Earlier in the week, a question was raised in one of the astronomy discussion forums I monitor.
"Can large ground-based telescopes or the Hubble Space Telescope resolve stars in such distant galaxies that those stars are likely to not exist anymore?"
This is actually several questions lined up in a deceptively simple package. How far away (or back in time) can HST or any large telescope image individual stars in distant galaxies? What kind of stars would we be able to see? In the course of normal stellar evolution, would these stars live, evolve and die in the time it has taken their light to reach us from the galaxy they reside in?
A few people chirped in with the suggestion that supernovae can be seen at great distances, but supernovae is kind of a cheat answer to the question, because we are seeing the results of a catastrophic explosion, not really an individual star. True, we can see supernovae in galaxies billions of light years distant, and obviously they no longer exist in their nascent form since they went supernovae. But, I don't think that is what was being asked.
Cepheid variables have been used to measure distances to galaxies by HST and ground based telescopes. These tend to be yellow giant and super giant stars 40-180 times the radius of the Sun, and thousands of times more luminous, so we can see them from millions of light years away. Before HST we were able to measure Cepheid variables in galaxies out to about 12 million light years. But what about now?
Pamela L. Gay, PhD astronomer at SIUE said, "I'm not sure with it's newest instruments, but the Hubble Key Project observed Cepheids, and they went out as far as 20 million parsecs."
20 million parsecs is about 65 million light years. So Hubble can measure individual Cepheids at least that far.
Above, are 1999 images of a Cepheid variable in M100 at a distance of about 50 million light years. You can see it at minimum, in between and at maximum light.
Luminous Blue Variables (LBVs) are hypergiant stars with masses approaching the theoretical limit for stars (150 solar+). They can shine across vast cosmic distances.
These are short lived stars as stars go, because they are so massive. The more massive a star is, the brighter it will shine, but also the faster it will burn itself out. Faint red dwarfs may live longer then the Universe is old, slowly and miserly burning their interior elements for tens of billions of years. LBVs may only survive a few million years.
Doug Welch, PhD in Physics and Astronomy at McMaster University said, "We can see such stars in the Coma cluster of galaxies which is about a third of a billion light years away, so I'd say the limit is about 1 billion light years with existing technology."
1 billion light years! Certainly, any LBVs we can see a billion light years away no longer exist today. The further back in time we look, the higher the probability the individual stars we CAN see in far off galaxies are no longer there, since they tend to be the "live fast, die young" animals in the stellar zoo.
Al Holm, PhD, Branch Chief for Data Processing and Archival Services at the Space Telescope Science Institute in Baltimore, confirmed what I had learned. Al wrote, "By 1999, Cepheids had been observed to a distance of 65 million light-years. Our instrumentation has improved since then, but I'm not sure what has been done to extend the distance scale to more distant galaxies.
Looking at the question in another way, Tom Brown recently used the ACS to take deep exposures of stars in the halo of M31. His plots show stars down to about 31.5 mag. In theory, at that apparent magnitude you might be able to see Eta Carina in its bright phase out to a couple billion light-years. The problem would be blending of images at that distance. To be seen, the star would have to be well away from any neighbors, but LBVs are young and embedded in clusters so they would fail that criterion. "
We've come full circle in our investigation at this point. Eta Carina is expected to go supernova in a million years or less. So not only can we image supernovae in distant galaxies, we may be able to see LBV supernovae progenitors like Eta Car at great distances!
No matter what instrument is used, the star needs to be in an uncrowded region of the galaxy in order to be imaged cleanly, and it can't be embedded in nebulosity or be a member of a cluster.
The answer then is, given the best possible conditions, with Hubble's modern equipment, we can see individual massive bright stars out to a billion light years or more. Since these massive stars tend to live for millions of years, not billions of years, massive stars visible at great distances have probably lived and died in the time it took their light to get here from there. Or, as Doug said, "ANY stars bright enough to be seen at 100's of millions of years distance are "currently" toast."