1. Temperature: This is determined by the color of the star. Hotter stars emit more blue light, while cooler stars emit more red light.

2. Luminosity: This refers to the total amount of energy a star emits per second. It is determined by the star's size and temperature.

3. Spectral Class: This is based on the absorption lines in a star's spectrum, which reveal the chemical composition and temperature of the star's outer layers. The spectral classes are organized by temperature, from hottest to coolest: O, B, A, F, G, K, and M.

These three criteria form the basis for the Hertzsprung-Russell (H-R) diagram, which plots stars according to their luminosity and temperature.