Based on what you have learned in previous chapters, which of the following measurements would we use to determine which galaxies are moving toward or away from the milky way?
a. the total luminosity of the galaxy
b. the width of spectral lines within the galaxy’s spectrum
c. the doppler shift of spectral lines within the galaxy’s spectrum
d. the parallax angle of the galaxy

the answer es c, the doppler Shift of spectral lines within the galaxy's spectrum

Explanation:

This is due to redshift, red approach or redshift, occurs when electromagnetic radiation, usually visible light, which is emitted or reflected from an object, is shifted to red at the end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared to the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of electromagnetic radiation. Instead, the decrease in wavelength is called blue shift. Any increase in wavelength is called "redshift", even if it occurs in electromagnetic radiation of non-visible wavelengths, such as gamma rays, X-rays and ultraviolet radiation. This designation can be confusing since at longer wavelengths than red (eg infrared, microwave and radio waves), "redshifts" move away from the red wavelength. So when talking about frequencies of waves smaller than red continues to mean that the wavelength tends to lengthen and not resemble red.

A redshift can occur when a light source moves away from an observer, corresponding to a Doppler shift that changes the perceived frequency of the sound waves. Although the observation of such redshifts, or their counterpart, towards blue, has numerous terrestrial applications (eg Doppler radar and radar gun),  astronomical spectroscopy uses Doppler redshifts to determine the movement of distant astronomical objects. This phenomenon was first predicted and observed in the 19th century when scientists began to consider the dynamic implications of the wave nature of light.

c. the Doppler shift of spectral lines within the galaxy's spectrum

Explanation:

we use this method to determine which galaxies are moving because galaxies moving away will always be a blue shift on the spectrum and galaxies moving closer will always have a red shift.

the device is called “ rocket engine”