During which stage of a star's life cycle is it considered stable?

A star is considered stable during its Main Sequence phase. In this stage, the star is in hydrostatic equilibrium, meaning that the gravitational force pulling inward is balanced by the pressure produced from nuclear fusion occurring in its core. This nuclear fusion, primarily converting hydrogen into helium, provides a steady source of energy that sustains the star's size and temperature.

During the Main Sequence phase, which can last for millions to billions of years depending on the star's mass, the star maintains a relatively constant luminosity and temperature. The balance between these opposing forces allows the star to remain in a stable state, creating a predictable life cycle.

While other stages like Red Giant, White Dwarf, and Protostar involve various instabilities, like rapid expansion, collapse, or gravitational changes, the Main Sequence represents a long-duration phase where the star exists in a stable environment, continuously fusing hydrogen until that fuel begins to deplete. This stability is a defining characteristic of stars during this critical phase of their evolution.