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A Are we now alone in the Universe?

  1. Apr 3, 2016 #1
    Alpha centauri A & B is about 4.5 light years away from us. That means, if one of your friend is flashing a light in there visible enough to be recognized here on Earth, it takes about 4.5 years before you would receive that signal.

    What are the chances that those stars, galaxies and constellations are still in there this very moment?
  2. jcsd
  3. Apr 3, 2016 #2


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    Where do you think they might have gone in the intervening years?
  4. Apr 3, 2016 #3


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    If you look at the farthest galaxies billions of light years away, I think it's quite likely (with the exception of red dwarfs) that most of the light emanating from those galaxies comes from stars that have since burned out.

    We can use spectra to classify stars, and see where they are in their life cycle at the time they emitted their light. A blue supergiant might only live for a few million years, so the blue supegiants we see in our nearest galaxy, Andromeda, might have burnt out already too.

    In our own galaxy, it's a numbers game.
    At 100,000 light years in diameter, any one star is most likely going to burn for at least another 100,000 years, and so probably won't've burned out by the time we see it.
    However, with hundreds of billions of stars in our galaxy, the odds are nearly 100 percent that dozens of stars have burnt out in our own galaxy while their light is still reaching us.

    That being said, our nearest neighbors (the Centauris) are all either main sequence stars like our sun, or red dwarfs that will last a thousand times longer than our sun. I think you can count on them being here for a few more billion years.
  5. Apr 3, 2016 #4
    Possibility is they could be gone by now.
  6. Apr 3, 2016 #5

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    That's really not an answer.
  7. Apr 3, 2016 #6
  8. Apr 3, 2016 #7


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    Average stars, like our sun, have a life expectancy in the billions of years. Massive stars, like blue giants, live only for a few millions of years. Low mass red dwarfs, none of which can be seen with the unaided eye, can live for trillions of years and are the most common stars in the universe. A typical galaxy has a small number of huge stars, a large number of average stars and a huge number of red dwarfs. So the odds are many of the stars that produced the light we now observe from distant galaxie are still pumping out photons.
  9. Apr 3, 2016 #8
    So, can it dismiss 13.82 billion years age of the universe?
  10. Apr 3, 2016 #9


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    No. While we do have evidence of stars over 13 billion years old, we have no credible evidence of stars older than the universe - e.g., black dwarfs.
  11. Apr 3, 2016 #10
    In the case of the Alpha Centauri system, 4.5 light years is a trivial distance (in cosmic terms).
    We know what kind of stars they are (the two big stars are fairly Sun-like, Proxima is a red dwarf).
    They will continue to exist for billions of years.
  12. Apr 4, 2016 #11
    The age of the universe will be the same everywhere (for the most part, General Relativity can complicate things.) So if our galaxy is still here, it's a good bet that all the others are there too. They've just evolved since the light that we currently see left them.

    Andromeda is 2 million light years away, almost every star we see in it, is still there. A million years is nothing compared to the life of the average star. The average star is smaller and will greatly outlive our sun.
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