Astronomers can tell whether a star is approaching or receding from Earth by

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Astronomers can tell whether a star is approaching or receding from Earth by observing the Doppler effect, which is a change in the wavelengths of light emitted by the star as it moves either closer or farther away from us. This change in wavelength is known as the star's redshift (if it is moving away) or blueshift (if it is moving closer). Temperature and absorption spectra do provide information about stars, but they are not directly related to determining their motion relative to Earth.

Astronomers Can Tell Whether a Star Is Approaching or Receding