The Astronomy Book

133

Milky Way

institutions. Under the supervision
of its director, Edward C. Pickering,
the Observatory employed many
men to build equipment and take
photographs of the night sky,
and several women to examine
photographic plates taken from
telescopes throughout the world,
measure their brightness, and
perform computations based on
their assessment of the plates.
These women had little chance
to do theoretical work at the
Observatory, but several of them,
including Williamina Fleming,
Henrietta Swan Leavitt, Antonia
Maury, and Annie Jump Cannon,
nevertheless left a lasting legacy.
Henrietta Swan Leavitt, who had
originally joined the Observatory
as an unpaid volunteer in 1894,

eventually became the head of

the photographic photometry

department. This mainly involved

measuring the brightness of stars,

but a specific aspect of Leavitt’s

work was to identify stars that

fluctuate in brightness—known

as variable stars. To do this,

she would do a comparison of

photographic plates of the same part

of the sky, made on different dates.

Occasionally she would find a star

that was brighter on different dates,

indicating that it was a variable.

Cluster variables

A specific task that Leavitt took

on was to examine some of the

photographic plates of stars in the

Small Magellanic Cloud (SMC) and

the Large Magellanic Cloud (LMC).

At the time, the SMC and LMC

were thought to be very large star

clusters within the Milky Way,

which itself was assumed to

comprise the entire universe.

Today, they are known to be

relatively small, separate galaxies

that lie outside the Milky Way.

The Magellanic Clouds are visible

to the naked eye in the night sky

of the southern hemisphere, but are

never visible from Massachusetts,

where Leavitt lived and worked.

Therefore, although she examined

numerous photographic plates

of the LMC and SMC obtained

by astronomers at an observatory

in Peru, it is highly unlikely that

she ever physically observed

them in the sky.

After several years’ work,

Leavitt had found 1,777 variables in

the SMC and LMC. One particular

kind that caught Leavitt’s attention,

representing a small fraction of all

the variables she had found (47

out of 1,777), was of a type called

a Cepheid variable. Leavitt called

them “cluster variables”—the term

Cepheid variable was introduced ❯❯

See also: A new kind of star 48–49 ■ Stellar parallax 102 ■ The star catalog 120–21 ■ Analyzing absorption lines 128 ■
Nuclear fusion within stars 166–67 ■ Beyond the Milky Way 172–77 ■ Space telescopes 188–95

THE RISE OF ASTROPHYSICS

Earth

Large
Magellanic
Cloud
Small Magellanic Cloud

One of the most striking

accomplishments of Miss

Leavitt was the discovery

of 1,777 variable stars in

the Magellanic Clouds.

Solon I. Bailey

Colleague of Leavitt

The Cepheid variable stars that

Leavitt studied are in the Magellanic

Clouds, known today to be galaxies

outside the Milky Way. The Large

Magellanic Cloud is about 160,000

light-years away; the Small Magellanic

Cloud is about 200,000 light-years away.

Both are part of the Local Group galaxy

cluster that includes the Milky Way.