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Milky way star count
This is often stated like between 200 and 400 billion stars. How does
one estimate this kind of figure? Is worked on mass alone or extrapolated brightness of milky way in night sky or a little of each? cheres!! S-S |
#2
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Starlight-Starbright wrote:
This is often stated like between 200 and 400 billion stars. How does one estimate this kind of figure? Is worked on mass alone or extrapolated brightness of milky way in night sky or a little of each? Star counts is one method. It has been used since Galileo first realized the Milky Way was composed of stars. William Herschel & Jacobus Kapteyn were famous early star counters trying to get the shape and density of the Milky Way. They counted stars in hundreds of regions in the sky. Star counting using CCD imaging and computer counting and more detailed models of the Galaxy are used nowadays. Unfortunately I'm not personally aware of a recent reference. I did find: "Star Counts Redivivus,part I, A new Look at the Galaxy at Faint Magnitudes", Neill Reid, and S.R. Majewski, The Astrophysical Journal, 409, 635, 1993 ... Also, given the mass of the Sun (msun) and its orbital speed (vsun = 220 km/s) then the mass of the material (M) within the radius(rsun = 8 kpc) of the Suns orbit about the center of the Milky Way can be determined from Kepler's 3rd Law of (planetary, or in this case solar) motion. Well, one needs to assume (1+msun/M)~1 which is very accurate. One finds that M ~ 9 x 10^(10) msun, or about 10^(11)msun, or 100 billion Sun masses, or 100 billion Suns ... it is a "stretch" calling this 100 billion stars of course ... The Sun though is not an average star ... I don't know what an "average" star would be for the Milky Way (K type?, maybe M spectral class) but anyway the number of stars estimate goes up from there ... also we did not include any mass of the Galaxy beyond the Suns orbit (or effects of dark matter) ... all the estimates have large errors associated of course...only approximate numbers are known ... cheres!! S-S |
#3
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Starlight-Starbright wrote:
This is often stated like between 200 and 400 billion stars. How does one estimate this kind of figure? Is worked on mass alone or extrapolated brightness of milky way in night sky or a little of each? cheres!! S-S Oh, Okay, here is another method: Assume the Galaxy is a cylinder :-) with radius (a) of 50000 LY and thickness (h) of 1000 LY ... then its volume (V) is pi*a^(2)*h = 7.85 x 10^(12) cubic LY's ... now form an equivalent volume in the form of a cube with (n-1) uniformly spaced stars on a side. The spacing between the stars is "d" LY. So the length of a side of the cube is d(n-1) and the equivalent volume is [d*(n-1)]^(3) = d^(3)*(n-1)^(3). But (n-1)^(3) = N is the total number of stars. So now we have a way to estimate N, because N = V/d^(3) = pi*a^(2)*h / d^(3). Of course we don't know "d" which is the average distance between stars in the Galaxy assuming an equivalent uniform distribution of stars in a cylindrical shaped galaxy :-) . However, lets assume d ~ 4 LYs, then N = 1.2 x 10^(11) stars, or 120 billion stars ... |
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