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

How big is milky way ?

  1. Apr 20, 2003 #1
    measured from one end to another end, how many kilometerS?

    if we are also in the milky way, how can we see milky way?
     
  2. jcsd
  3. Apr 20, 2003 #2
    I'll leave it to someone else to look up the exact figures and do the conversion but I think I can help on the second part.

    When you look up at the night sky and see the Milky Way, that's the center of the galaxy where all the stars are relatively densely packed. So seeing that band of stars from our vantage point is kind of like seeing the skyline of a big city from a less congested part of it.
     
  4. Apr 20, 2003 #3
    The Milky Way is roughly 100,000 light years in diameter, one light year being 9.5 x 10^15 meters or (according to my calculator) 9.460 x 10^17 kilometers or 946,000,000,000,000,000 kilometers.

    We can see the Milky Way much like you could see a balloon if you were inside of it.

    EDIT: 90,000 lyr's is 8.514 x 10^17 kilometers. <----converted for marcus' post.
     
    Last edited: Apr 20, 2003
  5. Apr 20, 2003 #4

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    There is a good website about the Milky Way Galaxy with lots of facts and photographs.

    http://www.anzwers.org/free/universe/milkyway.html

    It has specs, like 90 thousand lightyear diameter and
    the sun being 26 thousand lightyear from the center
    The same site has some maps that you can zoom into or
    back out of---like this which shows a neighborhood of
    the sun.

    http://www.anzwers.org/free/universe/2000lys.html


    You ask how we can "see" it and we cannot see it in the sense of getting an outside perspective. We can only see it from inside and IMAGINE what it would look like from outside.

    this site has some history of how and when people began to
    map out the structure of the galaxy. It is not easy to do from the inside. Now it is possible with infrared telescopes and such to see objects in the very center of the galaxy. But in 1950s they couldnt do that and they were mapping the galaxy with radio telescopes. The problem is that dust obscures a lot of the detail in the main plane of the galaxy so that visible-light telescopes are not so good for mapping the structure and people use radio and infrared wavelengths. A lot of inference and guesswork based on what other galaxies look like. I don't mean to put it down but it seems a lot harder to visualize one's own galaxy than it is to map other galaxies.

    because its hard to do, the history of how people first did it can be fairly interesting. I will see if there is something online.
     
  6. Apr 20, 2003 #5
    Do you think we may have several solar systems in milky way that can also harbor living things like the earth!

    I think it is possible!
     
  7. Apr 20, 2003 #6
    BTW,
    how may galaxies do we have that resemble milky way?
    Are they bigger? What're their names?
     
  8. Apr 21, 2003 #7

    russ_watters

    User Avatar

    Staff: Mentor

    The nearest such galaxy is Andromeda. It is roughly similar to the Milky Way in size and shape. Two closer galaxies, the large and small Megallanic Clouds are closer to us but smaller - and are visible to the naked eye in the southern hemisphere.

    There are an estimated 100 billion galaxies in the known universe.
     
  9. Apr 21, 2003 #8
    It is quite possible! :smile: I would like to think there is life elsewhere in our galaxy.
     
  10. Apr 23, 2003 #9
    Sorry, not quite the truth of it, the reason why we can see the milky way galaxy, is because our Sun is near the outer edges of it, in one of the spiral arms that it has.

    So when we look up at night, we are simply looking towards the center of it, and, at all of the matter that lies past that center.

    (Of what we can see, cause we cannot see alot of the matter, not enough light emitted from it, or, no light emission at all, ie; planets)

    There is a nice downloadable program called 'Celestia', available, for free, at this link, *MILKYWAY* that will give you a guided tour of the near regions, and let you conduct yourself around the near regions if you have a reasonably good video graphics card. (the requirements are posted there)

    EDIT* SP
     
  11. Apr 23, 2003 #10
    if there isnt life in our galaxy i hearby declair that whatever technology they have at the time when it is eventually worked out can bring me back to life and that each and everything on the earth can give me 10 spanks anywhere with a cricket bat!

    besides! I have seen other life forms from our galaxy, there on TV all the time! in star trek, stargate and such forth! gawd, do you not watch tv ?? :P
     
  12. Apr 24, 2003 #11

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    The Sun is not near the outer edges by any stretch of interpretation. It's normally considered to be about 3/5 of the way out. The brilliant milkyway band of stars which you see in the Winter sky is the galaxy *outwards* from us and it looks about as full of stars as the band you see in Summer which is inwards. For a long time people could not figure out where the center was and one of the models of the galaxy (1900-1920 IIRC) had us in the center.

    QUOTE]Originally posted by Mr. Robin Parsons
    So when we look up at night, we are simply looking towards the center of it, and, at all of the matter that lies past that center.
    [/QUOTE]

    This is not true, as a rule. The only time when one is looking towards the center of our galaxy is on summer nights when Scorpio and Sagitarius are visible and one is looking at a spot in Sag that is right near the tip of the Scorpion's tail.

    It took a lot of work for people to figure out where the center of the galaxy is. Optical telescopes were only seeing a few thousand lightyears into the plane, because of dust. The first real mapping of the spiral arms, and location of the center, was I believe in the 1950s with neutral hydrogen (21 cm) radio astronomy. Another way the center was located was by observing the distribution of globular clusters.

    What I said, that you were replying to, was:

    "... we cannot see it in the sense of getting an outside perspective. We can only see it from inside and IMAGINE what it would look like from outside."

