The offset of the sodium lines in the spectrum of a distant star is most likely explained by the Doppler effect, which relates to the motion of the star relative to us. If the lines are shifted toward the blue end of the spectrum (shorter wavelengths), this indicates that the star is moving toward us. Conversely, if the lines are shifted toward the red end of the spectrum (longer wavelengths), it indicates that the star is moving away from us.
Since you mentioned the offset of the sodium lines, the correct interpretation would depend on whether the lines are blue-shifted or red-shifted:
- If the sodium lines are blue-shifted: The star is moving toward us.
- If the sodium lines are red-shifted: The star is moving away from us.
Thus, the most likely explanation for the offset of the sodium lines would be that the star is either moving toward us or away from us, depending on the direction of the shift.
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