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What “reactor” usually means:
>Engineering definition (strict). “A nuclear reactor is a device used to initiate and control a sustained nuclear chain reaction.”
>Functional definition (broad). Any bounded system that sustains nuclear reactions and produces energy—e.g., the natural fission zones found two billion years ago at Oklo—can be called a reactor.

Arguments for calling the Sun a reactor
>Continuous self-sustaining fusion. In its 15 million °C core, hydrogen fuses into helium, releasing ~3.8 × 1026 W of power.
>Built-in regulation. Gravity acts like a “natural control rod”; core expansion cools the plasma, throttling the fusion rate, and vice-versa.
>Scientific usage. Astrophysicists freely describe stars as “natural fusion reactors,” the same way they speak of Oklo as a natural fission reactor.
>Proof of concept. The Sun shows that fusion can run stably for billions of years—precisely what terrestrial fusion programs hope to mimic.

Arguments against calling the Sun a reactor
>No human control. The standard definition stresses deliberate initiation and control; we can neither start nor stop the Sun.
>Not a manufactured device. A reactor is normally a purpose-built vessel or structure; the Sun is an astronomical body.
>Energy isn’t harnessed in situ. Reactors confine energy so humans can extract it efficiently; the Sun radiates its energy into space indiscriminately.
>Different mission. Reactors are designed for electricity, propulsion, or isotope production, whereas the Sun’s “mission” is simply to exist as a star.

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