What single factor dictates a star's placement on the main sequence and its subsequent lifespan?
Answer
Mass
Mass is the supreme factor that determines a star's entire main sequence existence, dictating the core temperature and pressure required, which in turn controls the rate of hydrogen fusion.

#Videos
What Defines A Main Sequence Star? - YouTube
Related Questions
What is the primary energy-generating process defining a main sequence star?What term describes the state of balance between inward gravity and outward fusion pressure in a star?What single factor dictates a star's placement on the main sequence and its subsequent lifespan?Approximately what fraction of all stars in the universe are currently on the main sequence?Which graph is used by astronomers to plot stellar properties where the main sequence appears as a distinct diagonal band?What happens to a star's lifespan as its mass increases significantly?What are the massive, hot, and very bright stars found at the upper-left end of the main sequence called?Which fusion reaction does a star like our Sun primarily use during its main sequence phase?What happens to a star immediately after it depletes the hydrogen fuel supply in its core?What classification is given to stars that are too small to ignite sustained hydrogen fusion?What does a star like the Sun evolve into immediately after leaving the main sequence?