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Thread: Sources of Milky Way luminosity by spectral class?

  1. #1
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    Sources of Milky Way luminosity by spectral class?

    The number of 100 000 million "stars" or several times that has been often thrown about when discussing the size of Milky Way.

    However, when looking at absolute magnitude of Milky Way...

    it is -20,8.

    Which converts into just 18 000 million times the brightness of Sun.

    How much of this actually originates from Sun-like stars, like G0V to G9VI?

    And how much of Milky Way´s brightness comes from other groups:
    1. hot dwarfs, O0V to F9V
    2. late dwarfs, K0V to M9V
    3. super-to subgiants, I-IV

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    Quote Originally Posted by chornedsnorkack View Post
    The number of 100 000 million "stars" or several times that has been often thrown about when discussing the size of Milky Way.

    However, when looking at absolute magnitude of Milky Way...

    it is -20,8.

    Which converts into just 18 000 million times the brightness of Sun.

    How much of this actually originates from Sun-like stars, like G0V to G9VI?

    And how much of Milky Way´s brightness comes from other groups:
    1. hot dwarfs, O0V to F9V
    2. late dwarfs, K0V to M9V
    3. super-to subgiants, I-IV
    I cannot point you to something on the Milky Way.

    However, the general question is well-studied, and goes by the name stellar population synthesis models. Here is a typical intro page:
    http://vivaldi.ll.iac.es/proyecto/tr...thesis-models/

    As noted, these models are pretty good in the optical/visual, less so in the UV and IR. Also, galaxy spectra can be modeled successfully with more than one stellar population (not really surprising).

    I would think that getting good population models for the MW would be very difficult without some bold assumptions.

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    Quote Originally Posted by chornedsnorkack View Post
    The number of 100 000 million "stars" or several times that has been often thrown about when discussing the size of Milky Way.

    However, when looking at absolute magnitude of Milky Way...

    it is -20,8.

    Which converts into just 18 000 million times the brightness of Sun.

    How much of this actually originates from Sun-like stars, like G0V to G9VI?

    And how much of Milky Way´s brightness comes from other groups:
    1. hot dwarfs, O0V to F9V
    2. late dwarfs, K0V to M9V
    3. super-to subgiants, I-IV
    Really good question! I also don't know the answer offhand, but a few things which may be relevant:

    The mass of the Milky Way is 1.5 trillion solar masses, not 100 million.

    That number doesn't mean that there are actually 1.5 trillion suns in the galaxy, just that is the total mass. Most of that mass is not in the form of stars, it's mostly in the form of dark matter and even with the normal matter the majority is not in stars, but in interstellar gas. With all that I've seen the estimates for the actual number of stars to be 100-200 billion, not million. So I think that just made the problem a lot worse for your question. lol.

    Also, most stars are not sun-like. Despite what we were taught as kids our sun is not an average star, it's actually on the large side for a star. Most stars are smaller then our sun, and that means they are dimmer. So that would help with explaining the apparent lack of brightness of the Milky Way, but I'm not sure by how much.

    This article seems to be discussing what you are asking, but they conclude that there is actually a slight excess of surface brightness for the milky way compared to how many stars we estimate it has. I'm not sure why their calculation was different from yours, but it seems like they are talking about exactly what your asking so it should help: http://adsabs.harvard.edu/full/1941ApJ....94..259K

    I'll be interested to hear the explanation from others!

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    Quote Originally Posted by Dave241 View Post
    With all that I've seen the estimates for the actual number of stars to be 100-200 billion, not million. So I think that just made the problem a lot worse for your question. lol.
    I did say 100 000 millions, not 100 millions.
    Quote Originally Posted by Dave241 View Post
    Also, most stars are not sun-like. Despite what we were taught as kids our sun is not an average star, it's actually on the large side for a star. Most stars are smaller then our sun, and that means they are dimmer. So that would help with explaining the apparent lack of brightness of the Milky Way, but I'm not sure by how much.
    I tried summing up for solar neighbourhood, within 5 pc of Sun:
    https://forum.cosmoquest.org/showthr...f-nearby-stars
    Combined brightness:
    2 stars earlier than Sun, A and F classes, Sirius and Procyon combined - 32,5 times as bright as Sun.
    3 Sun-like, G class stars being Sun, Rigil Kentaurus A and Tau Ceti compare to 2,97 times as bright as Sun
    The over 50 stars dimmer than Tau Ceti, starting with Rigil Kentaurus B, combine to 1,1 times the brightness of Sun.

