Since we observe a number of very old, Methusalah-type Stars in the Milky Way and Andromeda and nearby galaxies, would we expect to be able to observe similar stars in far away galaxies, if only our instruments were sensitive enough to discern such stars in such far away galaxies?
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1$\begingroup$ Milky Way's hardly exceptional, so I don't get your point. $\endgroup$Mithoron– Mithoron2025-11-29 21:00:25 +00:00Commented yesterday
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$\begingroup$ Sorry Mithoron, you're right, I was getting at if Methusala was roughly 13+ billion years old, but we could identify a similar star in a galaxy 10 billion light years away, or even 5 billion light years away, that would challenge our idea of a 13.8 billion year old universe. So I was wondering if such a star could only be observed locally, and never in a far away galaxy, even if we had good enough instruments to detect such a star so far away, because there wouldn't have been enough time yet for a star to get that old so long ago. $\endgroup$mdswartz– mdswartz2025-11-29 21:39:09 +00:00Commented yesterday
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1$\begingroup$ Indeed, and many very distant galaxies are entirely composed of metal poor stars. However, as light will have taken many billion years to reach us, we see these "methusula stars" when they were still quite young. We don't see evidence of very old stars in very distant galaxies, for the reason you imply $\endgroup$James K– James K2025-11-29 21:39:55 +00:00Commented yesterday
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$\begingroup$ Thank you James K, what you've said sounds right, but I've read articles about discovery of outlier galaxies at high redshift, metal-rich like the Milky Way, so I decided to ask the question, even though the answer is seemingly self-evident. What if we could discern individual stars in those outlier high redshift metal-rich galaxies, what might we see? No answers yet. $\endgroup$mdswartz– mdswartz2025-11-29 22:17:56 +00:00Commented yesterday
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1$\begingroup$ Probably. The Milky Way is a pretty old galaxy so it naturally has a population of older stars, but I don't see why a similarly aged galaxy couldn't have similarly aged stars. Of course, this would ONLY be the case if we were to observe one in real time. If you studied Methuselah from 5 Gly away, it would be seen as ~8 billion years old due to the travel of information across distances. But others have explained this $\endgroup$4NT4R3S– 4NT4R3S2025-11-30 03:34:41 +00:00Commented yesterday
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2 Answers
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The only stars that can be resolved and identified in galaxies outside the local group, are massive, luminous and young.
Would we expect to see 12-13 billion year old lower mass stars in distant galaxies? No, because the universe isn't old enough.
However, even if one were to hypothesise that our current best estimate of the age of the universe is a large underestimate and such "Methuselah stars" were then a possibility, one would have to explain where they all went in the local group. Low mass "Methuselah stars" can be very long lived, so if there are 12 billion year old stars in galaxies that are 5 billion light years away then there should be 17 billion year old stars found locally (and there aren't).
I suppose one could argue that star formation started in our Galaxy 12 billion years ago so there obviously couldn't be 17 billion year old stars in it. However, there is no evidence for significantly earlier (as in billions of years earlier) star formation anywhere we look.
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No, there is lots of evidence that the universe is about 13.8 billion years old and the oldest stars formed shortly after that time. Evidence for this includes the lack of very old, cold white dwarfs, and the mass distribution of population II stars. It follows that if we were able determine the age of stars in a galaxy at a light-travel distance of 10 billion light-years, then the stars would all be less than 3.8 billion years old. There would be no old Methusela-type stars in that galaxy, there might be lots of young(ish) metal-poor stars.
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$\begingroup$ Thank you James K, I was also wondering about extremely low mass, long cooled white dwarfs that didn't experience tidal stripping of material in a tight binary system, indicating great age, but it seemed that even if such a star could exist, it couldn't be detected, and I know we haven't detected any despite detection of a number of cold white dwarfs. I agree with your answer but accepted ProfRob's answer as the most correct. $\endgroup$mdswartz– mdswartz2025-11-29 23:26:06 +00:00Commented yesterday