Stars are classified by several factors, but the most fundamental is their spectral type. This system, known as the Harvard Spectral Classification, categorizes stars based on their surface temperature.

Here's a breakdown of the classification system:

O: The hottest stars (25,000 K and above)

B: Hot stars (10,000 - 25,000 K)

A: Moderately hot stars (7,500 - 10,000 K)

F: Intermediate temperature stars (6,000 - 7,500 K)

G: Sun-like stars (5,200 - 6,000 K)

K: Cooler stars (3,500 - 5,200 K)

M: The coolest stars (2,000 - 3,500 K)

Each spectral type is further subdivided into ten classes using numerical suffixes (e.g., A0, A1, A2, etc.).

Other classifications:

* Luminosity class: Based on a star's size and luminosity. It's denoted by Roman numerals (I - V) with I being the most luminous and V being the least. Our Sun is classified as a G2V star.

* Chemical composition: While the spectral type is primarily based on temperature, it also reveals the composition of a star's atmosphere.

* Age: Stars evolve over time, and their properties change. Astronomers can estimate the age of a star based on its location on the Hertzsprung-Russell diagram and other factors.

* Mass: A star's mass is a key factor in its evolution and lifespan. While not directly part of the spectral classification, it's an important characteristic used to understand stars.

The combination of these classifications helps astronomers understand a star's properties, evolution, and even its history.