1. Temperature: This determines the color of a star. Hotter stars are blue, while cooler stars are red. Temperature is often represented using a spectral type, which goes from hottest to coolest as: O, B, A, F, G, K, and M.

2. Luminosity: This refers to the total amount of energy a star radiates. Luminosity is often represented using a luminosity class, which goes from most luminous to least luminous as: Ia+, Ia, Ib, II, III, IV, V.

By combining these two variables, astronomers can create a Hertzsprung-Russell (H-R) diagram, which plots the temperature of a star against its luminosity. This diagram is a powerful tool for understanding the life cycle of stars and their properties.