Wednesday, November 17, 2010
Allan Sandage, Astronomer, Dies at 84; Charted Cosmos’s Age and Expansion
Allan R. Sandage, who spent his life measuring the universe, becoming the most influential astronomer of his generation, died Saturday at his home in San Gabriel, Calif. He was 84.
The cause was pancreatic cancer, according to an announcement by the Carnegie Observatories, where he had spent his whole professional career.
Over more than six decades, Dr. Sandage was like one of those giant galaxies that sit at the center of a cluster of galaxies, dominating cosmic weather. He wrote more than 500 papers, ranging across the cosmos, covering the evolution and behavior of stars, the birth of the Milky Way galaxy, the age of the universe and the discovery of the first quasar, not to mention the Hubble constant, a famously contested number that measures the rate of expansion of the universe. Dr. Sandage pursued the number with his longtime collaborator, Gustav Tammann of the University of Basel in Switzerland.
In 1949, Dr. Sandage was a young Caltech graduate student, a self-described “hick who fell off the turnip truck,” when he became the observing assistant for Edwin Hubble, the Mount Wilson astronomer who discovered the expansion of the universe.
Hubble had planned an observing campaign using a new 200-inch telescope on Palomar Mountain in California to explore the haunting questions raised by that mysterious expansion. If the universe was born in a Big Bang, for example, could it one day die in a Big Crunch? But Hubble died of a heart attack in 1953, just as the telescope was going into operation. So Dr. Sandage, a fresh Ph.D. at 27, inherited the job of limning the fate of the universe.
“It would be as if you were appointed to be copy editor to Dante,” Dr. Sandage said. “If you were the assistant to Dante, and then Dante died, and then you had in your possession the whole of ‘The Divine Comedy,’ what would you do?”
Dr. Sandage was a man of towering passions and many moods, and for years, you weren’t anybody in astronomy if he had not stopped speaking to you. In later years, beset by controversy, Dr. Sandage withdrew from public view. But even after retiring from the Carnegie Observatories and becoming ill, he never stopped working; he published a paper on variable stars only last June.
In 1991, Dr. Sandage was awarded the Crafoord Prize in astronomy, the closest thing to a Nobel for a stargazer, worth $2 million.
Wendy Freedman, his boss as director of Carnegie as well as a rival in the Hubble constant question, referred to him on Tuesday as the last giant of 20th-century observational cosmology. “Even when we had our scientific differences, I got a kick out of him,” she said. “His passion for his subject was immense.”
James Gunn, an astronomer at Princeton, said of Dr. Sandage in an e-mail message, “He was probably (rightly) the greatest and most influential observational astronomer of the last half-century.”
Allan Rex Sandage was born in Iowa City, Iowa, on June 18, 1926, the only child of an advertising professor, Charles Harold Sandage, and a homemaker, Dorothy Briggs Sandage. The stars were one of his first loves; his father bought him a commercial telescope.
After two years at the University of Miami, where his father taught, Allan was drafted into the Navy; he resumed his education at the University of Illinois, earning a degree in physics.
In 1948 he entered graduate school at the California Institute of Technology, where an astronomy program had been started in conjunction with the nearby Mount Wilson Observatory, home of Hubble, among others.
As a result, Dr. Sandage learned the nuts and bolts of observing with big telescopes from the founders of modern cosmology, Hubble; Walter Baade, who became his thesis adviser, and Milton Humason, a former mule driver who had become Hubble’s right-hand man.
In the years before World War II, there had been a revolution in the understanding of the nature and evolution of stars as thermonuclear furnaces burning hydrogen into helium and elements beyond. Astronomers could now read the ages of star clusters from the colors and brightness of the stars in them.
For his thesis, Dr. Sandage used this trick to date a so-called globular cluster, known as Messier 3, as being 3.2 billion years old, which meant that the universe itself could not be younger than that. In fact, Hubble’s own measurements of the cosmic expansion suggested an age of about four billion years — remarkably, even miraculously, consistent.
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. [See earlier post for another view]
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 the University of Indiana 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.
But Hubble had made mistakes — he saw bright patches of gas as stars, for example — and as Dr. Sandage and Dr. Tammann delved into the subject in a series of papers, the problematic constant came down and the imputed age of the universe rose.
In 1956, Dr. Sandage suggested that the Hubble constant could be as low as 75 kilometers per second per megaparsec. By 1975 the value, they said, was all the way down to 50, corresponding to an age of as much as 20 billion years, comfortably larger than the ages of galaxies and globular clusters.
This allowed them to conclude that the universe was not slowing down enough for gravity to reverse the expansion into a Big Crunch. That was in happy agreement with astronomers who had found that there was not enough matter in the universe to generate the necessary gravity.
As Dr. Sandage wrote in The Astrophysical Journal in March 1975, “(b) the Universe has happened only once, and (c) the expansion will never stop.”
“So the universe will continue to expand forever,” Dr. Sandage said in an interview, “and the galaxies will get farther and farther apart, and things will just die. That’s the way it is. It doesn’t matter whether I feel lonely about it or not.”
Shortly thereafter, however, their results on the Hubble constant came under attack by rival astronomers, who said that Dr. Sandage and Dr. Tammann had overestimated the distances to galaxies — a crucial part of the equation for the constant — making the universe appear bigger and older than it really was. The universe, they said, was really about 10 billion years old.
Stung by the criticisms, Dr. Sandage retreated from public view, even while he and Dr. Tammann redoubled their efforts to measure the troublesome constant, always getting a low value. As the groups shot back and forth at each other, the universe, as reflected in newspaper headlines, boomeranged back and forth from 10 billion to 20 billion years.
In 2001, a team led by Dr. Freedman, using the Hubble Space Telescope, reported a value of 72 kilometers per second per megaparsec, in good agreement with measurements of relic radiation from the Big Bang that give an age of 13.7 billion years for the cosmos full of dark energy and dark matter, and a Hubble constant of 71, which most astronomers now accept.
To the frustration of colleagues, Dr. Sandage, also using Hubble, kept getting a lower value.
We may never know the fate of the universe or the Hubble constant, he once said, but the quest and discoveries made along the way were more important and rewarding than the answer anyway.
“It’s got to be fun,” Dr. Sandage told an interviewer. “I don’t think anybody should tell you that he’s slogged his way through 25 years on a problem and there’s only one reward at the end, and that’s the value of the Hubble constant. That’s a bunch of hooey. The reward is learning all the wonderful properties of the things that don’t work.”