32 ASTRONOMY • JUNE 2020
Hubble continued imaging Cepheid
variables in other spiral nebulae, such as
M33 in Triangulum, demonstrating that
they, like Andromeda, are so far away that
they must be distant galaxies. Hubble’s
observations indicated that galaxies are
the basic units of stars, gas, and dust in
the universe, and that they exist on a fan-
tastic scale. He had many doubters, chief
among them Shapley, but pushed on. The
findings of the confident 35-year-old were
subsequently splashed on the front page of
The New York Times by November 1924.
Egged on by supporters, he sent a paper
summarizing the results to be read at
the winter meeting of the American
Astronomical Society, the professional
organization of astronomers, on New
Year’s Day 1925. After the distinguished
professor Henry Norris Russell of
Princeton University read the paper
aloud at the gathering, galaxies were on
their way to becoming widely accepted.
A breakthrough with
galaxy colors
Several more years led to another huge
advancement. A galaxy’s spectrum is
a picture of the collected light from all
of its stars and gas. In 1929, Hubble
and other astronomers recorded many
spectra of galaxies and noticed that
most appeared to be shifted toward the
red end of the spectrum, increasing the
wavelength and lowering the frequency
of their light. This was an effect first
noticed years earlier, in 1912, by Vesto
M. Slipher, an astronomer at Lowell
Observatory in Arizona.
You experience this effect, called a
Doppler shift, with sound every time an
ambulance with its loud siren passes you.
As it approaches, the siren seems high-
pitched (because it has a short wavelength
and high frequency of sound), and when it
passes and heads away from you, the pitch
drops lower (increasing the wavelengthand lowering the frequency of sound). The
same thing happens with light. When
objects are moving toward us, the fre-
quency of their light shifts higher, toward
the blue end of the spectrum. When they
are moving away from us, their light shifts
lower, toward the red end of the spectrum.
Consequently, the “redshift” of the spectra
of distant galaxies indicates that the gal-
axies are moving away from us. And this
means that the universe is not only
immensely bigger than previously
thought, but also that it is expanding to
become even bigger as time marches on.Here comes the Big Bang
Hubble’s work, building on the earlier
studies of Slipher and astronomer Milton
Humason, showed that, generally speak-
ing, all galaxies are moving away from
each other over time. Hubble also found
that redshifts can be used to calculate
distances to galaxies.The magnificent edge-on Sombrero Galaxy
One of the greatest edge-on galaxies in the sky, and the one most people say looks like a flying saucer, is the Sombrero Galaxy (M104) in Virgo. This galaxy consists
of a large rotating disk with a prominent dust lane edging it, consumed by a glowing halo of gas and stars. It lies 10 million light-years away and is about 49,000 light-
years across — half the size of the Milky Way. NASA AND THE HUBBLE HERITAGE TEAM (AURA/STScI)