On Cepheid distances in the $H_0$ measurement

Understand

Classical Cepheids were the first stellar standard candles and have played a crucial role for astronomical distance measurements ever since the discovery of the Leavitt law (period-luminosity relation).

• Enormous improvements in distance accuracy have been achieved since Hertzsprung's first application of Leavitt's law to measure the distance to the Small Magellanic Cloud in 1913, notably in very recent years thanks to a large data set of highly accurate space astrometry from the ESA mission Gaia.
• Complemented by homogeneous space photometry, Cepheids enable the most accurate distance estimates to galaxies hosting type-Ia supernovae up to approximately 70 Mpc distant.
• Here, I review the history of Cepheid distance measurements, open questions on the side of stellar astrophysics, and recent studies seeking to quantify and mitigate systematics with a view to further improve the accuracy on the Hubble constant.