Haven't The Stars In The Oldest Galaxies Died By Now?

If it has taken 13 or so billion light years for the light from the most distant observable stars to reach Earth, is it not probable that those stars no longer exist but have “gone nova”?

Originally posted on Forbes!

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.

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

A lone source shines out brightly from the dark expanse of deep space, glowing softly against a picturesque backdrop of distant stars and colorful galaxies. This scene shows PGC 83677, a lenticular galaxy — a galaxy type that sits between the more familiar elliptical and spiral varieties in the Hubble sequence. Image credit: ESA/Hubble & NASA | Acknowledgements: Judy Schmidt (Geckzilla)

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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How Can We Count Galaxies If They're Evolving Or Dying Out?

Does the estimate of the number of galaxies in the universe only contain those that exist now, or those that we can see now that their light has traveled to us? I mean, some of what we see now must have evolved, and died out. When you look into the sky are you seeing the same thing repeating, like a star for example, only at different times and distances?

Originally posted at Forbes!

Trying to count the total number of galaxies in the universe is a difficult task, made harder by the part where no one wants to spend an infinite amount of time counting galaxies. Instead, what we usually do is count the number of galaxies in a very small area of the sky. Usually what happens is that we point a telescope at a very empty, dark patch of sky, and wait for a while. We’ve done this a few times with Hubble, creating what we now call the Deep Fields. We now have the Hubble Deep Field, the Hubble Ultra Deep Field, and the Hubble eXtreme Deep Field. (Once more, astronomers prove themselves eminently practical namers.) Once we have a really deep image, we can then assume every other patch of the sky is roughly going to look the same (as far as we can tell, a valid assumption). We can then multiply the number of galaxies in that one piece of sky by the fraction of sky we looked at, and get a very rough estimate of the total number of galaxies. Hey presto: several hundred billion galaxies!

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind’s deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light from Hubble’s new infrared camera, enabling new studies of the earliest galaxies in the universe. The XDF contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Like photographers assembling a portfolio of best shots, astronomers have assembled a new, improved portrait of mankind’s deepest-ever view of the universe. Called the eXtreme Deep Field, or XDF, the photo was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. The Hubble Ultra Deep Field is an image of a small area of space in the constellation Fornax, created using Hubble Space Telescope data from 2003 and 2004. By collecting faint light over many hours of observation, it revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light from Hubble’s new infrared camera, enabling new studies of the earliest galaxies in the universe. The XDF contains about 5,500 galaxies even within its smaller field of view. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Image Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

But you’re absolutely on the money with your suggestion that we can only see the galaxies whose light has reached us. This fundamentally limits our observing, so that the galaxies which are most distant from us are also very far removed from our current time.

And here we begin a giant puzzle, because the most distant galaxies, which we see the farthest in the past, are very different from the galaxies we see in our local, nearby universe, which we see much closer to our current time. This means, again as you suggest, that the ancient galaxies we can spot in images such as the Hubble Deep Fields, must have evolved and changed between the time when the light we observe left them, and now. Galaxies which were once independent will have merged in the billions of years which have passed while that light was making its long way to us. Some galaxies will have used up or lost the gas they need to create new stars – one of the few ways a galaxy can “die out”, though its existing stars will disagree with you on how dead they are. Some galaxies will have uneventful evolutions, though they will still evolve. At a base level, galaxies will be creating more stars over time and adding to their own mass, though the number of new stars they make each year will drop over time.

It is known today that merging galaxies play a large role in the evolution of galaxies and the formation of elliptical galaxies in particular. However there are only a few merging systems close enough to be observed in depth. The pair of interacting galaxies picture seen here — known as NGC 3921 — is one of these systems. NGC 3921 — found in the constellation of Ursa Major (The Great Bear) — is an interacting pair of disc galaxies in the late stages of its merger. Observations show that both of the galaxies involved were about the same mass and collided about 700 million years ago. You can see clearly in this image the disturbed morphology, tails and loops characteristic of a post-merger. The clash of galaxies caused a rush of star formation and previous Hubble observations showed over 1000 bright, young star clusters bursting to life at the heart of the galaxy pair. Image credit: ESA/Hubble & NASA

It is known today that merging galaxies play a large role in the evolution of galaxies and the formation of elliptical galaxies in particular. However there are only a few merging systems close enough to be observed in depth. The pair of interacting galaxies picture seen here — known as NGC 3921 — is one of these systems. NGC 3921 — found in the constellation of Ursa Major (The Great Bear) — is an interacting pair of disc galaxies in the late stages of its merger. Observations show that both of the galaxies involved were about the same mass and collided about 700 million years ago. You can see clearly in this image the disturbed morphology, tails and loops characteristic of a post-merger. The clash of galaxies caused a rush of star formation and previous Hubble observations showed over 1000 bright, young star clusters bursting to life at the heart of the galaxy pair. Image credit: ESA/Hubble & NASA

Untangling the complex line which can connect a nearby galaxy to the sort of galaxy it might have been, billions of years ago, is a whole subfield of astronomy, under the moniker of galaxy evolution.

It’s important to keep in mind that it’s not quite as simple as seeing the same things repeated over and over again. The galaxies we see much earlier in their lives than our own are truly, physically, very far away, which is why we see them so far removed in time. They will be evolving over time in their own physical space, but they should evolve into something that looks like the galaxies near us. Distant galaxies seem to be the same everywhere we look, so we shouldn’t be looking at a special group of distant galaxies that would evolve in a unique way. They’re not the same galaxies as the ones that built our own galaxy, but they should be pretty similar. It’s up to us to learn what the pathway between ancient and current day must have been.

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