1. Temperature: Stars are classified by their surface temperature, which is directly related to their color. Hotter stars are blue, while cooler stars are red. This is represented by the spectral classes, with O being the hottest (blue) and M being the coolest (red).

2. Luminosity: This refers to the total amount of energy a star emits. Luminosity is influenced by both the star's temperature and size. More luminous stars are brighter, and less luminous stars are dimmer.

3. Spectral Class: Stars are grouped into spectral classes based on their absorption lines (the dark lines in their spectra). These lines indicate the elements present in the star's atmosphere and their relative abundance. The spectral classes are O, B, A, F, G, K, and M, with O being the hottest and M being the coolest.

4. Size: Stars come in a wide range of sizes, from tiny neutron stars to giant supergiants. Size is an important factor in determining a star's luminosity and lifespan.

5. Composition: Stars are primarily composed of hydrogen and helium, but they also contain trace amounts of other elements. The composition of a star can affect its evolution and lifespan.

These characteristics are interconnected and used together to understand a star's lifecycle, properties, and evolution.