalyst components of enzymes.

I am not convinced. All five of those elements tend to float. Earth, still our only example of life, is very depleted in carbon and nitrogen. Type II supernovas generated most of the phosphorus but the ratio of type II to type I was much higher in the early universe. All of the alpha elements were in concentrations excessive for life long before there is enough material to form rocky planets. Enough of each element would float into a crust anytime you have enough metals to make a surface that is solid and also supports liquid water oceans on top of that surface.

Plate tectonics are driven in part by fission reactions. For that we needed mostly uranium, thorium, and potassium-40. The radiogenic heat also helped thaw thick glaciers and let life survive a snowball earth. Life on a rocky planet closer to a star may not need the radioactive heat but it will die when the star ages and the oceans evaporate. The convection in the mantle is also thought to generate Earths magnetic field. The magnetic field reduces the loss of the nitrogen and oxygen to space.