240 EDWIN HUBBLE
universe—if you could figure out
the star’s absolute magnitude
from its variability period, then
the star’s distance from Earth
could be calculated from its
apparent magnitude. The first
step in figuring this out was
to calibrate the scale, which
was done in 1913 by Swedish
astronomer Ejnar Hertzsprung.
He figured out the distances to
13 relatively nearby Cepheids
using the parallax method (p.39).
Cepheids were immensely
bright—thousands of times more
luminous than our Sun (in modern
terminology they are “yellow
supergiants”). In theory, then, they
were an ideal “standard candle”—
stars whose brightness could be
used to measure huge cosmic
distances. But despite the best
efforts of astronomers, Cepheids
within the spiral nebulae
remained stubbornly elusive.
The Great Debate
In 1920, the Smithsonian Museum
in Washington DC hosted a
debate between the two rival
cosmological schools, hoping to
settle the issue of the scale of
the universe once and for all.
Respected Princeton astronomer
Harlow Shapley spoke for the “small
universe” side. He had been the
first to use Leavitt’s work on
Cepheids to measure the distance
to globular clusters (dense star
clusters in orbit around the Milky
Way), and discovered that they
were typically several thousand
light years away. In 1918, he had
used RR Lyrae stars (fainter stars
that behave like Cepheids) to
estimate the size of the Milky Way
and show that the Sun was
nowhere near its center. His
arguments appealed to public
scepticism toward notions of an
enormous universe with many
galaxies, but also cited specific
evidence (later to be proved
inaccurate), such as reports that
over many years some astronomers
had actually observed the spiral
nebulae rotating. For this to be
true without parts of the nebula
exceeding the speed of light,
they must be relatively small.By measuring the light from Cepheid
variable stars in the Andromeda nebula,
Hubble established that Andromeda
was 2.5 million light years way—
and was a galaxy in its own right.The “island universe” supporters
were represented by Heber D. Curtis
of the University of Pittsburgh’s
Allegheny Observatory. He based
his arguments on comparisons
between the rates of bright “nova”
explosions in distant spirals and in
our own Milky Way. Novae are very
bright star explosions that can
serve as distance indicators.
Curtis also cited the evidence
of another, crucial factor—the high
redshift exhibited by many spiral
nebulae. This phenomenon had
been discovered by Vesto Slipher
of the Flagstaff Observatory,
Arizona, in 1912—apparent through
distinctive shifts in the pattern of a
nebula’s spectral lines toward the
red end of the spectrum. Slipher,
Curtis, and many others believed
that they were caused by theWe are reaching
into space, farther and
farther, until, with the
faintest nebulae that can
be detected...we arrive
at the frontier of the
known universe.
Edwin Hubble