For 10 points, name these objects whose precise variability makes them viable for use as "standard candles" in determining the size of the universe.
These objects are the basis for the science done by the Key Project of the Hubble Space Telescope. Together with SX Phoenicis and Delta Scuti objects, among others, these astronomical bodies inhabit the instability strip. Sandage's method is used to remove bias from a relation relevant to these objects, and determining the zero-point of that relation is part of the mission of the Hipparcos satellite. The cluster-type variety of these objects belongs to population II and is exemplified by RR Lyrae stars. Interstellar reddening of these stars can be measured from their color-period relation, which is independent of distance, and their period-luminosity relation was first calculated by Henrietta Levitt. For 10 points, name these objects whose precise variability makes them viable for use as "standard candles" in determining the size of the universe.
These objects are the basis for the science done by the Key Project of the Hubble Space Telescope.
Cepheid variable stars (prompt on "variable")