Iron

Stars start their life as huge spheres of hydrogen gas. But the pressure at the center of the sphere should be much greater than the pressure at the surface, right? When the pressure at the center reaches a certain level, the hydrogen gas starts fusing together forming heavier helium gas (then all the elements up to iron). This fusion process releases energy. This released energy heats up the inner core and pushes the surrounding shell of gas outwards. But just like when you on Earth throw a ball upwards it has to fall back to you, the outer shell of the star has to fall back to the core. The only difference is that the fusion in the core of the star makes the core denser; that is the same core mass will slowly occupy smaller volume. A smaller core radius and same mass leads to stronger gravitational forces. If the same thing were happening to Earth's radius then every time the ball returns to you it would feel a little bit heavier. A time comes when the ball becomes so heavy that it breaks your arm. But this is what really happens in the core of stars. A time comes when the gravitational forces in the core become so strong that they fuse the electrons with the protons and turn them into neutrons. This breaks down all atoms!

Up until the discovery of Pulsars it was thought that no atomic nucleus can survive this gravitational collapse at the core. It was thought that the remnants of those collapsed cores consist of some electrons, some protons but predominantly neutrons (thus the name neutron stars). However the discovery of radio waves in pulsars means that neutron stars contain rotating iron cores! This means that the iron survived! Today we know that iron has the strongest bound nucleus amongst all elements.

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