If some super evil villain decided to launch this so called catalyst into the sun, it would most definitely cause some issues. The core of the sun already has a fusion reaction going. As this catalyst enters the suns atmosphere, the temperature would be enough to try to fuse hydrogen around it.
As Renan pointed out, it would require a pressure, because as the material tried to fuse it would create its own pressure pushing away all other matter. But as it sank further, the pressure would be enough to sustain such a fusion reaction.
Now, with the amount of energy required to initiate fusion is now so low and the energy released stays the same, they sun would consume its nuclear fuel substantially faster, and probably create significantly more pressure due to the increased released energy.
I don't have the numbers, but I would bet that the sun would either burn out quickly or blow itself apart. Now for the question on it being earth, there is not many places in which sufficient quantities of hydrogen exist at those temperatures. It would make fusion reactors easier to start, but just as hard to maintain the reaction.