- The Guardian, Wednesday 4 February 2009
The renowned Austrian-American astrophysicist Edwin E Salpeter, who has died aged 83, spent essentially his entire career at Cornell University, in Ithaca, New York, beginning as a postdoctoral researcher in 1949 and retiring as JG White professor emeritus of physical sciences in 1997. A self-described generalist, by moving effortlessly between fields he demonstrated how ideas and techniques from one area of physics can be usefully applied to other areas.
Born in Vienna, Salpeter fled with his parents to Australia as the Nazis rose to power, earned his bachelor's and master's degrees at Sydney University, then, with an overseas scholarship, a doctorate from Birmingham University, before joining Cornell. His early career as a nuclear theorist was capped in a 1951 paper with his fellow Cornell professor and Nobel laureate Hans Bethe, which derived what has become known as the Bethe-Salpeter equation.
Up to that point, there had been no quantum mechanical treatment of the formation of bound states of elementary particles, such as electrons and their antiparticles, positrons, that incorporated the effects of special relativity in a consistent fashion. The Bethe-Salpeter equation provides a consistent, relativistic formulation of the problem and, although it cannot be solved exactly, it can be used to determine many properties of such bound states and, in various limits, good approximate solutions can be obtained that are important in applications ranging from nuclear physics to condensed-matter physics.
Shortly thereafter, Salpeter followed the precedent of his mentor, Bethe, and began to think about how to apply nuclear physics ideas to an understanding of the stars. In 1938, Bethe had uncovered the processes that make the sun shine, by the nuclear fusion of four protons ultimately to form helium. However, it remained a mystery how stars might synthesise elements heavier than helium as they exhausted their hydrogen fuel.
Salpeter discovered a remarkable mechanism involving the fusion of the nuclei of three different helium atoms into the nucleus of carbon, through the initial formation of a metastable nuclear state in the collision of two helium nuclei, which subsequently captures another helium nucleus before it decays. This process, known as the Salpeter process, laid the foundation for all subsequent studies of stellar nucleosynthesis of essential elements.
As his career moved further into astrophysics, Salpeter's research areas continued to broaden. In almost every area he explored, his contributions were seminal. He proposed an "initial mass function" that could describe the distribution of stars that first formed as hydrogen, collapsed to form a protogalaxy and then fragmented into smaller regions that collapsed to form stars of differing masses. An understanding of the distribution of this first generation of stars is essential for understanding not only the subsequent evolution of the galaxy, but also the expected distribution of elements produced during stellar evolution and released into the galaxy as some stars explode at the end of their lives.
In 1964, Salpeter, along with the Russian physicist Yakov Zeldovich, made a bold prediction: black holes could be observable - not directly, of course, but Salpeter and Zeldovich argued that the gas that streams towards a black hole would be sufficiently heated before disappearing inside the hole to emit energetic X-rays which could then be detected. Three decades later, the Hubble telescope was able to verify their predictions.
Throughout his career, Salpeter provided a remarkable role model. One of the first to treat stars as nuclear physics laboratories, he spurred a tradition that continues today. The universe continues to provide a laboratory for testing ideas at the forefront of physics.
While the areas he pioneered, including stellar nucleosynthesis and X-ray astronomy, subsequently resulted in the awarding of numerous Nobel prizes to others, Salpeter himself was awarded the academy's alternate prize, the $500,000 Crafoord prize, in 1997, which he shared with Sir Fred Hoyle for their work on nuclear reactions in stars.
A humble, unpretentious man with a good sense of fun, he encouraged interactions with scientists of all ages. Any visitor to Cornell would find him eager to meet and talk, and inevitably would come away from the discussion delighted and enlightened.
His first wife, Miriam, a neuro-biologist, died in 2000 and he is survived by his second wife, Lhamo, two daughters and four grandchildren.
• Edwin Ernest Salpeter, astrophysicist, born 3 December 1924; died 25 November 2008