248
See also: John Michell 88–89 ■ Albert Einstein 214–21 ■ Paul Dirac 246–47 ■
Fritz Zwicky 250–51 ■ Stephen Hawking 314T
he 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 burned-out cores of
sunlike 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 (p.230)—that no
two particles can occupy the same
quantum state—comes into play.Forming a black hole
In 1930, Indian astrophysicist
Subrahmanyan Chandrasekhar
figured 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 spaceknown 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. ■THERE IS AN UPPER
LIMIT BEYOND WHICH
A COLLAPSING STELLAR
CORE BECOMES UNSTABLE
SUBRAHMANYAN CHANDRASEKHAR (1910–1995)
IN CONTEXT
BRANCH
AstrophysicsBEFORE
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 spiraling into what is
probably a black hole—the first
such object to be confirmed.The black holes of nature are
the most perfect macroscopic
objects in the Universe.
Subrahmanyan
Chandrasekhar