Date context
1889–1953
Working at Mount Wilson
Hubble joined Mount Wilson Observatory in 1919, using its large reflecting telescopes to study objects then called nebulae. Whether spiral nebulae lay inside the Milky Way or constituted distant stellar systems remained a major question in contemporary astronomy.
Cepheids in Andromeda
Beginning in 1923, he identified Cepheids in photographs of Andromeda and used their periods to estimate its distance. It lay well beyond the Milky Way. Later recalibrations changed the numerical distance without overturning the conclusion that Andromeda was a separate galaxy.
Distances and recession velocities
His 1929 paper compared distances with velocities inferred from spectra, finding a general trend of faster recession at greater distances. It used velocity measurements by Slipher and others, while Humason contributed further observations. The underlying dataset was not the work of one observer alone.
Observation and cosmological interpretation
The relation became important observational evidence in the study of cosmic expansion, linking stellar distance indicators, spectroscopy and cosmological models. Later investigators revised the distance scale and expansion rate. The space telescope bearing his name was built long after his lifetime.