Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Beginners Questions about Astronomy

  1. Nov 14, 2007 #1
    Hello,

    I have a few questions for astronomers.

    1) When you look up at the sky at night with the naked eye, are all those stars within the milkyway galaxy? Or can we see past our own galaxy and see stars from other galaxies mixed in with stars from the milky way?

    2) I heard the universe is 78 billion light years across. What is at the edge of the universe? And what happens if you go past it? I mean, i hear a lot of people say that there is no other side and the universe is all that is. Its not like the big bang happened in a vacuum, there is nothing then it expands. But lets say we did fly out to the edge and kept going, do you just bounce off the boundary? Or is it some freaky geometry that you would end up back where you started after a while?

    3) Is the sun going to burn out in 5 billion years?

    Cheers
     
  2. jcsd
  3. Nov 15, 2007 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    All the stars you can see are in our galaxy.
    A few of the objects you can see with the naked eye are actually other galaxies - you can't normally see individual stars but you can see the combined output of all the stars. A supernova ( a star exploding ) in another galaxy can be bright enough to see.

    The universe is "finite but unbounded". It's all to do with 4 dimensions of space-time, it's a bit like there being no edge to the Earth. There are lots of books explaining it at all levels.


    Thats exactly what happended - in the beginning was nothing, which exploded!
    Rememebr before the big bang there wasn't vacuum - there wasn't anything, no time, no empty space no nothing...

    In most cosmological models yes.

    Yep, sorry but at least you got plenty of notice.
    What actaully happens is that as the sun uses up it's hydrogen fuel it changes to burning different elements and expands. this will happen in around 5 billion years.
    It will then eventually use up all available elelements and gradually shrink and cool into a ball of gas.
     
    Last edited: Nov 15, 2007
  4. Nov 15, 2007 #3

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    A small correction, the sun will become a red giant in about 4.5 billion years and fry planet earth. Mars will become rather toasty, but liveable for about 500 million years thereafter.
     
  5. Nov 15, 2007 #4
    Cool, thanks guys :)

    Just a few more questions.

    So can you see individual stars in other galaxies with a good telescope?

    Are there any good books explaining it to a layman? Especially the geometry side of things. Any suggestions would be appreciated.

    I doubt it :P

    As far as we know. However, there could be a multiverse or another universe in a different dimension or some sort of eternal expanision and collapse of the universe. As far as i know science has no means to know what happened before the big bang.

     
  6. Nov 15, 2007 #5

    russ_watters

    User Avatar

    Staff: Mentor

    For an amateur, only if they explode. I believe real observatories can recognize individual stars in the nearest galaxies.
    Many people get their first exposure to these ideas from "A Brief History of Time".
     
  7. Nov 15, 2007 #6

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The brightest stars in other galaxies (novas aside) are cepheids.
    They were seen in photographics plates of the galaxy M31 on the 100inch (2.5m) telescope in the 1920s.
    But you wouldn't be able to see them by eye even with a 10m scope today.
     
  8. Nov 16, 2007 #7
    Let me take a shot at the first three questions.

    1. You can see many things in the night sky that are not stars, but only one with the naked eye that is not of our galaxy, and that is our sister galaxy Andromeda. There are occasions that a very bright explosion will occur outside our galaxy that is viewable, these are GRBs, but my understanding is that you would have to be able to see in the Gamma ray spectrum to see it really good :-p. You would not be abnle to discern individual stars in Andromeda without a very powerful telescope (maybe a 10" Dob might get it if you are patient). Good eyesight would allow you to see (on occasion) other galaxies in Virgo, but you would not recognize them as something other than a star in most instances. Other items you see in the night sky are nebula, planets, moons, clusters (stars, yet of interest as a group), and various satalites and Unknowns (Ufos). The most amazing things to look at are in our galaxy if you are using you own eyes (unless your name is Gates or Buffet). Look into the skjy any night and you can see the Seven Sisters, the tip of Orion's Sword, or the Great red spot with little more than a pair of binocs.

    2. There is considered agreement in the science that the Universe by measurement is close to 15 billion years old (I have heard strong arguments for 16 and for 13.5). This is based upon Sephiad Variable stars and doppler shift. If you google those two terms you will learn more about the expanding universe than you probably want to know. Concerning the ends and what is there? I would guess that it is an expanding "space" existing under physical laws that opose the laws of our universe. Hubble detected the expanding universe with his first work on the doppler effect, and Hawking says it will end, so I probably shouldn't dissagree.

    3. Pretty close from what I understand. The length of the life of the sun as a yellow star (hydrogen burning) is based upon the known parameters of star evolution. Some is estimates based upon red and yellow stars in clusters and the age of those clusters yielding the age of the stars inside and the stage in their lives they are at. also closer estimates have been given based upon consumption of hydrogen, stellar mass, and gravitational effect which all have jobs in the ballance that produces a brilliant yellow light from our sun. How about we stick around and find out?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?