Final answer:
RR Lyrae and Cepheid stars are pulsating variable stars used by astronomers to measure cosmic distances by utilizing their predictable periods and luminosities, serving as cosmic 'standard candles'.
Step-by-step explanation:
RR Lyrae and Cepheid stars are pulsating variable stars that are crucial in astronomy for determining cosmic distances. These stars have predictable brightness variations that correspond to their intrinsic luminosity. The RR Lyrae stars, which are less luminous than Cepheids, have periods of less than 24 hours and are often used to measure distances within our galaxy. Cepheid stars have a well-defined period-luminosity relationship, and by measuring their periods, astronomers can ascertain their true brightness and thus calculate their distances. This method of using Cepheids and RR Lyrae stars as standard candles extends our ability to measure distances to over 60 million light-years, and for the most distant objects, up to almost 1 million light-years.