What is the current distribution of elements in the observable universe? Wikipedia lists the composition of the Milky Way, but I'm not sure how the values would change if intergalactic medium were included. My understanding is that the density of this matter has been determined, but not its elemental composition.
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4$\begingroup$ Hydrogen with some minor impurities... $\endgroup$– Jon CusterCommented Nov 9, 2022 at 17:05
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4$\begingroup$ @JonCuster Hydrogen and helium, with some minor impurities. $\endgroup$– David HammenCommented Nov 9, 2022 at 17:56
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1$\begingroup$ @DavidHammen - Good to 10%.... $\endgroup$– Jon CusterCommented Nov 9, 2022 at 18:04
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1 Answer
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The large majority of hydrogen from the big bang has not been turned into stars and this means the overall abundance is similar to primordial with a little bit more helium and a few tenths of a percent metals at most.
Fukugita & Peebles (2004) estimate that only 6% of the baryonic mass of the universe is in the form of stars and stellar remnants Most of it exists as a rarefied gas in between galaxies.
The majority of those stars are still alive and on the main sequence. Even if you look at the distribution of mass in stars, the median will still be in stars below a solar mass; so the total mass that has ever been in stars is only a little bigger than 6%.
Very low mass main sequence stars could turn all their hydrogen into helium eventually, but they are less than 10% through their main sequence lives. Solar mass stars burn quicker, but have radiative cores and cannot process more than about 50% of their material during their long lives. Stars of even higher mass can have lived and died, but are comparatively rare and only process about 20% of their material. Nevertheless, it is these stars that are responsible for enriching the interstellar medium.
Bottom line: Material in present day or previous stars forms about 10% of the baryonic mass of the universe. Stars born today have a metallicity of 2% by mass, 73% hydrogen and 25% helium. Stars born in the past have less metals, but may now have a bit more helium. Of the rest, maybe another 10% or so has been enriched to the same level as newborn stars by stars that have lived and died. The rest will be unprocessed gas from the big bang with about 76% hydrogen and 24% helium by mass.
Back of the envelope thinking then leads to the universe being close to the primordial abundances, but with a little more helium and maybe a few tenths of a percent of heavier elements, certainly much less than 1%.
In terms of the elemental mix of metals I think it will be similar to the mix in the current interstellar medium. Most of the mass in current stars will not have been processed into metals, only a bit more helium.
NB This answer assumes you want to know the abundances in the universe at the current epoch. If you want the observed abundances in the observable universe then the fraction of metals will be even lower since would be including younger, high redshift galaxies that are more metal-poor in general.
