SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/astro-blueshift-approaching

A galaxy's spectral lines are shifted toward shorter wavelengths. What does that tell you?

It is moving toward us (a blueshift).

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  1. It is moving toward us ✔
  2. It is moving away from us
  3. It is very hot

Why: An approaching source squeezes its waves, shortening the wavelength: a blueshift. A receding source stretches them: a redshift. The Andromeda Galaxy is one of the few blueshifted galaxies.

Huh, didn't know that: Strictly it should be called a violetshift, since violet is the short end of the visible spectrum.

The dad joke
Andromeda is blueshifted. It's coming over and doesn't even call first.
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References

Answer: It is moving toward us. An approaching source squeezes its waves, shortening the wavelength: a blueshift. A receding source stretches them: a redshift. The Andromeda Galaxy is one of the few blueshifted galaxies.

  1. OpenStax Astronomy 2e, 5.6 The Doppler Effect openstax.org/books/astronomy-2e/pages/5-6-the-doppler-effect
    When the source of waves moves toward you, the wavelength decreases a bit.
    Checked 2026-10-10.

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Test yourself in the game

Astronomy & SpaceA star moves sideways across our line of sight, not toward or away. What Doppler shift do we see?Astronomy & SpaceA spectral line shifts by 1/1000 of its wavelength. How fast is the source moving along our line of sight?Astronomy & SpaceHubble's law says a galaxy's recession speed is proportional to what?

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