Why are methods to determine the distances to galaxies termed the "distance ladder"? Explain the term with examples

Question

What will be an ideal response?

Answer 1

Distances in space are difficult to measure. Any method of measuring distance is effective for a certain range of distances, and each step depends on the step below it.Cepheid variable stars are very useful, but they do not appear close to the earth, and cannot be seen at far distances. The distance to Cepheid variables depends on our understanding of the luminosity of stars in the H-R diagram, and so forth.The Hubble law is extremely useful at measuring far distances, but is only useful for galaxies that are some distance from the Milky Way. The Hubble law was determined using Cepheid variables, which cannot be seen at far distances from the Earth. The scale of the Hubble relation depends on comparing the distances calculated for galaxies that have Cepheid variables.(Other examples will be valid and should be judged on their correctness and connection to other steps.)