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Can we see any galaxies/stars that are newer than our own galaxy?

As light takes (c) amount of times to reach us - so relatively speaking, light which would have left the newer galaxy (and far enough from us) still would not have reached us. Therefore, can we ever see a galaxy that is newer than our own one. If we can see a new galaxy that is newer than our own one, how can we determine its age as light would not have reached us yet.

EG : Light travels from galaxy (x) to us (y) and it has taken 100 million light years. However if (x) was less than 100 million years younger than us, how could it have reached us and can we determine it and its age ?

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Possible duplicates: physics.stackexchange.com/q/25205/2451 and links therein. –  Qmechanic Mar 27 '13 at 21:48
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2 Answers

We certainly can see galaxies that are younger than our own. And we can see galaxies as they appeared when they were much younger than our own galaxy is now.

Our galaxy is over 10 billion years old, though the thin disk formed a bit later. (By comparison, the Sun and Earth are less than 5 billion years old).

If there's a galaxy that formed, say, 4 billion years ago, and it's 1 billion light-years away, then we'll be able to see it as it was 1 billion years ago (when it was 3 billion years old).

A galaxy that formed at the same time but that's currently 6 billion light-years away would not be visible; we can see its birth if we wait 2 billion years.

More generally, if we see an object x light-years away, we're seeing it as it was x years ago. If it's less than x years old, we can't see it at all. If it's more than x years old, we're seeing it as it was when it was x years younger than it is now.

But I think (I'm not sure of this) that most galaxies formed at about the same time. Our own, for example, formed not long after the Big Bang. But even if all galaxies formed at the same time, if we look 10 billion light years into space, we can see galaxies as they were when they were much younger.

I'm oversimplifying a bit. Relativity tells us that simultaneity is a more complicated concept than we might think. It's not necessarily meaningful to say that something 6 billion light-years away happened 4 billion years ago. And the expansion of the Universe messes things up a bit as well.

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To begin with your EG; No object of age $x$ years can be seen unless it is nearer than $x$ light years.

Consider a supernova, this is where a star reaches the end of it's main sequence and explodes, briefly shining as bright as a whole galaxy. We occasionally see these events in other galaxies. But the day we see the light arrive at Earth is not the day the star died, that time could be several thousands/millions of years in the past.

So next time your looking up at the night sky remember that some of those stars no longer exist...

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All the stars that are visible in the night sky are within a few hundred or thousand light-years. Since stars last for millions or billions of years, it's unlikely that any of them have died since they emitted the light we're now seeing. Betelgeuse is the most likely exception, but even that's unlikely; it's less than 1000 light-years away and is likely to become a Type II supernova any time in the next million years. On the other hand, the Pillars of Creation probably no longer exist. –  Keith Thompson Feb 23 '12 at 20:22
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