Whatever happened, any scenario involving the Sun going through its red giant phase would have to involve an explanation of how Mercury still exists. The reason is that when the Sun goes red giant, its radius will exceed that of Mercury's, Venus' and even Earth's orbits. The planets will just be melted, Their mass added to that of the star.
But Mercury's destruction obviously didn't happen in the Simulant Future.
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<Right, so wtf happened? It didn't collapse, it didn't go supernova. How far into the future is this any how?>
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How about the vex somehow used it to power their machinery and exhausted it? Their weapons are powered with the energy of other stars, afterall.