The brightness of RS Puppis, one of the brightest Cepheid variable stars, changes at a constant rate on a six-week cycle. Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration

The brightness of RS Puppis, one of the brightest Cepheid variable stars, changes at a constant rate on a six-week cycle. Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration

The brightness of RS Puppis, one of the brightest Cepheid variable stars, changes at a constant rate on a six-week cycle. Credit: NASA, ESA, Hubble Heritage Team (STScI/AURA)-Hubble/Europe Collaboration