The main sequence.
When it is a young adult
The "star life cycle" refers to stars. Earth is not a star.
Nobody "invented" it, the life cycle of stars happens naturally.
main sequence stars
In the nebula!
Interphase
main sequece
The smaller a star is, the longer its life cycle.
the answer is a protostar
The more the mass the shorter their life cycle (the more quickly they use their fuel)
The most important factor is the stars mass.
Stars are "born", mature, reach old age, then "die".