If a star's radial velocity is -50 km/s, the frequency of its light would appear to be (higher or lower) than its true frequency. We usually say that the star's wavelength is (blue or red) shifted.

If a star's radial velocity is -50 km/s, the frequency of its light would appear to be higher than its true frequency. We usually say that the star's wavelength is blue shifted.

Explanation:

Radial velocity is defined as the velocity of how an object is seen by the axis of a circle.

Then, if the radial velocity is -50km/s, this means that the radius is decreasing, and then that the star is coming towards the viewer.

as the star is coming towards the viewer, we will see a shorter wavelength, which implies that the frequency would appear to be bigger as it really is.

and this is called a blue shift, because the blue light is the visible light with the biggest frequency, while the red light is the visible light with the smallest frequency, then we have:

If a star's radial velocity is -50 km/s, the frequency of its light would appear to be higher than its true frequency. We usually say that the star's wavelength is blue shifted.