What type of star did Hubble use to measure the expansion rate of the universe?

Hubble used Cepheid variables to help measure the expansion rate of the universe. These pulsating stars have a dependable relationship between their intrinsic brightness and the timing of their brightness changes, making them valuable cosmic distance markers.

Astronomers first determine how bright a Cepheid appears to be and how rapidly it pulses. Because its true luminosity can be inferred from its period, the difference between its true and apparent brightness reveals its distance. Hubble’s sharp view above Earth’s atmosphere made it possible to identify Cepheids in relatively nearby galaxies.

Those galaxies could then be compared with their measured redshifts, which indicate how quickly they are receding. This relationship helped refine the Hubble constant, the value describing the universe’s present expansion rate. Hubble observations also supported the conclusion that the expansion is accelerating, a result associated with dark energy.

Type Ia supernovae are another important distance indicator, but they are explosive stellar events rather than the pulsating stars named in this question. RR Lyrae stars are useful standard candles too, though they are generally less luminous and serve different distance-measurement roles.