30 ASTRONOMY • JUNE 2020
in his fourth year as a staff astronomer
at Mount Wilson. He relished using the
100-inch Hooker Telescope to study
his favorite subject, the fuzzy nebulae
— mysterious, glowing gas clouds that
appeared scattered across the sky.
No one fully understood these nebu-
lae, although they were suspected to be
the birthplaces of stars. Using his mam-
moth telescope in rural Ireland in the
mid-19th century, the adventurous ama-
teur astronomer William Parsons, Third
Earl of Rosse, had first sketched nebulae
with spiral structures that looked like
faintly glowing whirlpool patterns.
But nearly a century later, little more
was known about them. Hubble was
interested in cracking the code of nebu-
lae, particularly spiral nebulae. His Ph.D.
work had centered on the topic. These
nebulae’s spiral shapes suggested that
they were rotating, but they otherwise
mystified Hubble and other astronomers.
On the night of October 4, 1923,
Hubble used the Hooker Telescope to take
a 40-minute exposure of one of his favor-
ite nebulae: the Great Nebula in the
Andromeda constellation. This spiral-
shaped cloud was large, bright, and faintly
visible to the naked eye as a fuzzy smear
of light for those located away from the
city lights of Los Angeles. The night had
very poor “seeing” when he took the
exposure, because Earth’s atmospherewas relatively turbulent, and so the star
images were not perfectly small dots.
Nevertheless, Hubble’s examination of the
photographic plate he had made revealed
a suspected nova: an exploding star. It
was exciting to record such a relatively
rare event inside one of the spiral nebulae.
Hubble photographed the Andromeda
Nebula again the next night, hoping for
a better-quality image of the suspected
nova. The resulting photographic glass
plate, exposed the night of October 5/6
and designated H335H, would become
one of the most celebrated in all of scien-
tific history. On it, Hubble successfully
recorded the nova again. But before he
could analyze it further, his periodicTHE PHOTOGRAPHIC PLATE THAT DEFINED GALAXIES
On October 5/6, 1923, astronomer Edwin Hubble made an exposure of
the “Andromeda Nebula” with the 100-inch Hooker Telescope at Mount
Wilson Observatory near Los Angeles. He was initially excited, believing
after analyzing the plate that he had recorded a nova — an exploding
star. He marked the star, which lies between two tick marks he drew on
the glass plate, with an “N.” The famous plate, designated H335H, would
help to unlock one of the universe’s biggest secrets.
A short time later, Hubble realized he had not seen a nova, but a vari-
able star of a particular type called a Cepheid, with very well-known
characteristics. He then wrote “VAR!” on the plate. Because Cepheids’
absolute magnitudes and light curves were well known, Hubble coulduse the variable he’d found to determine the distance to its home in
Andromeda. Astonished, Hubble found that the Andromeda Nebula was
actually a distant galaxy, perhaps a million light-years away — far larger
than astronomers believed the entire universe was at the time. With this
plate, he unlocked the nature of galaxies. (It turns out, we now know, that
the true distance to the Andromeda Galaxy is 2.5 million light-years.)
The image on the left shows the entire plate; the image on the right
zooms in on Hubble’s notes. The plate belongs to the collection of the
Carnegie Observatories in Pasadena, California, and is one of the most
famous and influential images in the history of science. IMAGES COURTESY OF THE
CARNEGIE OBSERVATORIES/CINDY HUNT