127
See also: Ole Rømer 58–59 ■ Edwin Hubble 236–41 ■ Geoffrey Marcy 327T
he color of light depends
on its frequency, which
is the number of waves
per second. If something moving
toward us is emitting waves, the
second wave will have a shorter
distance to travel than the first
wave, so it will arrive sooner than it
would if the source were stationary.
Thus the frequency of waves
increases if the source and receiver
are getting closer to each other, and
decreases if they are moving apart.
This effect applies to all types of
wave, including sound, and is
responsible for the changing pitch
of a siren as an ambulance passes.
To the naked eye, most stars
appear to be white, but through
a telescope many can be seen to
be red, yellow, or blue. In 1842, an
Austrian physicist named Christian
Doppler suggested that the red color
of some stars is due to the fact that
they are moving away from the
Earth, which would shift their light
to longer wavelengths. Since the
longest wavelength of visible light is
red, this became known as redshift
(as illustrated on p.241).The colors of stars are now
known to be mainly due to their
temperature (the hotter the star,
the more blue it appears), but the
movement of some stars can be
detected through Doppler shifts.
Binary stars are pairs of stars
orbiting each other. Their rotation
causes an alternating redshift and
blueshift in the light they emit. ■A CENTURY OF PROGRESS
ON THE COLORED
LIGHT OF THE
BINARY STARS
CHRISTIAN DOPPLER (1803–1853)
IN CONTEXT
BRANCH
Physics
BEFORE
1677 Ole Rømer estimates
the speed of light by studying
Jupiter’s moons.
AFTER
1840s Dutch meteorologist
Christophorus Buys Ballot
applies the Doppler shift to
sound waves, as does French
physicist Hippolyte Fizeau to
electromagnetic waves.
1868 British astronomer
William Huggins uses redshift
to find the velocity of a star.
1929 Edwin Hubble relates
the redshift of galaxies to their
distance from Earth, showing
the expansion of the universe.
1988 The first extrasolar
planet is detected, using the
Doppler shift of light from
the star that it orbits—the
star appears to “wobble” as
the planet’s gravitational
pull disrupts its rotation.
The heavens presented an
extraordinary appearance, for
all the stars directly behind
me were now deep red, while
those directly ahead were
violet. Rubies lay behind me,
amethysts ahead of me.
Olaf Stapledon
From his novel, Star Maker (1937)