159
Studying nebulae
Slipher’s initial work and research
were directed at the planets, but
from 1912, at Lowell’s request, he
began to study the mysterious
spiral nebulae. Lowell had a theory
that they were spirals of gas that
were coalescing into new solar
systems. He asked Slipher to
record the spectra of the light from
the outer edges of the nebulae,
to determine if their chemical
makeup resembled that of the
solar system’s gas giant planets.See also: The Milky Way 88–89 ■ Examining nebulae 104–05 ■ Properties of nebulae 114–15 ■ Measuring the universe
130 –37 ■ The shape of the Milky Way 164–65 ■ The birth of the universe 168–71 ■ Beyond the Milky Way 172–77
ATOMS, STARS, AND GALAXIES
The spectra of galaxies moving toward Earth exhibit “blueshifts” and those
receding from Earth exhibit “redshifts” because the light waves are squashed
or stretched when viewed from Earth. These are called Doppler shifts after the
Austrian physicist Christian Doppler, who first explained such phenomena.
Making small adjustments to its
mechanism, Slipher managed
to increase the sensitivity of
Lowell’s spectrograph, a complex
450-lb (200-kg) instrument
attached to the eyepiece of
the Observatory’s 24-inch (61-cm)
refractor telescope. During the
fall and winter of 1912, he
obtained a series of spectrograms
from the largest of the spiral
nebulae, which was located in
the constellation of Andromeda
and known at the time as the
Andromeda nebula.
The pattern of spectral lines
in the nebula’s spectrum (like
a fingerprint of its composition)
indicated a “blueshift”—they
were unexpectedly displaced
toward the short-wavelength/
high-frequency blue end of the
spectrum by what is known as
a Doppler shift (see diagram, left).
That could only mean that the
light waves coming from the
Andromeda nebula were being
shortened, or compressed, and
their frequency raised, because
the nebula was rushing toward
Earth at a considerable speed.
Slipher’s calculations revealed
that the nebula was approaching
at 200 miles per second (300 km
per second). Doppler shifts had
been measured for astronomical
bodies before, but shifts of this
size were unprecedented. Slipher ❯❯This galaxy is not moving relative to Earth.
Light waves coming from it are detected on
Earth at their normal, unaffected frequency.
Emission lines in the spectra of stationary
galaxies are consistent with the wavelengths
of the component gases in the galaxy.This galaxy is moving toward Earth. Light
waves coming from it are detected on Earth
as slightly shortened or of a higher frequency.
Emission lines in the spectra of approaching
galaxies are shifted toward the shorter, blue
wavelengths: this is a “blueshift.”This galaxy is receding from Earth. Light
waves coming from it are detected on Earth
as slightly lengthened or of a lower frequency.
Emission lines in the spectra of receding
galaxies are shifted toward the longer,
red wavelengths: this is a “redshift.”400
WAVELENGTH (nm)500 600 700400
WAVELENGTH (nm)500 600 700400
WAVELENGTH (nm)500 600 700Measurements of the
blueshifts and redshifts
of spiral-shaped nebulae
show that some are moving
toward Earth while others
are receding.If the spiral nebulae lie
within the Milky Way
Galaxy, they are moving so
fast relative to the rest of the
galaxy that they cannot
remain within it for long.Spiral nebulae may
be separate
galaxies outside
the Milky Way.