The diagonal band on the Hertzsprung-Russell diagram where most stars are located, representing stars that are fusing hydrogen into helium in their cores. Stars spend most of their lives in this stable phase of stellar evolution.
The term was introduced in the early 20th century when astronomers Henry Russell and Ejnar Hertzsprung independently discovered the relationship between stellar luminosity and temperature. 'Main' indicates it's the primary evolutionary phase, while 'sequence' refers to the orderly arrangement by mass.
The main sequence is like stellar middle age - it's the longest, most stable phase of a star's life where it's perfectly balanced between gravity trying to crush it and nuclear fusion trying to blow it apart! Our Sun has been a main sequence star for 4.6 billion years and has about 5 billion more to go.
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