John Bahcall, 70; Astrophysicist’s Research Helped Prove the Sun Is Powered by Nuclear Reactions
John Bahcall, the Princeton astrophysicist whose research was key to proving that the sun is powered by nuclear reactions and whose stature and influence in the astronomical community were instrumental in the creation and launch of the Hubble Space Telescope, died Wednesday in a hospital in New York City.
His family said Bahcall, 70, died of a rare blood disorder.
Physicist Hans Bethe, who died earlier this year, conceived and demonstrated in the laboratory the relatively simple set of nuclear reactions that scientists believe have powered the sun for billions of years. But before Bahcall, researchers had no direct experimental evidence demonstrating that the same reactions known to occur in the lab were also taking place in the sun.
In the early 1960s, Bahcall, then working at the Kellogg Radiation Laboratory at Caltech, performed a series of calculations showing that the sun should be emitting neutrinos that could be detected on Earth. Neutrinos are tiny, highly elusive subatomic particles with no electrical charge and the ability to pass relatively unscathed through even the densest of matter.
His work came to the attention of astrophysicist Raymond Davis Jr. of the Brookhaven National Laboratory, who began a lifelong collaboration with Bahcall. Davis constructed an elaborate neutrino detector in an abandoned gold mine in Lead, S.D. The detector was essentially a very large tank filled with a chlorine-based dry-cleaning fluid.
Most neutrinos striking the tank passed through it, just as they pass through other matter. But every now and then, a neutrino would strike a chlorine atom and convert it to a radioactive argon atom. Every month, the team would isolate a handful of the radioactive atoms, representing only the tiniest fraction of the neutrinos that passed through the tank.
In 1968, Bahcall and Davis announced that they had observed neutrinos from the sun, proof positive that, as Bahcall later wrote, “the nuclear reactions that produce the neutrinos also cause the sun to shine.” In proving that nuclear reactions fueled the sun, the team solved a mystery that had eluded scientists for more than a century.
But they also created another mystery, the so-called Solar Neutrino Puzzle, that would plague astronomers for more than three decades.
Davis observed only about a third as many neutrinos as Bahcall predicted should be seen. Few physicists initially took the discrepancy seriously, but Bahcall argued that it revealed a fundamental lack of understanding about the processes involved. He and others developed a variety of theories to explain the mystery, without much initial success.
Eventually, however, observations at other, newer neutrino observatories revealed that some neutrinos were undergoing a transformation during their journey from the sun to Earth, changing into a form that could not be detected at the Lead observatory.
Davis and physicist Masatoshi Koshiba of the University of Tokyo, who also built a neutrino detector, shared the 2002 Nobel Prize in physics for their work on the neutrino. Many researchers thought Bahcall should have received a share of the award as well, but he never indicated any bitterness that he did not.
Bahcall and physicist Sheldon Glashow of Harvard University also pioneered the field of neutrino astronomy, work that was triggered by the 1987 eruption of a massive supernova in a nearby region of the Milky Way. It was the first time a supernova had exploded close enough to the Earth for its neutrinos to be detected by observatories here.
The pair identified each of the 19 supernova neutrinos detected in observatories, noted the time of arrival of each one on Earth and calculated its energy level.
Had the neutrinos been more like normal particles, those with the highest energy, and thus with the highest mass, should have arrived at Earth first. But Bahcall and Glashow found that was not the case. In fact, there was no correlation between time of arrival and energy levels.
This observation led to fundamental discoveries in cosmology: that neutrinos, once thought to be without mass, did in fact have a very small mass and that the mass was too small for neutrinos to account for the so-called missing mass of the universe, the dark matter that accounts for more than half of the cosmos but that cannot be seen.
Bahcall’s most visible effect was probably on the development of the Hubble.
In 1944, Princeton astronomer Lyman Spitzer first suggested the idea of placing a large telescope in orbit, where its observations would not be impaired by the Earth’s atmosphere. When the technology to do so finally became available with the development of the Space Shuttle, Bahcall joined with Spitzer as a forceful advocate for the program, and the two appeared frequently before Congress to argue their case.
Bahcall “was really tenacious. He just kept going after it and wouldn’t give up,” Caltech astronomer Maarten Schmidt said Friday. That was crucial in his studies of neutrinos, “and it really paid off with the Hubble Space Telescope,” which was launched in 1990.
Bahcall was extremely disappointed when, in the aftermath of the shuttle Columbia’s destruction during reentry in 2003, then-NASA administrator Sean O’Keefe ruled out any further shuttle missions to repair Hubble, thereby dooming it to an early death.
When a National Academy of Sciences panel rejected O’Keefe’s reasoning last year, Bahcall was elated. “Finally, somebody told the king he didn’t have any clothes,” he said.
Even from his hospital bed, Bahcall stayed in touch with his students and continued to agitate for a future Hubble repair mission. NASA administrator Michael Griffin has said he will reexamine the Hubble issue.
Bahcall was also chairman of a National Academy of Sciences panel created in 1989 to chart the future of astronomy research for the coming decade. That report, published in 1991, came to be known as the Bahcall Report. Among other things, it recommended the production and launch of the Spitzer Space Telescope to study the universe at infrared wavelengths.
John Norris Bahcall was born in Shreveport, La., on Dec. 30, 1934. He enrolled in Louisiana State University thinking that he wanted to study philosophy and perhaps become a rabbi. Within the first year of college, however, he switched to physics and astronomy, convinced that they best suited a “quest for truth.”
He transferred to UC Berkeley, then received graduate degrees from the University of Chicago and Harvard. After short stints at Indiana University and Caltech, he spent the bulk of his career at Princeton’s Institute for Advanced Study.
In addition to his research, Bahcall took great pride in mentoring the students who passed through his laboratory, many of whom have now gone on to prominent positions. He also helped establish astronomy groups at the Weizmann Institute and Tel Aviv University in Israel.
On one trip to Israel, he met a young graduate student in physics, Neta Assaf. Although he spoke no Hebrew and she spoke very little English, he pursued her through more than a dozen rejections before she accepted a date. They were married within a year.
Neta Bahcall is now a cosmologist at Princeton.
The Bahcalls were the only astronomy couple who both were members of the National Academy of Sciences.
In addition to his wife, Bahcall is survived by son Safi, president of Synta Pharmaceuticals in Massachusetts; son Dan, a cognitive psychologist who lives in Berkeley; daughter Orli, an associate editor of the journal Nature Genetics, and a brother, Robert.
