1. Temperature: Measured in Kelvin, this is the most fundamental aspect, determining the star's color and spectral class (O, B, A, F, G, K, M, etc.). Hotter stars are blue, while cooler ones are red.
2. Luminosity (Absolute Magnitude): This refers to the star's intrinsic brightness, independent of its distance from Earth. It's measured on a logarithmic scale (smaller numbers are brighter).
3. Spectral Class: This refers to a star's chemical composition and temperature, categorized into letters like O, B, A, F, G, K, M, and further subdivided with numbers (e.g., G2).
4. Size (Radius): This indicates a star's physical dimensions, ranging from tiny white dwarfs to massive giants.
5. Mass: This is crucial for understanding a star's lifespan and evolution. Massive stars live fast and die young, while smaller stars have longer lifespans.
6. Composition: Stars are primarily composed of hydrogen and helium, but trace amounts of other elements can influence their appearance and evolution.
7. Age: This is determined by various methods like observing stellar clusters and studying star formation.
Therefore, while "four variables" is not entirely accurate, it might be referring to a subset of these key factors. For instance, astronomers often use temperature, luminosity, spectral class, and size as a starting point for classifying stars.
