Is mass all that is required to become a star? I mean, if there were a planet the size of the sun would it "ignite" regardless of the elements of which it is composed?
Pretty much yes.
It really is all down to mass.
Primarily stars are made of hydrogen but that's only because that was what was formed in the big bang.
If you did have a ball of rock the size of the Sun then there would be more than enough pressure in the interior to ignite the atoms there into nuclear fusion.
If you had a planet like Earth though with the iron core then you run into a different situation.
With iron it takes more energy to fuse the atoms together than the fusion releases so you'd need a net input of energy for normal fusion. So it's not going to happen in most parts of the iron core.
The outer parts of the "planet" (the outer parts of the core) that are made of rock etc... however will be under enough pressure to fuse and release energy.
In the deep interior a rough calculation shows that there is more then enough pressure considering the amount of mass you have in this enormous "planet" to enable the core to become degenerate.
This is where all the electrons get shoved into the protons and become neutrons because the gravitational pressure of all that overlying mass overcomes the electrical repulsion that keeps elecrtons from crashing into protons normally. This releases a collossal amount of energy which would almost certainly blow the whole thing to bits but assuming it didn't you'd have a neutron matter core surrounded by almost degenerate matter undergoing intense heating from the fusing outter core. This would force more and more matter to become degenerate and so the degenerate core would grow, releasing more energy as it went as more matter gets pressed down onto it.
The whole thing would basically melt in a very short period as we watched it and then there would be the most amazing explosion as the energy released by the formation of the neutron matter at the core formed.
Personally I'd be watching all of this from a perfectly black spaceship orbiting a completely different star while my friend Hotblack Desiato played his most recent composition on the stereo.