What factor almost entirely predetermines the future size of a star, determining if it becomes a giant or a supergiant?

Answer

Its initial mass

The initial mass of a star is the singular, fundamental determinant of its entire subsequent evolutionary track, including whether it expands into a standard giant or a truly massive supergiant. This initial mass sets the maximum gravitational pressure achievable in the core. Higher initial mass leads to much higher core temperatures and pressures during subsequent fusion stages, allowing the star to ignite heavier elements. For stars up to eight times the Sun's mass, this limit results in a red giant phase followed by white dwarf formation. Stars exceeding this $8 M_{\odot}$ threshold possess enough mass to sustain fusion through heavier elements, leading to the vastly larger volumes and different end-states characteristic of supergiants. This concept establishes a clear bifurcation point very early in the star's life cycle.

What factor almost entirely predetermines the future size of a star, determining if it becomes a giant or a supergiant?

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How Do Stars Become Red Giants Or Supergiants? - Physics Frontier

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