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

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

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 ...