RR Lyrae stars are old, pulsating stars that burn helium in their cores for fuel. Their progenitors were similar to our sun, but they are now much larger and brighter than our sun. We know of hundreds of thousands of RR Lyrae stars within the Milky Way, and we can use them to study how our galaxy has evolved since its formation. However, one fundamental measurement has so far eluded astronomers: there has been no reliable way to determine the masses of RR Lyrae stars—until now.
