The Astronomy Book

136

Following these calibrations,
Hertzsprung was able to establish
a system for determining the
distance to any Cepheid variable
from just two items of data—its
period and its apparent magnitude.

Further applications
It was not long before Leavitt’s
findings, tuned by the work of
Hertzsprung, led to further
important results in terms of
helping to understand the scale
of the universe. From 1914 to 1918,
the American astronomer Harlow
Shapley (who was also the first
person to show that Cepheid
variables are pulsating stars)
was one of the first to use the
newly developed concept that the
distances of variable stars could be
found from knowing their periods
and apparent brightness. Shapley
found that objects called globular
star clusters—all part of the Milky
Way—were distributed roughly in
a sphere whose center lay in the
direction of the constellation of
Sagittarius. He was able to
conclude from this that the center
of the Milky Way galaxy is at a
considerable distance (tens of
thousands of light-years) in the
direction of Sagittarius and that
the sun is not, as had previously
been supposed, at the center of the
galaxy. Shapley’s work, which led

to the first realistic estimate of

the true size of the Milky Way,

was an important milestone in

galactic astronomy.

Right up to the 1920s, many

scientists (including Harlow

Shapley) maintained that the

Milky Way galaxy was the whole

universe. Although there were

those that believed otherwise,

neither side could conclusively

prove their argument one way

or another. In 1923, however, the

American astronomer Edwin

Hubble, using the latest in

telescopic technology, found

a Cepheid variable in the

MEASURING THE UNIVERSE

Andromeda nebula, allowing its

distance to be measured. This

led directly to the confirmation

that the Andromeda nebula is a

separate large galaxy (and is now

called the Andromeda galaxy)

outside the Milky Way. Later,

Cepheids were similarly used to

show that the Milky Way is just one

of a vast number of galaxies in the

universe. The study of Cepheids

The star RS Puppis is one of

the brightest Cepheid variables

in the Milky Way. It is about 6,500

light-years from Earth and has a cycle

of variability lasting 41.4 days.

Leavitt left behind a legacy of
a great astronomical discovery.
Solon I. Bailey