My understanding--though it's from a Science Channel show so I'm not sure if it is correct--is that there is a fusion reaction happening in the center of the sun. Atomic nuclei are being created and fused into higher atomic weights from lower atomic weights. By the time this reaction gets to iron on the periodic table (billions of years), the iron atoms are too heavy to allow the reaction to continue, so the star collapses and goes supernova within a short time.
Is this true? If so, can a comet made of iron enter the sun's core and destroy it? How much iron would it take? Or do I just have an inaccurate understanding (entirely possible).
Note: I do not intend to try to destroy the sun.
Answer
First, a minor correction: iron is not too heavy to allow the reaction to continue, it is incredibly stable and therefore cannot produce anything else (it would take $\sim10^{22}$ years for it to decay into chromium and its binding energy is the highest per nucleon, so it would "cost" more to produce something heavier than iron has in it). Research shows that once silicon is produced in the core (silicon produces iron), the star has about a week before it blows up!
However, in order to produce that in the core, the sun needs to be about 6-8 times our suns mass. So rest assured that our sun will not go supernova (though it will go through a red giant phase and swallow earth in about 5 billion years).
The boiling point of iron is about 3000 K (5000 F) while the surface temperature of the sun is about 5500 K (10,000 F), so this comet-of-iron would evaporate en route to the sun's surface.
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