Doppler shift in astronomy — Astronomy, 14–17
Astronomers can tell whether a star or galaxy is moving towards us or away from us by tiny changes in the wavelengths of its light.
The idea
Light from a moving object is shifted slightly. Moving towards us makes its wavelengths shorter, shifting the light towards blue; moving away makes them longer, shifting it towards red. The shift reveals motion, even when the object is far too distant to watch move.
Why astronomers need it
Space is too large for astronomers to follow most objects across the sky. They needed a way to measure speed along our line of sight, not just position. The Doppler shift turns invisible motion into a measurable change in a spectrum.
A worked example
A spectral line normally appears at 500 nanometres. From a star it is measured at 501 nanometres, so it has shifted to longer wavelengths and the star is receding. For small speeds, the fraction 1/500 is about 0.002, so the speed is about 0.002 times light speed, or 600 kilometres per second.
The common trap
It is tempting to call every red shift a sign that an object is moving away. That is reasonable because motion really can produce a red shift, but expanding space can also stretch light between galaxies. Astronomers must identify which effect explains the measurement.
Where it is used
Astronomers use Doppler shifts to measure a star’s motion, map the rotation of galaxies and detect the tiny wobble caused by an orbiting planet. It also helps measure gas moving in nebulae and the speeds of jets from young stars.
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