Final answer:
To determine the distance to a galaxy using the Hubble-Lemaitre law, obtain the galaxy's redshift to find its velocity, then apply the formula v = H × d, where H is Hubble's constant, to calculate the distance.
Step-by-step explanation:
To use the Hubble-Lemaitre law to measure the distance to a distant galaxy, you should first obtain the spectrum of the galaxy. From the spectrum, you measure the redshift, which provides the galaxy's velocity (v) moving away from us. Knowing the velocity, you can use the Hubble-Lemaitre law (v = H × d), where 'H' is Hubble's constant and 'd' is the distance to the galaxy. Hubble's constant can be understood as the rate of expansion of the universe, typically measured in kilometers per second per megaparsec (km/s/Mpc). For example, if Hubble's constant is 70 km/s/Mpc, and a galaxy's velocity is determined to be 1400 km/s based on its redshift, then its distance would be 20 Mpc. It's important to note that this method works best for galaxies that aren't extremely distant, as the rate of expansion of the universe might have changed over time. For very distant galaxies, one must assume a cosmological model to convert large redshifts into distances accurately.