How a 16-year-old measures a galaxy's recession speed: redshift and velocity

One measurement to determine a galaxy's recession speed is the redshift of spectral lines, which gives velocity via Doppler shift; distance alone does not give speed.

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Measuring a galaxy's recession speed

For a 16-year-old learner: this describes the simplest way scientists estimate how fast a distant galaxy is moving away.

  1. Take a spectrum of the galaxy with a telescope and a spectrograph.
  2. Identify well-known spectral lines (like hydrogen) and see how much they shift toward longer wavelengths.
  3. Convert the shift to a velocity. For small redshifts, v ≈ z × c, where c is the speed of light (~300,000 km/s).
  4. Distance alone does not give speed. You can relate velocity and distance with Hubble's law v = H0 × d, but you do not obtain speed from distance alone.
  5. If you know the distance, you can estimate velocity by assuming a value for the Hubble constant, but the direct measurement comes from redshift.

Bottom line: redshift is the primary measurement for the speed of distant galaxies; distance helps relate velocity but does not replace the redshift measurement.

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