to produce amazingly detailed drawings
of what he saw. In particular, his obser-
vations of the Whirlpool Galaxy (M51),
the Triangulum Galaxy (M33), and M99
(NGC 4254) showed distinct spiral
structures. Without a proper way to
measure distances, astronomers could
only question whether these nebulae, like
stars and clusters, were within the Milky
Way. After all, if they were distant struc-
tures beyond the Milky Way, what did
that mean for our place in the universe?
Cosmic yardsticks
The debate about the physical nature
of the Milky Way continued into the
early 20th century. Two new technolo-
gies helped charge the discussion: spec-
troscopy and photography. The ability
to analyze starlight gave astronomers a
powerful new way of understanding the
chemistry of stars, while photography
augmented the limited light-gathering
ability of the human eye.
Armed with these tools, astronomers
Henrietta Leavitt, Edward C. Pickering,
and Ejnar Hertzsprung discovered and
defined a relationship between the
Cepheid variable stars remain important to
understanding the shape of the Milky Way.
Each dot in this image is a Cepheid whose
distance was measured by a team using the
Optical Gravitational Lensing Experiment
telescope (OGLE), at center, at Las
Campagnas Observatory in Chile. K. ULACZYK/J.
SKOWRON/OGLEperiod of dimming and brightening of a
class of stars called Cepheid variables. In
1908, Leavitt was studying variable stars
in photographs of the Large and Small
Magellanic Clouds sent to the Harvard
College Observatory, where she worked,
from Harvard’s observatory in Peru. She
noticed a rhythmic and predictable varia-
tion in brightness of these stars in the
Large and Small Magellanic Clouds,
which might last from a single day to
more than a month before repeating.
Furthermore, she discovered, the lon-
ger the period of variation, the brighter
the star appeared to be. Since all the stars
in the Small Magellanic Cloud are at
roughly the same distance, she reasoned
that the period of a Cepheid variable was
related to its true, intrinsic brightness.
Pickering, the observatory director,
suggested this period-luminosity relation
could be useful to determine the distri-
bution of star clusters and nebulae. And
Hertzsprung was able to calibrate this
technique by making independentdistance measurements to Cepheids
using the parallax method, seeing how
much they shifted against background
stars as Earth orbited the Sun.
Thus, by measuring the period of a
Cepheid, astronomers could know its
true brightness — and by comparing that
to its apparent brightness, calculate how
far away it was. Astronomers finally had
a reliable cosmic yardstick.
Around the same time, the young
astronomer Harlow Shapley began
measuring the distribution of globular
clusters — compact and dense spheres of
stars. By 1918, he had found that the
clusters centered around the constella-
tion Sagittarius, forming a halo around
the Milky Way. He also made improved
parallax measurements of Cepheid vari-
ables, which in turn improved the
calibration of Leavitt’s relation.
Using this data, Shapley not only
located the center of our galaxy — in
Sagittarius — but also showed that the
Milky Way was 10 times the size ofWilliam Herschel constructed
his map of the Milky Way with
measurements he called
“star-gages,” which he took by
pointing his telescope at
patches of sky and counting
the number of stars he saw.
The result is a cross section
of the Milky Way from Earth’s
vantage point. CAROLINE HERSCHEL