Allan Sandage

Allan Sandage (1926–2010) was an American astronomer who measured the universe’s expansion rate and developed methods for determining the ages of stars and galaxies.

Edwin Hubble: An American astronomer whose observations established the relationship between galaxies’ distances and their recession speeds. Sandage continued Hubble’s work on galaxy distances and cosmic expansion at Mount Wilson Observatory.
Hubble constant: The present-day rate at which the universe’s expansion increases distance between galaxies. Sandage devoted decades to measuring this quantity and favored values near 50 kilometers per second per megaparsec.
Surface brightness fluctuation: A distance-measurement technique based on pixel-to-pixel brightness variations in unresolved stellar populations. Sandage and collaborators used fluctuations in galaxy images to extend distance measurements beyond nearby galaxies.
Hubble–Sandage controversy: A decades-long dispute over the value of the Hubble constant and the universe’s expansion rate. Sandage’s low estimates clashed with higher values defended by Gérard de Vaucouleurs and others.
Walter Baade: A German-American astronomer who distinguished two stellar populations and revised estimates of the universe’s scale. Baade’s discovery of two kinds of Cepheid variable stars changed the distance scale Sandage inherited.
Cosmic distance ladder: A sequence of methods for measuring astronomical distances, each calibrated by a nearer method. Sandage calibrated successive distance indicators to estimate distances to remote galaxies.
Tully–Fisher relation: An empirical relation connecting a spiral galaxy’s luminosity to its rotational speed. This relation offered another way to estimate galaxy distances and test Sandage’s distance scale.
Gérard de Vaucouleurs: A French-American astronomer who studied galaxies and argued for a high value of the Hubble constant. De Vaucouleurs’s distance scale produced expansion-rate estimates far above Sandage’s.
Martin Schwarzschild: A German-American astrophysicist known for work on stellar structure and the evolution of stars. Schwarzschild collaborated with Sandage on using stellar evolution to estimate the ages of globular clusters.
Cepheid variable: A pulsating star whose period of brightness variation is related to its intrinsic luminosity. Cepheids helped Sandage calibrate galaxy distances and revise the expansion-rate estimate.

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