Now, to measure the masses of the other stars out there the first thing we need to find are some Binary (two) Systems - two or more stars in orbit around one another. It turns out that binary systems are quite common and make up more than 1/3 of the stars in the Galaxy.

Binary Systems

There are several kinds of binaries:

Mass and the HR Diagram

Measure the masses for as many stars as you can find in these systems and discover the Mass-Luminosity relation for main-sequence stars.

Mass-Luminosity Relation for main sequence stars

There is a pretty "steep" relationship between Mass and Luminosity in the sense that more massive stars are more luminous, thus

The Main-Sequence in the H-R Diagram is a Mass Sequence!

For each position along the main sequence there is a corresponding stellar mass:

The main sequence is a mass sequence

A Review of Star Properties: