Final answer:
Cepheid variable stars and RR Lyrae stars are essential for measuring stellar and galactic distances through the Period-Luminosity Relation.
Step-by-step explanation:
The major links in the distance chain involve using various methods to measure cosmic distances. One crucial link is the use of pulsating variable stars, such as Cepheid variables and RR Lyrae stars, which are integral to our study of the universe. These stars vary in brightness in a regular pattern, which allows astronomers to determine their luminosity and, subsequently, their distance from us.
Cepheid variable stars are particularly important because of the Period-Luminosity Relation. Their variability period correlates directly with their intrinsic luminosity; the longer the period, the brighter the star. This relationship enables the measurements of distances to these stars and to the galaxies they reside in, up to 60 million light-years away.