178
See also: Discovering white dwarfs 141 ■ Nuclear fusion within stars 166–67 ■
Supernovae 180–81I
n 1930, a young Indian student
named Subrahmanyan
Chandrasekhar calculated
that a star that ends its life with
a little more mass than the sun
cannot hold itself up against the
pull of its own gravity. This was
key to understanding the life cycles
of stars, and in particular the dim,
very hot stars called white dwarfs.
This type of star was known to bevery dense and made of compact
“degenerate” matter composed of
atomic nuclei and free electrons.
White dwarfs were prevented from
collapsing by a phenomenon known
as electron degeneracy pressure.
This meant that, when electrons
were packed very close together,
their movement was limited,
creating outward pressure.The Chandrasekhar limit
Chandrasekhar figured out that
electron degeneracy pressure
can prevent a white dwarf from
collapsing only up to an upper limit
for the white dwarf’s mass, which is
about 1.4 times the mass of the sun.
Today, it is known that the core of
a giant star at the end of its life will
collapse into a white dwarf if its
mass is below the Chandrasekhar
limit, but will collapse to an even
denser object—a neutron star or
a black hole—if its mass exceeds
the limit. This insight was largely
ignored by the scientists of the
day because neutron stars and
black holes were, at this time,
still purely theoretical. ■WHITE DWARFS
HAVE A MAXIMUM
MASS
THE LIFE CYCLES OF STARS
IN CONTEXT
KEY ASTRONOMER
Subrahmanyan
Chandrasekhar (1910 –1995)BEFORE
1914 Walter Adams details
the spectrum of 40 Eridani B,
an unusually faint white star.1922 Dutch astronomer
Willem Luyten coins the term
“white dwarf” for low-mass
white stellar remnants.1925 Austrian physicist
Wolfgang Pauli formulates the
Pauli Exclusion Principle, which
states that no two electrons
can occupy the same quantum
state. This leads to recognition
of the phenomenon of electron
degeneracy pressure.AFTER
1937 Fritz Zwicky
characterizes a type 1a
supernova as the explosion of
a white dwarf that exceeded
its Chandrasekhar limit.
1972 Astronomers find the first
stellar black hole candidate.The black holes of nature are
the most perfect macroscopic
objects there are in the
universe: the only elements
in their construction are our
concepts of space and time.
Subrahmanyan
Chandrasekhar