Transcript: Mass is a fundamental property of a star, but it can be difficult to measure. It’s a question of how do you weigh a star in empty space? Typically astronomers make a model of the star based on the knowledge of its energy source, and this can lead to an estimate of its mass. By direct observation the best situation is the case of a binary star where we can use the orbit to estimate the mass by an application of Kepler’s laws.