RR Lyrae stars are a type of pulsating variable star that are used as standard candles for measuring distances in the universe. Their pulsations are caused by the star's surface being heated and cooled by the star's own radiation, which causes the star to expand and contract. The period of these pulsations is related to the star's mass, so by measuring the period of an RR Lyrae star's pulsations, astronomers can determine its mass.

The relationship between the period of pulsation and the mass of an RR Lyrae star is given by the following equation:

$$ M \propto P^{3/2} $$

where:

- M is the mass of the star in solar masses

- P is the period of pulsation in days

This equation can be used to determine the mass of an RR Lyrae star by measuring its period of pulsation. For example, an RR Lyrae star with a period of 0.5 days has a mass of approximately 0.6 solar masses.

RR Lyrae stars are typically found in old stellar populations, such as globular clusters and dwarf spheroidal galaxies. They are also found in the Milky Way's halo. RR Lyrae stars are typically low-mass stars, with masses ranging from 0.5 to 1 solar masses.

The ability to weigh stars is important for astronomers because it allows them to determine the mass distribution of stars in the universe. This information can then be used to study the evolution of stars and galaxies.