What internal process marks the significant departure from the main sequence for a star entering the Bright Giant class?
Answer
Actively fusing helium in its core or having just begun doing so
The transition to the bright giant phase signifies a major shift in the star's internal energy generation mechanism, marking its departure from the main sequence where it primarily fused hydrogen in the core. A star that enters luminosity class II or III is actively moving past this initial phase. This means it is now either actively fusing helium in its core, or it has just initiated this process, following the exhaustion of its core hydrogen supply. This new energy source allows the outer layers to expand significantly, leading to the observed higher luminosity and increased radius characteristic of bright giants.
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