Cepheid Variables13.16 - Understand how Cepheid variables can be used to determine distances
A Cepheid variable is a star that pulses frequently. We see the light they produce dip and rise over a short period of time, returning to the same luminosity a few days later.
Cepheids have a period-luminosity relation. The longer the period between peaks of brightness the brighter the star is. From this we can figure out its apparent magnitude and from that how far away it is. Cepheids in other galaxies enable us to accurately measure how far away that galaxy is, something we would not be able to do with heliocentric parallax because the angles are very small.
They are thought to be youthful stars and are fondly called 'distance candles' by astronomers.
One of the most studied Cepheid variables is Delta Cephei in the constellation Cepheus.