NEWSFOCUS ASTROPHYSICS Thomas Gold explained as spinning neutron stars—sparked Bethe’s intense desire to under- A Towering Physicist’s Legacy stand the properties of superdense states of matter. Drawing from his deep well of nuclear Faces a Threatening Future physics, Bethe and colleagues wrote papers on the internal structures of neutron stars. On the centennial of Hans Bethe’s birth, his successors worry that cuts in long-planned They derived a likely radius of 10 kilometers, a projects will discourage the next generation of brilliant minds figure still in vogue. Soon after Bethe “retired” in ITHACA, NEW YORK—The language of astro- 1976, his friend Gerald Brown of the physics sizzles with alpha particles and State University of New York, Stony gamma rays. There’s a heavy dose of beta as Brook, piqued his interest with a well—Hans Bethe, that is, a giant of 20th cen- challenge to work out the nature of tury science, whose prowess in nuclear supernovae. The two scientists spent physics led to his fascination with combustion much of the next 3 decades ponder- in deep space. ing how giant stars blow up, their Bethe probed astrophysics at its purest prodigious outbursts of neutrinos, levels right up until his death last year at and binary systems of neutron stars age 98 (Science, 8 April 2005, p. 219). At a and black holes. recent meeting* here, speakers fondly These topics followed logically recalled his influence in the region where from Bethe’s work on the atom nuclear physics and astrophysics fuse, from bomb, says astrophysicist Stan neutrinos to supernovae, and ordinary stars Woosley of the University of Cali- to neutron stars. They also laid out key mys- fornia, Santa Cruz. “Stars are grav- teries that still tantalize scientists: How do itationally confined thermonuclear giant stars explode and forge the elements reactors,” and their demise is bomb around us? What happens when neutron physics on the grandest scale, stars or black holes crash? And what is the Woosley says. “And Hans was nature of the dark matter and dark energy really interested in the birth of the that suffuse space? elements, especially uranium,” he But as Bethe’s scientific descendents adds with a smile. marked what would have been his 100th birth- Bethe wanted to find the day on 2 July, they worry about their ability to essence of why a dying star’s core address such questions anytime soon. Cuts in implodes. His key contribution, the science program at NASA have cast a pall says Woosley, was to consider the over missions designed to turn the cosmos star’s entropy. As a star runs out of into a high-precision physics laboratory. The fuel and fuses heavier elements up damage to Bethe’s legacy could be serious, to iron, Bethe found, the outer lay- they warn. ers grow disordered while entropy “In the worst-case scenario, the young declines at the blazing core. “Hans people we need may feel hopelessness,” says liked to say [the core] had the Saul Teukolsky, chair of the physics depart- A rare spark. Hans Bethe, shown at age 90, calculated how stars entropy of an ice cube, even though ment at Cornell University—Bethe’s aca- burn—including the “carbon cycle” in background. it was 10 million times hotter than demic home for 70 years. “They may not enter hell,” Woosley recalls. Bethe calcu- the field at all.” proton-proton reaction that propels hydrogen lated that when the core collapses, it has too lit- fusion in the cores of modest stars like our tle entropy for iron nuclei to break up. Instead, Bomb physics, near and far sun. In the late 1930s, he was the first to they compress into the extraordinary densities Astrophysics was the alpha and omega of describe a separate fusion cycle involving of neutron stars. Bethe’s long career. Although his fame stems carbon, nitrogen, and oxygen atoms, which That collapse ignites an outward shock from leading the theoretical division at Los powers massive stars during their short lives. wave, which the great mass of the star quickly Alamos, New Mexico, during the Manhattan In 1967, Bethe received the Nobel Prize in snuffs out. Bethe believed neutrinos emitted Project, and his tireless advocacy of arms physics for that work. by the newborn neutron star would relaunch control once World War II was over, In the decades after World War II, Bethe’s the shock wave and drive the supernova blast, astronomers and physicists revere him as “the research focused on the theory of nuclear a scenario he published in his 80s with astro- guy who figured out how the sun works,” says matter and atomic physics. A highlight was a physicist James Wilson of Lawrence Liver- astrophysicist Michael Turner of the Univer- calculation of a subtle shift in the energy levels more National Laboratory in California. The sity of Chicago, Illinois. “You don’t need a of electrons in excited hydrogen atoms. That verdict is still out; the best computer models better legacy than that.” three-page paper, written on the train between have yet to blow up a simulated star in a Born in Strasbourg, Germany, and edu- New York City and Ithaca, set the stage for convincing way. cated at universities in Frankfurt and Munich, modern quantum electrodynamics. Solar neutrinos also captivated Bethe. the young Bethe spelled out the details of the Later in life, however, two catalysts drew With the late John Bahcall of the Institute for Bethe tirelessly back into astrophysics. The Advanced Study in Princeton, New Jersey, * Bethe Centennial Symposium on Astrophysics, Cornell 1967 discovery of pulsars—flashing deep- Bethe helped explain why underground detec-

CREDIT: MICHAEL OKONIEWSKI CREDIT: University, 2–3 June. space beacons that Bethe’s Cornell colleague tors on Earth observed only a fraction of the

