What observation, related to the Doppler effect, signifies that a star has a negative radial velocity ($V_r$)?
Answer
The star is moving toward the observer
In the standard convention used for measuring radial velocity ($V_r$), the sign indicates the direction of motion along the line of sight relative to the observer. A negative value for $V_r$ specifically denotes that the star is approaching the observer. This approach causes the light waves to be compressed, resulting in a spectral signature known as blueshift, where absorption lines shift toward the blue end of the spectrum compared to stationary sources. Conversely, a positive $V_r$ indicates recession, accompanied by redshift.
Related Questions
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