    This seems like an obvious point. It took radio astronomy to first see the spiral structure and map the arms. It was non-trivial to even locate the center. We do not get an outside perspective because we are pretty much inside the thing----in the plane of it and only about 3/5 of the way out from center. And its full of dust so optically we only see a few thousand LY in the plane. Looking from inside at something mostly obscured by dust means the overall structure has had to be inferred. Like the inside of the earth, it is imagined, not seen.
     
  13. Apr 24, 2003 #12
    So sorry, my references of such things I have not 'updated', as what I had remebered comes from a reference source that I had read as a kid, back in the 60's, and it told of the Sun being ~45,000 ly out in a ~100,000 ly galaxy.

    Just went to a more up to date reference source, they pin it at real close to central from the center. (7 to 8 kiloparsecs)

    But they do not seem to distiguish, as you did Marcus, seeing it in the summer, and winter skies, as being a different view, could you explain why?

    A re-read makes your statement of inner/outer 'perception' clearer, as I had found that statement, rather unclear, I suspect it is the 'Imagine' part, as mapping it gives up a very clear 'outside' view as it is mappable as it is not a 'solid'. (closed to our view)
     
  14. Apr 24, 2003 #13

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Indeed Mr. Robin, I spoke unclearly and should not have said "imagine" since I meant a rigorous construction and apologize for the misleading term. We both understand now.

    When Orion is on meridian (winter basically) and the citylights dont block it out you see this bright band of stars running roughly north and south past Orion's left shoulder. This is the plane of the galaxy in the outwards direction.

    If memory serves the outward direction is sort of between Orion's left shoulder and Gemini.

    Then let six months pass so you are looking in 180 degrees different direction and you see Scorpio in the sky (summer basically). Again you see this bright band running roughly north and south. Not exactly along the meridan but skewed from it some. The band runs between scorpio's tail and sagitarius and right there where the tail dips into the stripe of stars is the galactic center direction.

    The milkyway band forms a ring around us.
    In all directions in the plane we see a dense concentration of stars. So we see the stripe no matter whether we are looking outwards from the center (as we are sometimes at night) or inwards towards the center (as we do other times).

    There are times of year when, at say 30 N latitude) the ring is along the horizon. I am at 37 N latitude and I almost see this. It is wonderful. get in a dark open field and all around you is the ring of the milkyway. At midnight around March 20 or so would do.

    Then the inwards direction is to the southeast where scorpio has just risen and the outwards
    direction is to the northwest where orion has just set.

    It is a great galaxy and we are fortunate to be a part of it.
     
  15. Apr 24, 2003 #14
    Thanks Marcus, you must be telepathic, as while I as sitting at my first meal of the day today (two peanut butter and jam sandwiches with soup....my ONLY meal today,I missed the first one my bicycle broke) I realised why you had stated that, as it is in the circle that the planets makes around the Sun that affords us a different night time view, winter, and summer, I had missed that one.

    Thanks again!
     
  16. Apr 25, 2003 #15
    Quote - "Do you think we may have several solar systems in milky way that can also harbor living things like the earth!

    I think it is possible!"

    Scientists theorize that we have evolved over billions of years from blue green algae and such, and even though i do not believe in the theory of evolution, i understand their reasoning..

    Simple life began on earth because of the presence of oxygen, which came here on comets in the form of ice, water. Life adapted to oxygen because it is so efficient in the breakdown of energy. We see common examples of this in comparisons of anaerobic respiration vs. that of aerobic respiration in cells, where ATP net is totally in favor of oxygen....

    Scientifically, it is possible for other planets to harbor life, but you must also take other factors in to consideration:
    Time, planetary temperature, environmental hostility, etc..

    At the time of the formation of simple life on earth, elements such as carbon, nitrogen, hydrogen, sulfur, phosphorus, etc were present, and were therefore synthesized into the daily life of these organisms over a VERY long period of time. Perhaps if elements like arsenic and mercury (along with oxygen) were present instead of our organic elements, life may have adapted around those elements instead.
     
  17. Apr 25, 2003 #16

    marcus

    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    these are exciting things to speculate about. there was another thread here a few weeks ago which discussed reasons why multicelled life MIGHT be extremely rare *even if* planets superficially like the earth in temperature and gross chemical makeup are fairly common.
    The outstanding fact that several people pointed to is that life on earth seems to have gotten "stuck" at the singlecell level for a several billion years and only 600 million years ago suddenly began to have multicell lifeforms.

    my physics/chemistry handbook has a table of cosmic abundances of elements and carbon and oxygen and nitrogen etc because they are synthesised in cores of stars in great amounts and blown out profusely into space all the time are very abundant. so I accept that plenty of planets may have formed in circular orbits the right distance from their stars to have the right temperature range and be made of roughly the same mix of chemical elements, and they may also be geologically active and have volcanos and all that good stuff

    but there may be things and events necessary to multicelled life which we havent included in the picture or thought about enough,
    like getting stuck at a single celled level, and what event in the earth's history might have triggered multicell life 600 million years ago, if there was a trigger. and the role of mass extinctions----if they play a role in evolution----and what causes them. on that thread a few weeks ago people were discussing things like that, IIRC (if i recall correctly) you might even find it with a keyword search. pretty interesting. conceivable that multicell life is quite rare in this galaxy
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?