### mass function

(measure of binaries that constrains the masses)

For binary stars, the **mass function**
(or **binary mass function**) is a formula
of observable values that yields constraints
on the masses of the two stars if the radial velocities
can be determined through Doppler shifts.

P
——— V³
2πG

This value is equal to the following constraint on the masses:

m³
—————— sin³i
(M+m)²

**M** - mass of the same star as the velocity measurement.
**m** - mass of the other star.
**i** - inclination of the orbit with the line of sight.

The inclination is evident if it also transits,
or may be estimated by the "average" inclination.
For star population studies, there is the potential
that some useful statistics may be derived assuming the
inclinations are random.

Note that the phrase *mass function* is to indicate it
deals with physical masses. The two-word phrase is used in probability
(**probability mass function**) for an incompatible concept:
a function yielding the probability of some discrete random variable
taking on a given value.

(*binary stars,stellar mass,function*)
http://en.wikipedia.org/wiki/Binary_mass_function

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