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In a hot radiation-supported stellar core, electron-positron thermal pair abundance can grow as radiation energy is converted into pair rest energy. The stellar adiabatic exponent can fall below $4/3$, permitting contraction and rapid heating. If the resulting explosive nuclear fusion releases enough energy, it disrupts the star in a pair-instability supernova. Weaker events can produce pulses rather than full disruption, and still heavier cores can collapse without an explosion. Thus the thermal relativity scale alone neither establishes the instability nor predicts its outcome.

Temperature-sensitive nuclear fusion can heat material faster than cooling and expansion can regulate it, accelerating the burning. In a classical nova, hydrogen accumulated on a white dwarf ignites; initial electron degeneracy pressure helps suppress the usual expansion response. The eruption can eject the envelope without destroying the underlying white dwarf.