WWW.ASTRONOMY.COM 31observing run on the 100-inch telescope
ended and he had to leave to accommo-
date other observers.
At his office in Pasadena, away from
the mountaintop observatory, he contin-
ued studying earlier images taken by oth-
ers of the Andromeda Nebula region.
And then he made an unusual discovery.
A nova brightens dramatically and then
fades into oblivion. But the star he
recorded appeared on older plates,
brightening and fading regularly over
a period of 31 days. This star was not a
nova, then. It had to be some other kind
of star inside the Andromeda Nebula.
Hubble’s breakthrough
Suddenly, Hubble came upon the solu-
tion. He realized that he had made an
image of a type of star similar to a well-
known one in the constellation Cepheus.
On his photographic plate H335H, he
crossed out “N,” for nova, and wrote
“VAR!” instead, denoting a variable star.
Moreover, this star was a special type
of variable that brightened and faded
in a precise way. Astronomers had long
studied this kind of star, which came to
be known as a Cepheid variable (after
the star in Cepheus), and they knew how
intrinsically bright it was. By knowing
how bright the star really was, and by
measuring how bright it appeared to be
in the sky, Hubble could use the star as
a guidepost to gauge its distance.
This was a monumental realization.
Hubble calculated that, owing to the star’s
faint light, it must lie a million light-years
away — and so must the entire nebula
surrounding it. This meant that the uni-
verse stretches across a distance at least
three times larger than most astronomers
then believed. With his photographic
plate, Hubble had single-handedly reset
the size of the cosmos.Discovering the galaxies
Hubble’s discovery set off a firestorm of
activity among astronomers research-
ing other spiral nebulae. Countlessobservations followed, and follow-up
studies rolled on for many months as
bickering and soul-searching lit up the
world of professional astronomy. Adding
fuel to the fire was a debate staged
several years earlier, in 1920, between
two prominent astronomers of the day:
Harlow Shapley of Princeton University,
and Heber Curtis of the Allegheny
Observatory. Shapley believed that the
Milky Way Galaxy constitutes the entire
universe, while Curtis speculated that
spiral nebulae are galaxies separate from
the Milky Way — essentially “island
universes.” Though not everyone would
concede it yet, Hubble’s discovery seemed
to prove that Curtis was right.The Andromeda Galaxy
in black and white
A monochromatic image of the Andromeda Galaxy,
our galactic neighbor, reveals the intricate details of
its spiral arms, areas of swirling gas clouds near the
galaxy’s center, and two satellite companions, M32
(above, left of the center of Andromeda), and NGC 205
(below the galaxy’s center). TONY HALLASNGC 1530: A barred spiral galaxy
with a “mini-spiral” nucleus
Oriented nearly face-on to our line of sight, the barred
spiral NGC 1530 in Camelopardalis lies some
80 million light-years away. Its prominent bar
connects with large, well-defined spiral arms. The
galaxy’s center has a swirling pattern reminiscent of
the spiral form of a galaxy in itself. ADAM BLOCK/MOUNT LEMMON
SKYCENTER/UNIVERSITY OF ARIZONA