Main Sequence
Main sequence explained in an easy to understand way:
Imagine trying to organize a diverse group of people by age and height, noticing that most fit into a predictable pattern where older people are generally taller. This is similar to how stars in the universe are organized by their brightness and temperature, forming a predictable pattern known as the main sequence on the H-R diagram. Just as most people fall along a diagonal trend from younger and shorter to older and taller, about 90% of stars fall along a diagonal band on the H-R diagram, from hot and bright to cool and dim, illustrating a natural order in both scenarios.
Practice Version
Main Sequence: The distinct diagonal band on an H-R diagram that includes 90 of all stars main sequence. Main sequence stars are those that are in the stable phase of their lifecycle, fusing hydrogen into helium in their cores.