Why aren't galaxies brighter?

  1. L'homme des tÚlescopes
    L'homme des tÚlescopes

    So I had this thought the other day. Stars are big balls of burning gas millions of miles away. Correct? Galaxies, on the other hand, are massive groups of big balls of burning gas millions of miles away. So, if you think about it, a normal galaxies contains perhaps 200 million stars, so that 200 million times brighter than only one star. So they should be brighter, in fact, they should be so bright that you could see them with the naked eye, even in light polluted conditions. Even more, if a galaxy was 200 million times further away than a star, it would still be as bright. So, how come we can see stars from the other side of the universe with the naked eye (can we?) but we can't see galaxies that are so much brighter and some, much nearer?

  2. bladekeeper
    We cannot see stars from the other side of the universe naked eye. Any stars that we can see naked eye in the night sky are everyone well within the confines of our own galaxy.

    It takes a high powered telescope (Hubble) to resolve stars in another galaxy, and then likely only the Andromeda Galaxy. Supernovae explosions in other galaxies can be see as they are very much intensely brighter than a star.

    Other galaxies are just too far away to be naked eye bright. Their surface brightness is basically the same as photometric luminence and is constant with distance, i.e. as a galaxy becomes fainter with distance, it also becomes smaller in visual area.

    There is a great article written by Alan (KT4HX) over in the Deep Sky Objects forum as a sticky thread, titled "Why Can't I See That Galaxy". Give that one a read and see what you think.
  3. KT4HX
    The individual stars we see in our galaxy are significantly closer than any other galaxy. You said that galaxies are millions of miles away. That is vastly understated. They are millions and billions of light years away, not simply millions of miles. Light travels 6 trillion miles in one year (rounded off). For example, the bright and close galaxy M31 is about 2.5 MLY away. That equates to (2.5 x 106) x (6 x 1012) = 1.5 x 1019 miles. You take a bright star like Betelguese that is 640 light years away, that works out to (640) x (6 x 1012) or 3.84 x 1015. That is a very substantial difference. Considering the vast distances to other galaxies, the intervening dust in both our own galaxy and that of the galaxy we are looking at, plus the attenuation associated with our atmosphere, I think it is easy to understand the inherent faintness of such objects. Scientific notation didn't display properly!
  4. bladekeeper
    Excellent answer Alan!
  5. KT4HX
    Thanks Bryan. Sorry I came so late to the party. I wanted to be more verbose but there are character limits in these posts, and maybe that's a good thing! So I tried to trim it down yet still get the point across. Maybe I was successful. Now I see the scientific notation did display properly. When I first posted it didn't elevate the exponent above the base.
  6. j.gardavsky
    Hello L'homme des tÚlescopes,

    the essence of the answer to your question is in the Olber's Paradox, described and animated in https://en.wikipedia.org/wiki/Olbers'_paradox
    Otherwise, the nearby galaxies would have brightness comparable in lots of cases to that of our Milky Way, which can be seen with unaided eyes under the clear dark skies.

    With increasing distance, the red shift would let the galaxies appear yellow, and at even greater distances reddish. So the night skies would look like a carpet of increasingly smaller patches with someyellow to red colors seen through the telescopes.
    But it is the Olber's Paradox at work, which allows to see just the Andromeda Galaxy, and eventually the Triangulum Galaxy with unaided eyes, keeping the skies dark enough for a good sleep.


  7. bladekeeper
    Thanks JG! That's great information too!
  8. KT4HX
    Thank you JG for linking that article. I had not heard of the Olber Paradox. Makes for interesting reading.
  9. KenGS
    No one has mentioned the inverse square law yet. So the OPs example of a galaxy of 200 million stars being as bright as one star from 200 million times further away is incorrect. At 14000 or so times further away (the square root of 200 million) the spread of light from the galaxy would reduce its brightness to that of one star.
  10. mkeschinger
    Wouldn't time also be a factor in this? Given the expansion of the universe, over time (longer than our lives), is it safe to speculate that not only are we not seeing things from the proposed other side of the universe, but we are also not seeing light that has not reached our current range of "visibility" from even farther (or earlier) away ? Man, I love this stuff. Then we can get into different forms of matter, gravitational impact on ligt distorting our view the same way we "see" things in water...
    Ok, bed time...
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