Stars create heavier elements via nuclear fusion, the process whereby lighter elements are fused together to make heavier elements, subsequently releasing energy. The heavier the element, the less energy is released when they’re fused. The typical path stars take is by first fusing hydrogen into helium, then helium into carbon, carbon into oxygen, oxygen into neon, neon into silicon and then, finally, silicon into iron. Fusing iron requires more energy than is released, so it’s the last step in any stable nuclear fusion reaction. The majority of stars die before they reach the point where they start fusing carbon, but those who do get to this point, or further, typically erupt into a supernova soon after.
An iron star is a star which is composed purely of iron, but is paradoxically still releasing energy. How? Via quantum tunneling. Quantum tunneling refers to the phenomenon whereby a particle passes through a barrier it would otherwise be unable to transverse. To use an example: if I threw a ball at a wall, it would normally hit the wall and bounce back. But according to quantum mechanics, there is a small chance that the ball could pass through the wall, and hit the unsuspecting person on the other side.
That is quantum tunneling. Of course, the probability of this happening is infinitesimal, but at an atomic level it occurs relatively frequently, especially within huge objects such as stars. Normally, a large amount of energy is required to fuse iron, as it has a barrier of sorts which resists fusion, meaning that it requires more energy than it gives out. With quantum tunneling, however, iron can fuse without using any energy at all. One way of comprehending this is by imagining two golf balls slowly rolling towards each other and spontaneously merging when they collide. Usually this fusion would require a huge amount of energy, but quantum tunneling allows it to occur with practically none.
Since iron fusion via quantum tunneling is extremely rare, an iron star would need to have an extremely high mass to experience a sustainable fusion reaction. For this reason – and because iron is relatively rare in the universe – it is thought that it will take just under 1 Quingentillion years (1 followed by 1503 zeros) before the first iron stars appear.