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A general question about stars

  1. Sep 13, 2009 #1
    Somebody once told me that when you look into the night sky to view stars, you are actually looking at something that happened a long, long time ago (millions?, billions?, trillions of years ago?).

    I understand this has something to do with the speed of light and the distance of the stars from earth.

    I am familiar with c = 3*10^8m/s, and I'm sure with a bit of research I could find how far a star is from earth and do the basic math to find the time taken. I'm just wondering if the principles behind what I have been told are correct.

  2. jcsd
  3. Sep 13, 2009 #2


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    All the photons we currently receive from stars are old. It is a speed of light thing. Photons from the nearest star to earth were emitted about 4 years ago. The faintest starlight visible to the naked eye was emitted at least 3 million years ago. Photons from the most distant objects detected by the Hubble Space Telescope were emitted nearly 10 billion years ago.
  4. Sep 13, 2009 #3
    So I'm guessing brighter stars are the youngest ones, and dimmer ones are the ancient ones?

    I have no background in astro, just mathematics. These sort of things boggle the mind though, and at times almost borders on being as much a philosophical thing than an astro thing (well in my mind at least)
  5. Sep 13, 2009 #4


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    This kind of a generalization is only very loosely true. It's a stellar population thing, basically. Because dim stars tend to live very long, there's a high likelihood that these categories of stars will be older than those which shine very brightly (since they die quickly). But in terms of a single star on the main sequence, luminosity actually increases as a star ages. So it's a complicated thing and difficult to generalize.

    Also in regards to how long it takes for a given photon to reach us from a star, you may also want to account for how long it takes a photon starting in the core of a star to actually escape. This number is somewhere around the order of 10,000 years, I believe. Although if you want to get technical about it, it wouldn't be the same photon since it just gets absorbed and re-emitted many times, but that's more semantics than anything :)
  6. Sep 13, 2009 #5
    cool, thanks for the replies guys.
    So generally, its safe to say that when you are viewing a star, you are viewing a photon of light that was first emitted anywhere between 4 and 3 million years ago (to the naked eye as said)?
  7. Sep 13, 2009 #6

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    Not really. The brightness we perceive (apparent magnitude) is a combination of how bright the star truly is (absolute magnitude) and and its distance from us. Incredibly bright (absolute sense) stars are inherently young for the simple reason that to be that bright they need to be very, very big. Compared to our sun, large stars burn very brightly, have very short lives -- and die hard.

    However, amongst stars of the same mass, type, and metalicity, it is the older stars that burn more brightly. Our Sun, for example, is about twice as luminous as it was when it first formed.
  8. Sep 13, 2009 #7

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    The 2000 or so stars we can see with our naked eyes are all fairly close to us. Most are within a few hundred light years from the solar system. Aludra, a blue supergiant, is 3400 light years away. There are a few cataloged (but unnamed) naked eye stars even further than that. The only really remote things we can see unaided are galaxies and supernovae.
  9. Sep 14, 2009 #8
    hmm so either way, we are still viewing something that happened in the past, right in front of our eyes in our perception of the "present".

    That's mainly what interests me, the fact that it is xxxxx years ago just makes it further impressive
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