How do we determinate the mass of stars

[Bjarne]

How do we determinate the mass of stars (or planet) in other solar systems?

[Arch2008]

Astronomers use the mass-luminosity relation to calculate an approximate mass.
http://en.wikipedia.org/wiki/Mass-luminosity_relation
Also, the masses of binary stars can be determined directly with Kepler's laws.
http://outreach.atnf.csiro.au/education/senior/astrophysics/binary_mass.html
An exoplant's mass is determined by it's parent stars doppler shift as the star "wobbles".
http://planetquest.jpl.nasa.gov/science/finding_planets.cfm

[smallphi]

The mass of a planet or a star can be infered by observing a body orbiting around it - you have to measure the orbital period (time) and the dimensions of the orbit (radius for circles, semimajor axis for ellipse) and then use Newton's law for gravitation to calculate the gravitating mass. The astronomy textbook example is calculating the masses of two stars that orbit around their common center of mass - the so called binary stars. We know the gravitating mass of the Sun from the periods and distances to its planets.

I think mass of star can be infered from it's spectrum by modelling the processes inside it but that is model dependent, let astronomers say how much its reliable.

[Chronos]

Interfereometry is the usual measurement method.

[qraal]

Interfereometry is the usual measurement method.

For size, not mass.

[Chronos]

Mass measuirements using interfereometry is pretty mainstream these days. See, for example: http://www.iop.org/EJ/article/1538-3881/120/2/1106/990454.web.pdf?request-id=a02eb948-b3dc-4629-8fa1-f6a4ec77fe22

[Arch2008]

Luminosity measurements using interferometry may be mainstream, however the mass is determined using the mass-luminosity relation (MLR), as your link notes.

[Chronos]

Agreed, I dont see the issue here.