A forming star, known as a protostar, doesn't truly become "stable" in the same way a mature star does. Instead, it goes through a series of stages as it accumulates mass and heats up.

Here's a breakdown:

Early Stages:

* Pre-stellar core: A dense, cold cloud of gas and dust collapses under its own gravity. This stage is very long, lasting millions of years.

* Protostar formation: As the core collapses, it heats up and becomes a protostar. This is marked by the emission of infrared radiation.

Main Sequence Star Formation:

* Hydrostatic equilibrium: Eventually, the core becomes hot enough and dense enough for nuclear fusion to begin. This is the point where the outward pressure from fusion balances the inward pull of gravity. This is the critical point where the protostar becomes a main sequence star.

* Stability: The star now burns hydrogen into helium in its core, releasing energy that keeps the star stable.

Important Notes:

* Variable timeline: The exact time it takes for a protostar to become a main sequence star depends on its initial mass. Massive stars form much faster than smaller stars.

* "Stable" is relative: While a main sequence star is considered stable compared to a protostar, it still undergoes evolution over millions or billions of years. Stars change as they age, burning through their fuel and eventually evolving into giants or even exploding as supernovae.

Key takeaway: A protostar becomes "stable" when it reaches hydrostatic equilibrium, where the outward pressure from nuclear fusion balances the inward pull of gravity. This marks the transition from a protostar to a main sequence star.