At the time, astronomers were also still debating whether the universe had had a Big Bang and a beginning at all, not to mention whether it would have an ending as well. An opposing view championed by the British cosmologist Fred Hoyle held that the universe was eternal and in a “steady state,” with new matter filling in the void as galaxies rushed away from one another.

Choosing between these models was to be the big task of 20th-century astronomy, and of Dr. Sandage. In 1961 he published a paper in The Astrophysical Journal showing how it could be done using the 200-inch telescope. He described cosmology as the search for two numbers: one was the cosmic expansion rate, known as the Hubble constant; the other, called the deceleration parameter, tells how fast the expansion is being braked by cosmic gravity.

That paper, “The Ability of the 200-inch Telescope to Discriminate Between Selected World Models,” may well have been “the most influential paper ever written in any field even close to cosmology,” Dr. Gunn said. It was to set the direction of observational cosmology for 40 years, ruling out the Steady State and the Big Crunch and culminating in the surprise discovery in 1998 that the expansion is not slowing down at all but speeding up.

Meanwhile, Dr. Sandage investigated the birth of the galaxy. By analyzing the motions of old stars in the Milky Way, he, Olin Eggen of Caltech and Donald Lynden-Bell of Cambridge showed in a 1962 paper that the Milky Way formed from the collapse of a primordial gas cloud probably some 10 billion years ago. That paper still forms the basis of science’s understanding of where the galaxy came from, astronomers say.

In 1959, Dr. Sandage married another astronomer, Mary Connelly, who was teaching at Mount Holyoke and had studied at Indiana University and Radcliffe, but did not pursue further research. He is survived by her and two sons, David and John.

It was measuring the cosmic expansion that was the most backbreaking part of fulfilling Hubble’s legacy. In an expanding universe, the speed with which a galaxy flies away from us is proportional to its distance. The constant of proportionality, the Hubble constant, is given in the mind-numbing terms of kilometers per second per megaparsec. Hubble’s original estimate of his constant of 530 meant that for every million parsecs (3.26 million light years) a galaxy was farther away from us, it was retreating 530 kilometers per second (around 300 miles per second) faster.

Hubble’s original estimate, however, corresponded to an age for the universe of only 1.8 billion years, at odds with both geological calculations of the Earth’s age and Dr. Sandage’s later estimate of the ages of star clusters.