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Fred Hoyle and Martin Schwarzschild

1994 Balzan Prize for Astrophysics (Evolution of Stars)

For their pioneering contributions to the theory of stellar evolution, upon which the modern development of the field is founded.

Among Fred Hoyle’s (1915 – 2001) most important work is the demonstration that all of the elements from carbon and heavier elements can be produced by nuclear reactions in stars. He laid the foundation of this work in 1946, when it was still believed that all the elements might have been produced at the creation of the Universe. In 1954, he predicted an excited state of the carbon nucleus (subsequently confirmed). The existence of this excited state was necessary if the carbon produced from helium was not to be turned into oxygen immediately, leading to a Universe in which life would be impossible. His classical paper, with E.M. Burbidge, G.R, Burbidge and W.A. Fowler, in 1957, identified all the crucial nuclear reactions and suggested at what stages in stellar evolution they would occur. Later, with Fowler, he pioneered investigation of how the evolution of a star would lead to a supernova explosion, which is the main mechanism by which highly processed material is returned to the interstellar medium. Together with Fowler, he also demonstrated that the age of the Milky Way could be estimated by a comparison of the theoretical production of heavy radioactive elements with observation. Hoyle also recognised that all of the observed helium could not have been produced in stars and, with Tayler and with Wagoner and Fowler, he showed that production in the early Universe was the probable solution.

Hoyle’s other work on stellar evolution includes studies of the accretion of interstellar matter by stars. Although it is now recognised that accretion is unimportant in normal stars, the ideas from these papers still figure prominently today in studies of various objects in which accretion discs are present. Also, with Lyttleton, he suggested that in red cool giant stars there is a discontinuity of chemical composition between central core and outer envelope, which could explain the large radii of such stars.

Hoyle’s most important paper on stellar evolution is his joint paper with Martin Schwarzschild on the evolution of ordinary low mass stars to very luminous giants. This was a key paper in providing an understanding of the physical processes in stellar evolution.
With Hazelgrove, he also showed that the ages of stars clusters could be deduced from a study of red giant evolution, and he found an age of the Milky Way comparable with his result from radio-active elements. Finally with Fowler, he pioneered a study of the evolution of extremely massive stars where the energy derives from gravitation, which is a forerunner of current models of active galactic nuclei.
Hoyle’s work, which in total covers much more than the fields of nucleosynthesis and stellar evolution, has been characterised by a high degree of imagination and originality. With this, a deep knowledge of physics and strong mathematical ability has been allied. His work has had a major influence on the development of astrophysics in the second half of the twentieth century.

Martin Schwarzschild’s (1912 – 1997) major contributions to stellar structure and evolution have been concerned with the structure of giant stars and also with the properties of the outer layers of stars in which radial transport of matter is important. His theoretical work has been complemented by observational studies of stars and the Sun. Schwarzschild’s 1958 textbook, Structure and Evolution of the Stars, has been particularly influential.
Schwarzschild’ s early work on inhomogeneous models for the structure of giant stars with Li Hen and with Oke paralleled similar work by Hoyle and collaborators. A break-through came with a paper with Sandage which included the first explicit time-dependent evolution of a star with a hydrogen exhausted core from the “main sequence” of hydrogen burning equilibrium states to the sequence of giant stars. The culmination of this series is the important joint paper with Hoyle which explained the evolution along the whole of the giant sequence as a series of equilibrium stages, and provided important clues to the structure of “horizontal branch” blue stars in globular star clusters.

With Härm, Schwarzschild gave a first discussion of the later evolution of low mass stars; in particular, discovering thermal instabilities up to the stage where mass loss transfer them to the stage of a “planetary nebula” – a star surrounded by a glowing shell of gas.
Schwarzschild pioneered the use of balloon-borne telescopes for precise imaging of the Sun, planets and stellar systems. By lifting the telescope to more than 25.000 meters, above most of the Earth’s fluctuating atmosphere, he could obtain much better resolution of structures on the solar surface and thus give conditions for models of the Sun and other low mass stars. Pioneering work with Barbara Schwarzschild discovered differences in chemical composition between stars that move past our neighborhood in the Milky Way at high or low velocities, providing important clues for the nucleosynthesis studies of Hoyle.

Schwarzschild’s work on the evolution of stars required physical insight, mathematical expertise, imagination, and precision in detail. It included the use of the electronic computer that, by the mid- 1950’ s, had reached a stage of development, and could efficiently be used in astrophysical applications. Schwarzschild’s mastery of the computer as a research tool paid enormous dividends, both in the work just cited and in subsequent papers. Schwarzschild’s research covers many fields in astronomy and astrophysics. He has dedicated the last two decades to galactic dynamics, a field in which he has made contributions of comparable importance to those in stellar evolution.