The Mira Distance Ladder

Here we provide a review of Mira variables, their basic properties, and Period-Luminosity Relations with an emphasis on their role in measuring the Hubble Constant. The usage of multiple independent distance indicators and methods is crucial to cross-checking systematic uncertainties in distance measurements and in reinforcing previous findings of the Hubble tension. To this end, Mira variables serve as an alternative Type Ia Supernova calibrator to the more commonly-used Cepheid variables or Tip of the Red Giant Branch method. They also have the potential to expand the number of local SN Ia calibrators by calibrating previously-inaccessible SNe Ia. Short-period ($P \lesssim 400$ d) O-rich Miras are a ubiquitous older population that can reach galaxies not hosting the younger Cepheids variables or out of reach to the old but fainter Tip of the Red Giant Branch. With the current and upcoming focus on infrared observations, Miras, which can be discovered and characterized using exclusively near-infrared and infrared observations, will be particularly useful in obtaining distances to astrophysical objects. Long-period Miras ($P \gtrsim 400$ d) are highly luminous variables that have the potential to measure $H_0$ directly, excluding Type Ia SNe altogether in the distance ladder.