Do stars made by universe getting smaller or larger in mass? Do stars made by universe getting smaller or larger in mass compared to stars made 180 million years after the Big Bang? If they are getting smaller, is this due to the fact that the universe is expanding and the interstellar mass that made stars is getting less denser?
The size of stars depends on the amount of non-hydrogen elements (&quot;metals&quot;) affecting the opacity of the gas that forms them. Metals make it easier to radiate away heat when a gas cloud is collapsing, making it easier for a large amount of gas to accrete (although it may mainly act by increasing the minimum mass of stars that form). There is a fair disagreement about whether the first stars (&quot;population III&quot;) were very massive (hundreds of solar masses) or actually small, but I think the rough consensus is that very massive stars were likely. Once they emerged and died, they added metals to the interstellar medium and star formation eventually became more like the present.
The expansion of the universe does not have much effect once gas become trapped in dark matter halos and start to form galaxies.
Once the star has formed opacity and mass determines its structure and luminosity. It turns out that for increasing metallicity the maximum stable mass of a star decreases (section B.2). For the eventual metallicity at the end of the stelliferous era Adams and Laughlin estimate a maximum mass star as only 30 solar masses.