A PARADIGM SHIFT 239
more limited in extent argued that
the spirals might be suns or solar
systems in the process of formation,
in orbit around the Milky Way.
Stars with a pulse
The answers to this long-standing
puzzle came in several stages, but
perhaps the most important was the
establishment of an accurate means
of measuring the distance to stars.
The breakthrough came with the
work of Henrietta Swan Leavitt, one
of the team of female astronomers
at Harvard University who were
analyzing the properties of starlight.
Leavitt was intrigued by the
behavior of variable stars. These
were stars whose brightness
appeared to fluctuate, or pulse,
because they periodically
expanded and contracted as they
neared the end of their lives. She
began to study photographic plates
of the Magellanic Clouds, two small
patches of light visible from the
southern sky that look like isolated
“clumps” of the Milky Way. Each
See also: Nicolaus Copernicus 34–39 ■ Christian Doppler 127 ■ Georges Lemaître 242–45
The universe is big...
and getting bigger.A Cepheid variable
is a star whose brightness
we can know for certain,
which means that
we can calculate
how far away it is.If the Cepheid variable
is millions of light years
away, it must be
in a galaxy far
outside our own.The light coming
from other galaxies may
be blueshifted (moving
toward us) or redshifted
(moving away from us).The light of every distant
galaxy is redshifted, and
the farther the galaxy, the
greater the redshift.Henrietta Leavitt received little
recognition in her lifetime, but her
discoveries relating to Cepheid variable
stars were the key that allowed
astronomers to measure the distance
from Earth to faraway galaxies.
of the clouds, she found, contained
huge numbers of variable stars, and
by comparing them across many
different plates, she not only saw
that their light was varying in a
regular cycle, she could also figure
out the period of the cycle.
By concentrating on these small,
faint, isolated star clouds, Leavitt
could safely assume that the stars
within them were all at more or
less the same distance from Earth.
Though she could not know the
distance itself, this was still
enough to assume that differences
in the “apparent magnitude”
(observed brightness) of the starswere an indication of differences in
their “absolute magnitude” (actual
brightness). Publishing her first
results in 1908, Leavitt noted in
passing that some stars seemed to
show a relationship between their
variability period and their absolute
magnitude, but it took another four
years for her to figure out what this
relationship was. It turned out that,
for a certain type of variable star
known as a Cepheid variable, stars
with greater luminosity have longer
variability periods.
Leavitt’s “period-luminosity”
law would prove the key to
unlocking the scale of the ❯❯