A supernova occurs due to the core collapse in a medium to high-mass star. As the core contracts and temperatures rise, nuclear fusion processes form iron. Since iron fusion absorbs energy, an energy deficit occurs, causing the core to contract further under gravity. This rapid contraction halts abruptly when the core's density reaches a critical point, releasing an immense amount of energy that expels the star’s outer layers into space in a spectacular explosion. The observable effects include a sudden increase in brightness, the release of a vast amount of energy, and the dissemination of heavy elements into space.

Planetary nebulae are formed during the late stages of low-mass star evolution. As a low-mass star exhausts its core hydrogen, it transitions into a red giant, with its core contracting and the outer layers expanding. When the core temperature rises sufficiently, helium begins to fuse into carbon. During this stage, the star experiences pulsations, and these pulsations, combined with intense stellar winds, cause the outer layers to be expelled. The exposed hot core emits ultraviolet radiation that illuminates the ejected gas, creating a planetary nebula. It's a transient phase, and the core eventually cools and fades as a white dwarf.

The fate of a star post-supernova is determined by the mass of its core. Neutron stars form when the core's mass is between about 1.4 and 3 solar masses. In this case, neutron degeneracy pressure counteracts gravitational collapse, resulting in a highly compact and dense star. If the core’s mass exceeds around 3 solar masses, the gravitational force overwhelms all repulsive forces, leading to a complete gravitational collapse and the formation of a black hole. Therefore, while both neutron stars and black holes can originate from supernovae, the core mass is the deciding factor in their formation.

Stars transition between lifecycle stages due to internal nuclear reactions that alter their structure and composition. For instance, a star moves from the main sequence to the red giant phase when its core hydrogen is depleted. The core contracts and heats up, enabling helium fusion while the outer layers expand and cool. These internal changes are driven by the balance between gravitational forces and radiation pressure. As nuclear fuel is consumed, gravitational forces cause the core to contract, raising its temperature and pressure, which in turn initiate new nuclear fusion processes, marking the star's entry into a new lifecycle stage.

The initial composition of a protostar plays a crucial role in determining the star's evolution and fate. A protostar rich in hydrogen and helium will have a distinct evolutionary path compared to one with a significant amount of heavier elements. The initial composition influences the star's nuclear fusion processes, temperature, and luminosity. For instance, a protostar with an abundance of heavier elements may undergo different nuclear reactions, leading to a varied luminosity and temperature profile. Such compositional variations can also impact the star's lifespan, the nature and duration of its main sequence phase, and its ultimate transformation into a red giant, supernova, or other end states.