Final answer:
The difference between the apparent magnitude and the absolute magnitude is known as the distance modulus of cepheid variables. By measuring the period and apparent magnitude of a cepheid variable, astronomers can calculate its absolute magnitude and then use the difference between the two to determine its distance from Earth.
The correct answer is option a. measuring the distance modulus of cepheid variables.
Step-by-step explanation:
The difference between the apparent magnitude and the absolute magnitude is known as the distance modulus of cepheid variables. Cepheid variables are a type of star that has a known correlation between their period and their absolute magnitude.
By measuring the period and apparent magnitude of a cepheid variable, astronomers can calculate its absolute magnitude and then use the difference between the two to determine its distance from Earth.
The correct answer is option a. measuring the distance modulus of cepheid variables.