The final collapse of a star. The process begins when red giants with masses less than or equal to about three solar masses do not have internal temperatures that are sufficiently high to ignite any further nuclear fusion reactions after the completion of the helium burning stage. For these relatively small stars, shedding of the outer layers begins and the mass is reduced by some 50%.

The material that was once the outer layers of the star surrounds a small collapsing core. The radiation from this core can be very intense and the resulting glow, caused by the ionization of the surrounding material, is known as a planetary nebula. The core continues to undergo gravitational collapse until equilibrium is reached with the Fermi pressure. This Fermi pressure is generated by the electrons being confined to smaller and smaller spaces as a result of the gravitational collapse. Electrons, which are responsible for most of the volume in ordinary matter, when confined in this way generate a pressure because as fermions they are subject to the Pauli exclusion principle and therefore have an appreciable mutual repulsion. The equilibrium between gravitational collapse and the Fermi pressure is reached when the core has a diameter of about 1% of the sun. This implies a very high density of about 1000 kg/cm³. At this stage the star is classed as a white dwarf and appears on the bottom-left quarter of the Hertzsprung–Russell diagram. White dwarfs gradually become dimmer as they cool and eventually reach a temperature equilibrium with the surrounding vacuum.

If at the white dwarf stage a star has a mass exceeding 1.4 solar masses, it is said to have exceeded the Chandrasekhar limit. For stars of this mass, the electron Fermi pressure is not large enough to counter the continued gravitational contraction. Electrons and protons in the resulting dense matter then combine in an interaction similar to the radioactive transformation known as electron capture. Neutrons are formed in this electron-proton fusion and further collapse may be halted (if the star’s mass is not so large) by a Fermi pressure generated by the tightly packed neutrons (as neutrons are also fermions). The core is then said to consist of neutronium, since it is exclusively made of neutrons.

The final collapse to the neutronium stage is very rapid and accompanied by an extreme rise in temperature. This rapid contraction is stopped as suddenly as it began by the neutron Fermi pressure, which generates an intense radiation pressure causing the star to explode. The resulting explosion is called a supernova. Supernovae can be so intense that over a short period (typically a few days) they have been known to emit as much radiation as an entire galaxy of stable stars. The extreme pressure and temperatures within a supernova explosion are sufficient for thermonuclear fusion of nuclei to form nuclei of elements heavier than iron. The debris from supernova explosions is therefore the source of all the elements with which we are familiar on earth. As a result of density fluctuations in clouds of debris of this type, gravitational collapse can begin once again and form a new generation of stars. At the centre of some supernovae there may be a residual highly dense core. This remnant, composed of neutrons, is called a neutron star. Theoretically, neutron stars can rotate at a very high rate and in doing so emit very regular radio-frequency pulses. Before the theoretical work that led to the idea of neutron stars, pulsars—pulsating sources of radio waves—had already been observed. The current explanation of pulsar radiation is that it originates from a rotating neutron star.

The Fermi pressure generated by the tightly packed neutrons within a neutron star is the only influence preventing further gravitational contraction. However, if the mass of the neutron star is above about three solar masses, then the neutron Fermi pressure is not sufficient to prevent a further collapse. As the radius of the star reduces, the gravitational field becomes increasingly strong until eventually the radius of the star falls to lower than a limit, called its Schwarzschild radius (τ‎_s). At this point the equations of general relativity are only valid for an observer outside the Schwarzschild radius. The boundary that delimits this region of validity is called the event horizon.

The gravitational fields within the event horizon are so large that not even electromagnetic radiation (including light) can escape. The star is then said to have collapsed to a black hole. It is thought that the collapse of a large star to a black hole is accompanied by a gamma-ray burst. Black holes are still only theoretical constructs although many observations suggest that they do exist. For example, the intense radiation emitted from quasars may be produced as a consequence of matter falling into black holes. Other examples that support the existence of black holes include the detection of very massive but invisible partners in binary systems and the rapidly orbiting gases around galactic centres. Both phenomena could be explained by the presence of centrally directed forces resulting from a black hole. Cygnus X-1 is believed to be an example of a binary system in which one member is a main-sequence star and the other a black hole. As charged gases from the main sequence star spiral in towards the black hole, they reach very high speeds and can emit X-rays.