    For 1/500 000 000 total brightness of Milky Way, solar neighbourhood has a total of just 3 Sun-like stars, 60 stars of all kinds and 24 solar masses.
    For Milky Way, that would give mass of mere 12 000 million solar, and 30 000 million stars total, not 100+ millions. Because most of the brightness comes from Sirius.

    But what are the actual averages for Milky Way?

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    Quote Originally Posted by chornedsnorkack View Post
    I did say 100 000 millions, not 100 millions.
    Lol, I didn't notice that until you pointed it out. My mind kept readying 100 million, not 100,000 million (which would be 100 billion, so yes that was correct).

    I've actually not seen it written this way often, so it is a bit confusing at least for me to see it like that. For instance, even though I knew thats how you write numbers when I read this:

    For Milky Way, that would give mass of mere 12 000 million solar, and 30 000 million stars total, not 100+ millions.
    My brain again read "12 million" and "30 million" instead of how you actually wrote it. Perhaps its just me.

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    This is a complex subject, but one relatively easy way to investigate is to use the Besancon model of the galaxy. There's a nice website which will allow you to choose some region of the sky and ask "how many stars of each spectral type are there in this direction, within some range of distances?" Go to

    https://model.obs-besancon.fr/

    I suggest you play with this model -- you may be able to answer your questions for yourself.

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    Dave241:
    I try to say it that way because, back in the day, when I would read something from the UK it would say "half a billion stars in the Local Group" and I would think "that's off by a factor of a thou- oh, British English".
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by Tom Mazanec View Post
    Dave241:
    I try to say it that way because, back in the day, when I would read something from the UK it would say "half a billion stars in the Local Group" and I would think "that's off by a factor of a thou- oh, British English".
    Both usages actually come from the French, who in the 16th century coined billion, trillion, quadrillion to denote the second, third and fourth powers of a million - a pretty logical usage subsequently adopted in Britain. Then, for some reason, French mathematicians changed their minds during the eighteenth century to the slightly odd "how many times do we multiply a thousand by a thousand" thing - which is when America adopted the French usage. Then the French changed back again.

    Meanwhile, we have a perfectly good word for a thousand million, milliard, which everyone seems to have forgotten in the excitement.

    Grant Hutchison

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    Is a billiard 10^15?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Quote Originally Posted by chornedsnorkack View Post
    The number of 100 000 million "stars" or several times that has been often thrown about when discussing the size of Milky Way.

    However, when looking at absolute magnitude of Milky Way...

    it is -20,8.

    Which converts into just 18 000 million times the brightness of Sun.

    How much of this actually originates from Sun-like stars, like G0V to G9VI?

    And how much of Milky Way´s brightness comes from other groups:
    1. hot dwarfs, O0V to F9V
    2. late dwarfs, K0V to M9V
    3. super-to subgiants, I-IV
    No-one has measured the magnitude of the MW from outside -it is a calculated number ! So someone knows how to do it from estimates of the stellar populations etc.

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    Quote Originally Posted by chornedsnorkack View Post
    I did say 100 000 millions, not 100 millions.


    I tried summing up for solar neighbourhood, within 5 pc of Sun:
    https://forum.cosmoquest.org/showthr...f-nearby-stars
    Combined brightness:
    2 stars earlier than Sun, A and F classes, Sirius and Procyon combined - 32,5 times as bright as Sun.
    3 Sun-like, G class stars being Sun, Rigil Kentaurus A and Tau Ceti compare to 2,97 times as bright as Sun
    The over 50 stars dimmer than Tau Ceti, starting with Rigil Kentaurus B, combine to 1,1 times the brightness of Sun.

    For 1/500 000 000 total brightness of Milky Way, solar neighbourhood has a total of just 3 Sun-like stars, 60 stars of all kinds and 24 solar masses.
    For Milky Way, that would give mass of mere 12 000 million solar, and 30 000 million stars total, not 100+ millions. Because most of the brightness comes from Sirius.

