Massive stars undergo dramatic evolutionary pathways that differ significantly from Sun-like stars, progressing through extreme internal transformations, explosive endings, and the creation of dense stellar remnants that shape the structure and chemical composition of the universe. Their behaviour is governed by mass-dependent nuclear processes, rapid core evolution, and catastrophic gravitational collapse that produces some of the most energetic phenomena known in astrophysics.

Evolution of Massive Stars

Massive stars, typically with initial masses greater than about eight solar masses, follow a sequence of stages that reflects increasing central temperature and pressure. Their structure changes more rapidly and violently than that of low-mass stars, owing to the faster consumption of nuclear fuel and the enormous gravitational forces acting on their cores.

Red Supergiant Stage

As hydrogen in the core becomes depleted, the star can no longer sustain core hydrogen fusion, forcing structural readjustment. The outer layers expand and cool, forming a red supergiant, an extremely large and luminous star with a swollen outer envelope. During this phase, nuclear fusion continues in shells surrounding an increasingly compressed core.

Red supergiants often display complex internal layering, with different fusion reactions occurring simultaneously at varying depths.

Onion-shell structure in an evolved massive star, with concentric layers where different nuclei fuse. The innermost iron core is inert to further exothermic fusion, setting up imminent collapse. Labels highlight successive burning shells consistent with massive-star evolution. Source.

This multi-shell burning allows the synthesis of progressively heavier elements up to iron.

The star’s enhanced luminosity and extended radius result in significant mass loss. Stellar winds strip the outer atmosphere, expelling heavy elements into the surrounding interstellar medium, a process crucial for cosmic chemical enrichment.

End of Fusion and Core Instability

Fusion in the core proceeds through elements such as helium, carbon, neon, oxygen, and silicon. Each stage produces an inert core of the next heavier element. The ultimate result is the formation of an iron core, which cannot undergo exothermic fusion. Without an energy source to counteract gravity, the core becomes unstable.

Between the exhaustion of silicon fuel and the onset of collapse, only a brief period elapses. The core becomes supported solely by electron degeneracy pressure, which is eventually overwhelmed by gravity as the mass of the core approaches a critical limit.

Core-Collapse Supernova

The fundamental fate of a massive star is determined by the catastrophic collapse of its core followed by an outward explosion known as a Type II supernova. This event marks the destruction of the star’s outer layers and the birth of a compact remnant.

Stages of a core-collapse supernova: collapse of the iron core, formation and temporary stalling of the shock, and neutrino heating that revives the shock to expel the envelope. The final panel shows ejected material and a compact remnant. Labels (a–f) provide a clear, stepwise physical narrative. Source.

Collapse Phase

The collapse begins when electron degeneracy pressure fails. The star’s core contracts rapidly, and protons and electrons combine to form neutrons and neutrinos in a process known as electron capture. The density rises dramatically, creating a proto-neutron core. Neutrinos escape initially but soon become trapped as density increases.

Rebound and Explosion

When nuclear densities are reached, the collapse halts abruptly as the extremely compressed matter becomes incompressible. This sudden stop generates a shockwave propagating outwards. Although initially stalled, the shock is re-energised partly by neutrinos streaming from the core, enabling it to blast apart the star’s outer layers.

Bullet points highlighting key features of a supernova explosion:

• Rapid core collapse to near-nuclear density

• Shockwave formation and revival

• Ejection of outer layers at high velocity

• Intense neutrino emission transporting energy

• Production of heavy elements beyond iron

The supernova disperses newly formed heavy elements into space, seeding future star systems with essential materials.

Formation of Stellar Remnants

Neutron Stars

If the mass of the collapsed core is below a critical threshold, the result is a neutron star, an extraordinarily dense body composed almost entirely of neutrons. Typical neutron stars have masses around 1.4–2 solar masses and radii of only about 10 km.

Neutron stars may rotate rapidly and possess strong magnetic fields, giving rise to observable phenomena such as pulsars.

Diagram of a pulsar: a rotating neutron star whose magnetic axis and spin axis are misaligned, producing sweeping radiation beams. Field lines and beam geometry are clearly indicated. This includes some extra context about lighthouse-like pulsations, which is helpful but not required by the syllabus. Source.

Black Holes

If the core remnant exceeds the maximum supportable mass of a neutron star, gravitational collapse continues, leading to the formation of a black hole. No known physical pressure can halt this collapse.

Massive stars that produce black holes typically have particularly large initial masses or undergo significant fallback of material onto the remnant after the supernova.

Overview of Mass-Dependent Outcomes

• Moderately massive stars → red supergiant → supernova → neutron star

• Very massive stars → red supergiant → supernova → black hole

The pathway of a massive star is therefore determined chiefly by its initial mass and the behaviour of its collapsing core.