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I
n the 1780s and 1790s, British
astronomer William Herschel
cataloged large numbers of
nebulae and speculated that some
of these might be comparable in
size and nature to the Milky Way.
In his conjectures, Herschel was
following an earlier suggestion by
German philosopher Immanuel
Kant that nebulae might be large
disks of stars—“island universes”
independent of the Milky Way and
separated from it by vast distances.
In the 19th century, using improved
telescopes, British astronomer
Lord Rosse discovered that some
nebulae had “arms” arranged in
a spiral, while his compatriot
William Huggins found that many
nebulae consisted of a mass of stars.
However, aside from the fact that
they might contain stars, nebulae
were still little understood by the
turn of the 20th century, when a
young scientist from Indiana named
Vesto Slipher began to study them.The Lowell Observatory
From 1901, Slipher worked at the
Lowell Observatory in Flagstaff,
Arizona. The observatory had been
founded by American astronomer
Percival Lowell in 1894. Lowell hadSPIRAL GALAXIES
IN CONTEXT
KEY ASTRONOMER
Vesto Slipher (1875 –1969)BEFORE
1842 Austrian physicist
Christian Doppler proposes
the Doppler effect—a change
in the perceived frequency of
waves coming from an object
moving relative to the viewer.1868 William Huggins
determines the velocity of a
star moving away from Earth
using the Doppler effect.AFTER
1929 Edwin Hubble finds a
link between the recessional
velocities of spiral galaxies
and their distances.1998 Saul Perlmutter and his
colleagues discover that the
expansion of the universe has
been accelerating for the last
5 billion years.It seems to me that with this
discovery, the great question,
if the spirals belong to the
system of the Milky Way or
not, is answered with great
certainty: they do not.
Ejnar Hertzsprung
in a letter to Vesto Slipherselected the site because its high
altitude, at over 6,900 ft (2,100 m),
with few clouds, and its distance
from city lights, meant it guaranteed
good visibility almost every night.
Lowell’s venture marked the first
time an observatory had been built
deliberately in a remote, high place
for optimal observations.
Lowell initially hired him for
a short-term position, but Slipher
would remain for his entire career.
Lowell and Slipher worked well
together, with the unassuming new
recruit content to leave the limelight
to his flamboyant employer. Slipher
was a talented mathematician and
had practical mechanical skills,
which he put to use installing new
spectrographic equipment. He set to
work developing improved techniques
in spectrography—the separation of
light coming from celestial objects
into its constituent wavelengths,
and the measurement and analysis
of those wavelengths (p.113).Slipher used the 2 4 -i n (61-c m) Alvan
Clark telescope at the Lowell Observatory
to observe the spiral nebulae. Today,
people can use the original telescope
at the observatory’s visitor center.