175
at enormous speeds—speeds far
too high for them to be contained
within the Milky Way.
Hubble set out to see whether
there was a relationship between
the distances of spiral nebulae and
their velocity. His strategy was to
search for Cepheid variable stars
(p.138)—stars whose luminosity
changes predictably—within
nebulae and to measure their
distances from Earth. This
provided Hubble with his first big
discovery in the winter of 1923.
Beginning with photographic
plates of the closest and clearest
nebulae, Hubble spotted a Cepheid
variable on one of the first plates
he reviewed. The distances he
calculated for even relatively
nearby nebulae were so vast
that it effectively killed the Great
Debate immediately: NGC 6822
was 700,000 light-years away, while
M33 and M31 were 850,000 light-
years away. It was instantly clear
that the universe extended beyond
the Milky Way. Just as Curtis had
maintained, the spiral nebulae
were “island universes,” or “extra-
galactic nebulae” as Hubble termed
them. Over time, the term “spiral
nebulae” fell into disuse and they
are now simply called galaxies.In the realm of the nebulae
Hubble pressed ahead with
his program of measuring the
distances to galaxies beyond
the Milky Way. Farther out,
however, it became impossible
to pick out individual Cepheid
variables in such faint and fuzzy
distant galaxies. He was compelled
to fall back on indirect methods,
such as the so-called “standard
ruler” assumption: reasoning that
all galaxies of a similar type are
the same size allowed him to
estimate the distance to a galaxy
by measuring its apparent size
and comparing it to the expectedSee also: Measuring the universe 130–37 ■ The theory of relativity 146–53 ■ Spiral galaxies 156–61 ■
The birth of the universe 168–71 ■ Space telescopes 188–95 ■ Curtis (Directory) 337 ■ Arp (Directory) 339
ATOMS, STARS, AND GALAXIES
There is a linear relation
between velocities
and distance.Distant galaxies are
all moving away
from us.The farther away
the galaxy, the greater
its velocity.The universe
is expanding in
all directions.Hubble is seen here looking through
the lense of the Hooker Telescope
at Mount Wilson. It was here that he
measured galaxy distances and a
value for the expansion of the universe.“true” size. Thanks to Slipher’s
measurements, Hubble already
knew that light from most spiral
nebulae was redshifted. In addition,
the fainter spirals had higher values
of redshift, showing that they
were moving faster through space.
Hubble realized that, if there were
indeed a relationship between
a galaxy’s distance from Earth
and its recession velocity, these
redshifts would serve as a cosmic
yardstick, enabling the distances
of the very farthest and faintest
galaxies to be calculated, and
a ballpark figure to be put on the
size of the universe as a whole.
Meanwhile, Milton Humason,
the assistant astronomer at Mount
Wilson, checked Slipher’s redshifts
and collected new spectra from
distant galaxies. It was hard,
punishing work, and he and Hubble
spent many bitterly cold nights
in the observer’s cage at the top
of the tube telescope on Wilson
Mountain in California.
Hubble’s landmark paper,
“A relationship between distance
and radial velocity among
extra-galactic nebulae,” was ❯❯