How bright an object appears to us depends on its distance. For example, a 100 watt lightbulb looks very bright when observed from a distance of a few feet, but is barely visible from a distance of one mile. In practice, astronomers traditionally use the magnitude system for measuring star brightnesses. We thereby learn how much energy an object is emitting. This was critical in understanding how stars create energy through nuclear fusion in their cores.
Some stars exist in pairs called binaries.
The stars orbit around each other, bound together by their mutual attraction due to gravity. Knowing the masses of stars enables us to understand more about their nature. For example, if we know the mass of a star, we can make a computer model that describes the interior structure of the star by using a number of physical laws relating gas pressure, bouyancy, etc. The model tells us the density, temperature, and pressure at any place in the star, as well as how much energy is being produced by nuclear fusion at different places. It enables us to figure out how stars work.