Astronomy has lost another pioneering scientist: Allan Sandage, who died Saturday.
Sandage worked with Hubble himself on trying to determine the size and scale of the Universe. Perhaps his greatest contribution to astronomy was refining the Hubble Constant, a measure of how fast the Universe is expanding. In a seminal paper published in 1958, he set the expansion rate at 75 km/sec per megaparsec. That means that for every megaparsec (3.26 million light years) farther away a galaxy is, it appears to be moving away from us an additional 75 km/sec. So a galaxy 100 Mpc away would recede at 7500 km/sec.
This term relates to the age of the Universe. Until this new measurement, it was thought the Universe was much younger. The lower the number, the greater the age of the Universe, and Sandage's new number meant the Universe was about 15 billion years old. Later, Sandage thought the Universe might be even older, but using much improved methods over the years, we now think Hubble's constant is about 70 km/sec/Mpc, very close to what Sandage estimated in 1958.
I wish I had met Sandage, but I never did. His contributions to cosmology made him one of the greatest scientists of the 20th century.