Measuring the Masses of Stars


Measuring the mass of a star really means measuring the gravitational force of that star on another object.

For the Sun this means measuring the gravitaional force of the Sun on the planets. For other stars this means measuring the gravitational force of the star on another star: Binary Stars.

We must use Newton's law of gravity applied to Kepler's third law of planetary motion:

If we can measure the orbital period and the physical size of the orbit we can calculate the sum of the stellar masses. If we can measure the ratio of orbital sizes we can also get the ratio of masses, and hence each individual stellar mass.

To do this requires measuring

  1. the angular size of the orbits
  2. the orbital period
  3. the distance to the binary stars

Both (1) and (3) require that the binary star is close to Earth.



Different Types of Binary Stars:

VISUAL



SPECTROSCOPIC



ECLIPSING





What do we know about masses of stars?


More luminous stars are more massive:



Low mass stars are much more common: