IN CONTEXT

BRANCH

Astrophysics

BEFORE

19th century White dwarf stars are discovered when astronomers identify a star that has far more mass than its tiny size would suggest.

AFTER

1934 Fritz Zwicky and Walter Baade propose that explosions known as supernovae mark the deaths of massive stars, and the collapse of their cores form neutron stars.

1967 British astronomers Jocelyn Bell and Anthony Hewish detect rapidly pulsing radio signals from an object now known as a “pulsar” – a rapidly rotating neutron star.

1971 X-ray emissions from a source known as Cygnus X-1 are found to originate from hot material spiralling into what is probably a black hole – the first such object to be confirmed.

The development of quantum physics in the 1920s had implications for astronomy, where it was applied to the understanding of superdense stars known as white dwarfs. These are the burnt-out cores of Sun-like stars that have exhausted their nuclear fuel and collapsed, under their own gravity, to objects about the size of Earth. In 1926, physicists Ralph Fowler and Paul Dirac explained that collapse stops at this size due to the “degenerate electron pressure” that arises whenever electrons are packed together so tightly that the Pauli exclusion principle – that no two particles can occupy the same quantum state – comes into play.

Forming a black hole

In 1930, Indian astrophysicist Subrahmanyan Chandrasekhar worked out that there was an upper limit to the mass of a stellar core beyond which gravity would overcome the degenerate electron pressure. The stellar core would collapse to a single point in space known as a singularity – forming a black hole. This “Chandrasekhar Limit” for a collapsing stellar core is now known to be 1.44 solar masses (or 1.44 times the mass of the Sun). However, there is a middle stage between white dwarf and black hole – a city-sized neutron star stabilized by another quantum effect called “neutron degeneracy pressure”. Black holes are created only when the neutron star’s core exceeds an upper limit somewhere between 1.5 and 3 solar masses.

"The black holes of nature are the most perfect macroscopic objects in the Universe."

Subrahmanyan Chandrasekhar

See also: John Michell • Albert Einstein • Paul Dirac • Fritz Zwicky • Stephen Hawking