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Thread: 100 billion stars and 100 billion galaxies

  1. #1
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    100 billion stars and 100 billion galaxies

    How exactly did they figure that there are 100 billion stars in this galaxy and 100 billion galaxies in the universe?

  2. #2
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    Sampling and estimating counts based on gravitational effects?
    I don't think anyone really means "there are". It's more of an "there are estimated to be".

  3. #3
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    If that 100B turned out to be literally true I would have to really have a good think about my prejudices.

  4. #4
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    There are good estimates as to the total mass of the galaxy, and we have a reasonable census of the ratios of each mass range of star, and an estimate as to how much of the matter in the galaxy is in stars. The number 100 billion is actually highly dependent on where we draw the line on low mass stars and non-stellar objects.
    Forming opinions as we speak

  5. #5
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    On a tangent, counting/approximating stars seems relatively straight forward, but I hadn't really thought about the fact that there is the potential for so much non-stellar mass. Between recent estimates of ~stellar mass blackholes in the billions for the milky way, the even more recent understanding on planets outside the solar system and, pardon the pun, most nebulus estimates on gas and dust clouds, I don't see how good galactic mass estimates can result in good star counts. I'm not a cosmologist. Please forgive me when I ask how are galactic masses estimated?

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    The estimates I've heard are closer to 200 billion galaxies in the observable universe, and who knows how many more beyond that.
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  7. #7
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    Average guesimates are 10^22 stars in the knowable Universe. If that has been true for the past 13 billion years, and only 10^20 were high mass stars with an average life on main sequence of 10^8 years; then 130 times 10^20 compact stars are still around, but most are difficult to detect, even though they average 3 solar mass, each. The mass of compact stars possibly exceeds the present mass of main sequence stars. Neil

  8. #8
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    Quote Originally Posted by neilzero View Post
    ...then 130 times 10^20 compact stars are still around...
    What do you mean, compact stars? Aren't all stars "compact"?
    Everyone is entitled to his own opinion, but not his own facts.

  9. #9
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    Quote Originally Posted by neilzero View Post
    ... The mass of compact stars possibly exceeds the present mass of main sequence stars. Neil
    neilzero this is a thread hijack, and a non-mainstream conclusion. Would you like this moved to ATM, or are you unwilling to defend your idea?
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  10. #10
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    I assumed post 5 was correct in saying that black holes are not stellar mass, therefor neutron stars and white dwarfs are not stellar mass. Likely I worded that wrong and/or was thinking wrong. No need to move it to ATM. If high mass stars were as numerous over the past 13.7 billion years as what we estimate at present, and they became white dwarfs, neutron stars or black holes, one billion years (average) after their birth, then we can multiply the present estimated number of high mass mainstream stars by 13.6 to get the total number of compact stars of those 3 types. Perhaps 2 solar mass is closer to the correct average mass of compact stars. Does that have an arithmetic error, or are some of my assumptions unlikely? I suggested somewhat smaller numbers for the first 3.7 billion years in my previous post. You have my permission to edit both posts to make them mainstream. Neil
    Last edited by neilzero; 2013-Sep-28 at 07:26 PM. Reason: Added first sentence

  11. #11
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    Neil's speculation appears to be completely on-topic. It is
    part of the problem of defining what is meant by "stars" when
    the number of stars in the Milky Way, galaxies in general, or
    the Universe as a whole is expressed.

    I haven't tried to analyze his numbers, even roughly.

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    point of rockets is to explore them!" -- Kai Yeves

  12. #12
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    Quote Originally Posted by MichaelMotorcycle View Post
    how are galactic masses estimated?
    We infer the masses based on motions: stars within galaxies, or galaxies within clusters of galaxies. We can measure the radial velocities of objects with a spectrograph, estimate their tangential velocities, then, assuming that gravitational forces dominate the dynamics of these objects, use the motions to estimate the masses.

    For more details, read

    http://spiff.rit.edu/classes/phys443...tffj/tffj.html
    http://spiff.rit.edu/classes/phys443.../gal_dark.html

  13. #13
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    The Hubble Extreme imaging of a somewhat smaller region of the HUDF, but for a longer exposure period of time (23 days, I think), revealed about 5,500 galaxies in a very small unit area that, when extrapolated, produces the sum of 175 billion observable galaxies overall. So the 100 billion number is likely an earlier assessment.

    There was an article in Nature (2010) that claimed 3e23 for the number of stars in the universe by bumping the stars per galaxy from the 100 billion number since elliptical galaxies can have a trillion stars. Assuming there is a paper on this, it might be fun to dissect it here in this forum. Considering the earlier immaturity of the galaxies being observed, and in the count, the [added: the number of galaxies with a] 1 trillion star number in some galaxies must be excessive.

    Perhaps they are extrapolating the number of galaxies from the extreme HXF range of ~ z = 11 to another billion years or so for the earliest and more distant galaxies that we should, eventually, be able to add to the galaxy count.

    Nevertheless, if we use the HXF extrapolation of 175 billion galaxies and you allow me to use an average of, say, 115 billion stars, then we have a convenient number we can all remember.... 2e22.

    On the other hand, the 3e23 figure ain't bad, but I would expect it should get doubled so we can finally give a name to our universe: Avogadro!
    Last edited by George; 2013-Oct-02 at 06:54 PM.
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