WWW.ASTRONOMY.COM 59previous estimates. His observations also
placed our solar system far from the cen-
ter of the galaxy. Given the size of our
galaxy, Shapley was convinced that spiral
nebulae, like globular clusters, were all
part of the Milky Way.
The Great Debate
By the early 20th century, speculation
about spiral nebulae and the nature
of the Milky Way had reached a fever
pitch. Photography clearly showed these
nebulae had well-defined spiral structure
composed of countless stars, but there
were no good measurements of their dis-
tance to verify whether they were within
the Milky Way or not.
In April 1920, Harlow Shapley faced
off with Heber Curtis at the National
Museum of Natural History in
Washington, D.C., in a discussion called
the Great Debate. Shapley maintained
that spiral and all other nebulae were
part of the Milky Way, just like globular
clusters. But Curtis provided convincing
evidence that they were independent star
systems — “island universes,” as he
called them, a term coined by the
German philosopher Immanuel Kant.
Cepheid variables ultimately settled
the debate. A few years later, while using
the 100-inch Hooker telescope at Mount
Wilson Observatory in California,
Edwin Hubble found Cepheid variables
in the Andromeda spiral nebula. Using
Shapley’s calibration of Leavitt’s period-
luminosity relation, Hubble showed this
object was 900,000 light-years away, far
beyond the outskirts of Milky Way. (This
figure has since been refined to 2.5 mil-
lion light-years.) In a single measure-
ment, he proved that the Milky Way was
not the entire universe, but part of a vast
sea of island universes.
Radio astronomy rises
In the 19th century, astronomers were
puzzled by large areas along the Milky
Way almost devoid of stars. These so-
called coal sacks appeared as dark holes
against a starry background. At least one
astronomer speculated they might be
openings into heaven!
Because at that time the exploration
of the Milky Way was still restricted to
visible light, astronomers were unaware
that the coal sacks were huge clouds of
gas and dust blocking the light of distant
stars. New technology would need to
be developed before astronomers could
explore and understand these cold, dark
clouds running throughout the plane of
the Milky Way.
The birth of radio astronomy pro-
vided this new tool and led to the discov-
ery that the galaxy is filled not only with
dust, but also with tremendous amounts
of cold, neutral hydrogen gas. Most of
the time, a hydrogen atom’s proton and
electron spin in the same direction. But
sometimes, electrons f lip and spin in the
other direction. For any given hydrogen
atom, this only happens about once every
100 million years. When it does, energy
is emitted with a wavelength of 21 centi-
meters. These waves pass right through
the clouds of dust that hide visible light,
which has a much shorter wavelength.
When astronomers first detected
21 cm radiation in 1951, they began
using it to finally peer through these
clouds to build a fuller picture of our
Milky Way. By noting the distribution
of neutral hydrogen, astronomers could
map unseen portions of the galaxy, trac-
ing its spiral arms, where hydrogen is
concentrated.The shape of our galaxy
Over the past 70 years, a picture has
emerged of a massive galaxy with
four primary spiral arms. The Sun
is located 27,000 light-years from thegalactic center along the Orion Spur, a
smaller arm located between the Perseus
and Sagittarius arms. In recent years,
astronomers have discovered our galaxy’s
central bulge has a bar structure. And its
disk of gas and stars is slightly warped
and twisted, perhaps by gravitational
interaction with nearby dwarf galaxies.
There are deeper mysteries yet to be
resolved. For instance, Newton’s law of
gravity holds that stars and gas at a gal-
axy’s outer fringes should orbit slower
than objects closer to its center — but
instead, we observe that outer objects
move faster. This is true in our galaxy,
as well as others. The only explanation
— without modifying the law of gravity
— is the unseen existence of a great deal
more mass, probably in the form of dark
matter. But this dark matter has never
been directly observed.
To stand out under a dome of shining
stars and understand their true nature is
an amazing feat of human ingenuity. To
fathom the immensity of our galaxy and
our place in it is a striking act of human
imagination. Democritus envisioned vast
numbers of stars beyond his power to
see. And thanks to countless astronomers
who came after him, we have found our
true place in the galaxy we call home.In 2019, the OGLE team released a three-dimensional map of the Milky Way, as traced by over 2,400
Cepheids. The map shows how the outer portion of the galaxy’s disk is warped. This warping had
been previously detected, but this was the first time it had been mapped with direct distance
measurements to stars. JAN SKOWRON/OGLE/ASTRONOMICAL OBSERVATORY, UNIVERSITY OF WARSAW–600,000 ly 0 600,000 lySunA WARPED VIEW
Raymond Shubinski is a contributing
editor of Astronomy who is fascinated by
the history of astronomy.