160
asserted that “we have at the
present no other interpretation
for it. We may conclude that the
Andromeda nebula is approaching
the solar system.”
Discovering Doppler shifts
Over the next few years, Slipher
studied 14 more spiral nebulae and
found that nearly all were traveling
at incredible speeds relative to
Earth. Most remarkably, whereas
some were moving toward Earth,
most were showing redshifted
spectra, where the wavelengths
had stretched, meaning they were
moving away from Earth. The
nebula known as M104 (also called
NGC 4594), for example, was flying
away at an astonishing speed of
nearly 600 miles per second (1,000
km per second). Another called
M77, or NGC 1068, was receding
at 680 miles per second (1,100 km
per second). Altogether, out of
the 15 galaxies observed, 11 wereSPIRAL GALAXIES
significantly redshifted. In 1914,
Slipher presented his results to the
American Astronomical Society
and received a standing ovation.
By the time Slipher had
presented his next paper on
spiral nebulae in 1917, the ratio of
redshifted to blueshifted nebulae
had risen to 21:4. In this paper,
Slipher noted that the average
velocity at which they were
approaching or receding—
scientifically known as “radial
velocity”—was 450 miles per
second (700 km per second). This
was much faster than any star had
ever been measured moving
relative to Earth. Slipher found it
almost inconceivable that the spiral
nebulae could be passing through
the Milky Way at such speeds, and
he began to suspect that they were
not moving through the Milky Way
at all, stating: “It has for a long time
been suggested that the spiral
nebulae are stellar systems seen
at great distances ... This theory,The galaxy NGC 4565, which
Slipher established to be receding at
700 miles/s (1,100 km/s), is also known
as the Needle galaxy because of its
thin shape when viewed from Earth.Vesto Slipher Vesto Slipher was born on a farm
in Mulberry, Indiana, in 1875.
Soon after graduation, he started
working at the Lowell Observatory
in Arizona, where he would remain
for more than half a century. Most
of Slipher’s major discoveries
occurred in the earlier part of his
career. He began by investigating
the rotational periods of planets,
finding evidence, for example,
that Venus’s rotation is very slow.
Between 1912 and 1914, he made
his most significant discovery—
that some spiral nebulae are
moving at high speed. In 1914,
Slipher discovered the rotationof spiral galaxies, measuring
spin rates of hundreds of
miles per second. He also
demonstrated that gas
and dust exist in interstellar
space. Slipher was director of
the Lowell Observatory from
1926 to 1952. During this time,
he supervised a search for
trans-Neptunian planets,
which ledin 1930 to Clyde
Tombaugh’s discovery of Pluto.Key work1915 Spectrographic
Observations of Nebulae