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of Technology (Caltech) in Some researchers at the meeting pinned Pasadena, who motivated Bethe these sacrifices on NASA’s decision, after much to probe the issue. political intervention, to fund a repair mission for the Hubble Space Telescope and to push An endangered generation ahead with its planned successor, the costly The Laser Interferometer Space James Webb Space Telescope (JWST). “We Antenna, long under develop- could just see [Hubble] would be a $2 billion ment with the European Space drag on the program,” says Turner, who recently Agency as a sensitive partner to completed a 3-year stint as head of mathematical LIGO, is one of three major and physical sciences at the National Science space science missions planned Foundation. Given all of NASA’s other priorities, by the astrophysics community Turner says, moving forward with the repair in the next decade to peer without reconsidering its value within the entire further into Bethe’s realm. The suite of missions “was stupidity on stilts.” other projects are the four tele- But other scientists believe that prolonging scopes of Constellation-X, a Hubble’s life didn’t automatically take money high-resolution successor to the away from other programs because space Chandra X-ray Observatory; science isn’t a zero-sum game. Even with no and NASA’s share of the Joint servicing mission, “it’s not clear that money Dark Energy Mission, an effort would be made available for Explorers, or with the U.S. Department of Constellation-X, or anything,” says astronomer Energy to chart the weird Robert Kirshner of the Harvard-Smithsonian Ablaze with energy. Neutrinos from our sun drew Bethe’s focus late speeding-up of the universe’s Center for Astrophysics in Cambridge, Mass- in his career. growth and determine its cause. achusetts. He also notes that Bahcall led a But funding prospects are dim National Academies’ committee that in 2003 neutrinos predicted to stream from the sun’s (Science, 17 March, p. 1540). A tight NASA came out strongly in favor of at least one more core. Confirmation came from Canada’s Sud- budget, combined with massive cost overruns NASA repair flight to the telescope, after bury Neutrino Observatory in 2001: The parti- and a huge backlog of proposed projects, has extensive community input. cles have minuscule masses and oscillate left them competing for what could be only More vexing for the long-term health of among different “flavors.” That behavior, as one new NASA start for a major astrophysics astrophysics is the ballooning cost of JWST, Bethe and Bahcall foresaw a decade earlier, mission in the next 3 years. now pegged at $4.5 billion. “I worry that we’ve arises from unknown physics beyond today’s The pain spreads to NASA’s low-cost gotten ourselves into the SSC [Superconducting standard theory. Explorers, which many view as the field’s Super Collider] mentality, that we need $5 bil- Neutrinos are so elusive that physicists still soul. Often led by univer- lion to do what’s next, and everything else can have no direct evidence of Bethe’s carbon- sities, these missions go to hell,” says astrophysicist David Helfand nitrogen-oxygen cycle. That fusion draw students and yield of Columbia University. “We may suffer the should happen in our sun, albeit more outsized sci- same fate our particle-physics colleagues did sedately than in massive stars. “He 15 years ago,” he adds, referring to Congress’s would want us to verify that,” says decision in 1993 to cancel the partially physicist Wick Haxton of the Uni- built accelerator in Texas. Astro- versity of Washington, Seattle. physicist Shri Kulkarni of Cal- Doing so, however, will require sensitive new experiments— Where next? In the post-Bethe era, such as a proposal to place a astrophysicists face tough choices— 130-ton vat of liquid neon at and a hard act to follow. Sudbury to spot low- energy neutrinos. tech paints the situa- Bethe also did not tion bluntly: “Is a sin- live to see a test of a gle mission worth the claim that he, Gerald rest of astronomy?” Brown, and Chang- No one at the meet- Hwan Lee of Pusan National ing had a good answer, and University in Busan, Korea, made— there was no consensus on how the after Bethe turned 90—that binary systems community might gain the necessary political containing two black holes should be 20 times entific results, says astrophysicist Roger support for its priorities. Indeed, the room as abundant as systems with one black hole Blandford, director of the Kavli Institute for seemed infused with a wistfulness that Bethe and one neutron star. Their prevalence would Particle Astrophysics and Cosmology at Stan- couldn’t be there to rally his colleagues in their be good news for the Laser Interferometer ford University in Palo Alto, California. “You time of need. “The scope of problems he could Gravitational-Wave Observatory (LIGO), optimize the use of finite resources most effi- solve pretty much had no limit,” Brown wrote which seeks the space-rippling disturbances ciently by putting them into Explorer pro- last year in Physics Today, recalling his struggle caused by the mergers of such binaries. Two grams,” Blandford says. “Instead, NASA has to keep up with a friend 20 years his senior. black holes should make a more violent been starving them to death,” including cancel- “In that sense, I think [Bethe] was the most “splash” in the gravitational pond of space, says lation in February of the on-budget NuSTAR powerful scientist of the 20th century.”

theorist Kip Thorne of the California Institute mission to image high-energy x-rays. –ROBERT IRION LEWIS N. PAT CREDITS (TOP TO BOTTOM): NASA/ESA; ILLUSTRATION:

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