IN CONTEXT
Physics
1677 Ole Rømer estimates the speed of light by studying Jupiter’s moons.
1840s Dutch meteorologist ChristophorusBuys 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 colour of light depends on its frequency, which is the number of waves per second. If something moving towards 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 called Christian Doppler suggested that the red colour of some stars is due to the fact that they are moving away from the Earth, which would shift their light to longer wavelengths. As the longest wavelength of visible light is red, this became known as redshift (as illustrated).
The colours 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.
"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 Stapledoln
From his novel, Star Maker (1937)
See also: Ole Rømer • Edwin Hubble • Geoffrey Marcy