In article ,
Nicolaas Vroom writes:
To see how the steller masses are calculated see:
https://arxiv.org/abs/1607.04678
My impression is that this is not simple (Figure 9)

The basic method is to measure the luminosity of the stars and
multiply by a derived mass-to-light ratio. There are complications
in deriving M/L, the largest being what initial mass function one
assumes. You can think about that as a calibration factor, but
otherwise, if one has observations in the rest near-infrared, the
derived mass won't be wrong by more than a factor of 2 and usually
less.

The main problem around dark matter related to galaxies is in the name,
which is is confusing.
In simple language there are three types of matter: visible baryonic
invisible baryonic and non-baryonic.

The last two are more frequently called "baryonic dark matter" and
"non-baryonic dark matter." The phrase "dark matter" by itself can
mean the sum of these two or the non-baryonic component only,
depending on context. This isn't ideal, but the difference is only a
factor of 0.85 so not usually significant.

The reason why there is no non-baryonic matter in the solar system
is because all the planets (the movements) are accordingly (almost)
to Newton's law.

There's not much dark matter, but the value doesn't have to be
identically zero.

The question is if the same can be said for binary stars in our
galaxy, (or for clusters of three or 4 stars) of which the masses
accurately can be observed. If that is the case than, within such
clusters, there is no extra non-baryonic matter required.

I doubt one could tell for systems with few stars because the stellar
masses aren't known well enough. A better case is globular clusters,
which have no detectable dark matter. That's in contrast to low-mass
galaxies, which overlap the mass range of globular clusters, and do
have dark matter.

In the original document...
What is missing (?) is the ratio: visible-baryonic/invisible-baryonic

The velocity dispersion measures _all_ matter, from which the stars
are subtracted. Therefore in this context, "dark matter" means the
sum of the two dark components. This should be obvious from the
explanation of the methods, but as noted above, it hardly matters.

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