    But what are the actual averages for Milky Way?
    The solar neighbourhood is reportedly under-luminous compared to the average for the galactic radius. We're not in a spiral arm.

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    Quote Originally Posted by kzb View Post
    The solar neighbourhood is reportedly under-luminous compared to the average for the galactic radius. We're not in a spiral arm.
    I thought we were in the Orion Arm (or at least the Orion Spur)?
    SHARKS (crossed out) MONGEESE (sic) WITH FRICKIN' LASER BEAMS ATTACHED TO THEIR HEADS

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    Here is an artist's impression of a face-on view of our galaxy from the north, from what appear to be reliable sources.

    https://en.wikipedia.org/wiki/Milky_...annotated).jpg

    The Sun appears to be in a bright spot in what is described as the Orion Spur, which branches off the Sagittarius arm, which branches off the major Perseus arm. It appears from this view that our neighborhood out to several hundred light years is less intense than the brightest parts of the major arms but is more intense than the portions of the disk between the arms. The light in these regions is dominated by a relatively small number of high-luminosity bluish white stars. The central bar is more intense yet, but is dominated by a much larger number of closely spaced low-mass/low-luminosity old stars.

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    Quote Originally Posted by Hornblower View Post
    Here is an artist's impression of a face-on view of our galaxy from the north, from what appear to be reliable sources.

    https://en.wikipedia.org/wiki/Milky_...annotated).jpg

    The Sun appears to be in a bright spot in what is described as the Orion Spur, which branches off the Sagittarius arm, which branches off the major Perseus arm. It appears from this view that our neighborhood out to several hundred light years is less intense than the brightest parts of the major arms but is more intense than the portions of the disk between the arms. The light in these regions is dominated by a relatively small number of high-luminosity bluish white stars. The central bar is more intense yet, but is dominated by a much larger number of closely spaced low-mass/low-luminosity old stars.
    That picture was out of date before it was published. I wouldn't take it too seriously.

    There have been several publications on the luminosity of the Milky Way, many of them far more recent than 1941.

    A couple of papers reported the MW was a 2-sigma outlier, being too dim for its mass.

    Then another study came along to rescue the situation, finding that the solar neighbourhood is under-luminous compared to the average at this galactic radius. Using the solar neighbourhood to estimate the total luminosity therefore leads to an underestimate.

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    Quote Originally Posted by kzb View Post
    That picture was out of date before it was published. I wouldn't take it too seriously.

    There have been several publications on the luminosity of the Milky Way, many of them far more recent than 1941.

    A couple of papers reported the MW was a 2-sigma outlier, being too dim for its mass.

    Then another study came along to rescue the situation, finding that the solar neighbourhood is under-luminous compared to the average at this galactic radius. Using the solar neighbourhood to estimate the total luminosity therefore leads to an underestimate.
    Did I inadvertently link something published in 1941? When I clicked on my own link it took me right to the image published in 2008 by author R. Hurt of NASA/JPL-Caltech, and updated with information about the bar from ESO.

    That patch of light that includes the Sun definitely looks fainter than the fat parts of the major arms, and a circle around the center at the Sun's radius is largely in those bright regions.

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    Quote Originally Posted by Hornblower View Post
    Did I inadvertently link something published in 1941? (1) When I clicked on my own link it took me right to the image published in 2008 by author R. Hurt of NASA/JPL-Caltech, and updated with information about the bar from ESO. (2)

    That patch of light that includes the Sun definitely looks fainter than the fat parts of the major arms, and a circle around the center at the Sun's radius is largely in those bright regions. (3)
    (1) No but Dave241 did.

    (2) I believe that 2008 picture is out of date. The structure of the galaxy is still somewhat controversial, even the number of spiral arms is questioned.

    (3) Yes I think that is what they were saying in that paper I remembered. Extrapolating from the local stellar population to the whole galaxy will underestimate the total brightness. If you correct for that, you restore the MW from being a 2-sigma dim outlier back to a distinctly average galaxy for its size.

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    Found it:

    The surface brightness of the Galaxy at the Solar Neighbourhood

    https://core.ac.uk/download/pdf/44148460.pdf

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