It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million...
It’s not just probable; it’s a certainty! Most of the brightest stars have lifetimes that only keep them going for a few hundred million years. Even if the light caught in our detectors was generated in that star’s earliest days, by the time that light reached us, 13 billion years later, that star must be long gone. Our own Sun, which has a comparatively lengthy 8 billion year lifespan, wouldn’t have survived for the length of time it takes for the light from the most distant known galaxies to reach us.
What does that mean for the galaxy as we see it? For starters, it means that its population of stars has changed almost entirely since that light began its travels. The brightest stars have all died, exploding out to recycle their gas into their stellar neighborhood, and possibly triggering the formation of a new round of stars. In 13 billion years, this may happen many, many times. But the stellar recycling act isn’t perfect. For every large, extremely bright star that’s formed, we typically expect a number of smaller, fainter stars to also form. These smaller stars (like our Sun) live longer and don’t explode as violently (or at all) at the end of their lifetimes. Stars which are even smaller and redder than our own will live even longer, and may even persist for the entire length of the trip the light took to reach us. This means that over time, the galaxy will build up a reservoir of faint, reddish stars, which limits the amount of gas present in the galaxy available to make new stars. This sequence of events is one of the suggestions for how we end up with the giant elliptical galaxies in the nearby universe -- their population of stars is mostly red, and they seem to have very little gas.
But it’s not the only pathway open to that distant galaxy -- it’s also possible to refill the galaxy with gas, allowing the galaxy to continue forming the brightest, bluest, shortest-lived stars for a longer period of time. Depending on whether or not the galaxy finds itself surrounded by smaller, gas-filled galaxies, or on its own in a more lonely part of the Universe, that distant galaxy’s course will change again. If we could sit and watch that distant galaxy’s evolution for a few billion more years, we would be able to say for sure which pathway that galaxy was sent down. I don’t know about you, but I certainly don’t have a hundred million years to wait.
So without millions or billions of years to wait for updates from that galaxy, we’re a bit stuck. We effectively have a snapshot of this earliest galaxy as it was, and no ability to check what it did later, or how the galaxies around it changed with time. What we do have is another snapshot of the Universe later, where a different set of galaxies exist, with the same inability to watch where they go forward in their path through their own lives. Trying to piece together which galaxies in the distant universe might evolve into the galaxies we see at more recent times, at less extreme distances, is one of the fundamental puzzles of observational astronomy.
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