As a star similar to the Sun ages, what element is polluting the energy source at the core of the star?

Answer 1

Helium

Answer 2

Helium is being fused from hydrogen, being a heavier gas it will accumulate near the core of the sun and eventually starve the core from an adequate supply of hydrogen to maintain the fusion process, if the sun has used up enough of its total mass it will begin to expand into a red giant.

Answer 3

iron

since it is at the top of the binding energy curve
http://en.wikipedia.org/wiki/Image:Binding_energy_curve_-_common_isotopes.svg

it absorbs energy if it undergoes nuclear reactions

unlike the others
http://en.wikipedia.org/wiki/Binding_energy

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look at my first link. it shows helium can be fused to release energy. or this if you dont believe me
http://en.wikipedia.org/wiki/Triple-alpha_process

in fact fusion of anything up to iron releases energy. fusing iron TAKES energy
so
"what element is polluting the energy source at the core of the star?" iron!

you can even read the 'isotopes' part of this article
http://en.wikipedia.org/wiki/Iron#Isotopes
if you still dont believe me

"it [56Fe] is the endpoint of fusion chains inside extremely massive stars" it says. and why do you think that is the case?!

IRON! :)