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Stellar classification is a system used to categorize stars based on their spectral characteristics. This system helps astronomers understand the different types and life stages of stars. The spectral classification runs from O, B, A, F, G, K, to M, signifying a star's temperature and color. The hottest stars are classified as O-type, with blue hues indicating their high temperatures and masses.

• O-type stars are the hottest and most massive, often appearing blue in color, and have strong ionized helium lines.
• B-type stars are also hot and massive, but slightly cooler and less massive than O-type stars.
• A-type stars are white or bluish-white, very luminous, and usually show strong hydrogen lines.
• F-type stars are yellow-white and moderately hot, cooler than A-type stars.
• G-type stars, like our Sun, are yellow and have even cooler temperatures.
• K and M-type stars are cooler and appear orange to red, with M-type being the coolest and least massive.

Stellar classification is crucial for understanding how stars evolve and where they are in their lifecycle.

The main sequence is a continuous band of stars appearing on the Hertzsprung-Russell diagram, representing the primary phase of a star's life. In this phase, stars fuse hydrogen into helium in their cores, which provides the energy that makes them shine.
Stars spend the majority of their lives in the main sequence stage. The position on this band is determined by the star's mass and composition. O-type stars are found at the top-left end of the main sequence because they are the hottest and most massive. In contrast, cooler, less massive stars like M-types are at the lower-right end.
Joining the main sequence is a star's entry point in stellar evolution, and leaving it indicates that it has advanced to a new stage. The time a star remains on the main sequence depends heavily on its mass; massive stars exhaust their nuclear fuel rapidly, while smaller stars can remain stable for billions of years.

Star lifespan refers to the duration of a star's existence from its formation to the end of its stellar life. A star's mass primarily determines its lifespan. Massive stars burn through their nuclear fuel quickly due to their higher temperatures and pressures. For example:

• O and B-type stars, with masses ranging from 10 to over 100 solar masses, typically last only a few million years.
• A-type stars, with masses of around 2 to 3 solar masses, have lifespans of several hundred million years.
• F, G, K, and M-type stars are progressively smaller and can last billions of years. An F-type star might last up to 2 billion years, while K and M types can survive for tens of billions of years.

The absence of certain spectral types within a star cluster provides clues about its age. If only cooler, less massive stars remain, it indicates the cluster is older, as the more massive stars have already exhausted their fuel and evolved into later stages.