141
See also: Observing Uranus 84–85 ■ Refining star classification 138–39 ■
The life cycles of stars 178 ■ Energy generation 182–83I
n the first decade of the 20th
c ent u r y, A merican astronomer
Walter Adams developed a
method for calculating the absolute
magnitude of stars from the relative
intensities of particular wavelengths
in their spectra. One of the original
team members at the Mount Wilson
Observatory in California, Adams
used his method to investigate the
triple-star system 40 Eridani, which
contained a mysterious star that
seemed very dim but also very hot.White dwarf
The brightest of the three stars,
40 Eridani A, was being orbited
by a much dimmer binary pair,
40 Eridani B and C. Stars as
faint as 40 Eridani B and C were
expected to be of spectral class M,
meaning that their starlight is red,
indicating a relative coolness. 40
Eridani C fitted this profile, but 40
Eridani B was one of the whitest
and hottest types of star. When
Adams published the data in 1914,
astronomers were presented with
a puzzle: a star that hot had to be
getting its energy from somewhere.The answer could only be that,
although it was small (about the
size of Earth), its density must be
immense—about 25,000 times
that of the sun. 40 Eridani B was
the first white dwarf star to be
discovered. White dwarfs were
later shown to be the hot stellar
cores left behind when main
sequence stars run out of fuel
for nuclear fusion. ■THE RISE OF ASTROPHYSICS
A WHITE HOT
STAR THAT
IS TOO FAINT
DISCOVERING WHITE DWARFS
IN CONTEXT
KEY ASTRONOMER
Walter Adams (1876–1956)
BEFORE
1783 William Herschel
discovers 40 Eridani B and C.
1910 Williamina Fleming
answers an inquiry from
Henry Norris Russell about
the spectrum of 40 Eridani B,
confirming that it is a Type
A star.
AFTER
1926 British astronomer
Ralph Fowler applies new
ideas in quantum physics
to explain the nature of the
extremely dense material
in white dwarfs.
1931 Subrahmanyan
Chandrasekhar calculates
that white dwarfs cannot be
more massive than 1.4 times
the mass of the sun.
1934 Walter Baade and Fritz
Zwicky suggest that stars
too massive to become white
dwarfs form neutron stars.
Composed of material 3,000
times denser than anything
you have ever come across, a
ton of [this] material would be
a little nugget that you could
put in a matchbox.
Arthur Eddington
describing white dwarfs