In order to get a better idea of what stars are like, put them into groups of some sort. Then you can see how the other quantities differ among the various groups. Astronomers group stars into general types based on their temperature. Temperature is chosen because the color of a star depends on the temperature and color is a easily seen characteristic, regardless of the distance. However, using color as a temperature probe gives only a crude measurement of the star's temperature.
Astronomers use another method of determining the temperature more accurately.
It uses the strength of different absorption lines in a star's spectrum. Once astronomers developed this method, they began to look for correlations of temperature with other quantities such as mass, size, and luminosity in the hope that the underlying physical principles of stars could be understood. You will see how that important question can be answered. The strength and pattern of the absorption lines does vary among the stars. When scientists learned more about the physics of the atom, they discovered that the temperature of the star's photosphere determines what pattern of lines you will see.