What is the best way to determine a galaxy's redshift?

The most effective method to determine a galaxy's redshift is by taking a spectrum of the galaxy and measuring the difference in wavelength. When we observe a galaxy, its light is emitted across a range of wavelengths, and the spectrum reveals distinct absorption and emission lines corresponding to various elements and compounds present in the galaxy.

If the galaxy is moving away from us, these spectral lines will be shifted towards longer wavelengths, a phenomenon known as redshift. By precisely measuring the amount of this shift, astronomers can calculate the galaxy's redshift, which provides important information about its velocity and distance based on the expansion of the Universe.

This approach allows for a more detailed and accurate assessment compared to simply comparing photographs or using color filters, which do not provide the same level of specificity regarding the shifts in wavelengths. Furthermore, analyzing brightness over time focuses on luminosity variations, which, while helpful for certain studies, does not directly indicate a galaxy's motion relative to the observer.