Frank H Shu, made his most important contributions in the area of star formation. Since antiquity, human beings' intellectual curiosity has been drawn to the twinkling lights of countless stars. Our fascination about these heavenly bodies is not just limited to what they are and their role in the grand order of nature, but also extends to their origin and its implications for the very roots of our own existence. Although the present astronomical census reveals that the energy content of the Universe is mostly locked up in dark matter and dark energy, stars remain as the basic building blocks of galaxies, the predominant beacons of visible light from remote cosmic corners to our immediate neighborhood, and the sources of complex nuclei which are the essential ingredients of life and its supporting planetary habitats. In the forefront quests to investigate the emergence of structure on galactic scales and beyond as well as the origin of the solar system, a thorough understanding of the star formation process is essential.
The conventional concept is built on the hypothesis that stars form from dense clouds of interstellar gas which congregate, cool, and eventually collapse under their own weight. However, this idealized paradigm has been challenged in the past two decades to account for the observational discoveries of far more complex stellar nurseries which are shrouded with turbulent gas, threaded by chaotic magnetic fields, and stirred up by jets of gas emerging from newly forming stars. These observational breakthroughs are made possible by advances in high-precision and large-aperture ground-based instruments and the Hubble space telescope. The launch of space-borne infrared detectors was particularly important in providing astronomers with tools to pierce through the intervening clouds and Earth's atmosphere and to peek directly at star forming sites.
Frank Shu presented a universal framework for interpretation of these observations based on the omnipresence of disks around young stars. Moreover, he established a standard star-formation model. In this theory, gravity, magnetic fields and turbulence, together influence the flow of gas from the diffuse interstellar medium to dense cores of molecular clouds. As they collapse under the influence of gravity, clouds spin up and swirl into disks which channel gas to congregate at their centres. Eventually, protostellar objects emerge from the accumulated gas. Their magnetic fields become sufficiently strong to perturb the motion of nearby gas and to eject a fraction of it through collimated outflows. The outflowing gas imparts momentum to the surrounding clouds and quenches gas replenishment in the vicinity of the emerging stars.
Frank Shu's theory is also relevant to the origin of the solar system. He was the first to propose that protostellar disks reflect the light from the stars at their centre and are evaporated by it over a few million years. Most recently, he showed that some relics of smallest planet building blocks are repeatedly condensed and melted down in the proximity of the infant Sun, redistributed and moulded into primitive meteorites throughout the solar system. This theoretical prediction was recently vindicated by the fragments of Comet Wild 2 which were collected and brought back to Earth by a NASA sample-return star-dust mission.
In addition, Frank Shu laid the theoretical foundation to demonstrate, with C C Lin in 1964, that the spiral structures in galaxies and in Saturn's rings are density waves. His innovations also led to the insightful understanding of and a rigorous model for mass transfer in binary-star systems.
Astronomy Selection Committee
The Shaw Prize
16 June 2009, Hong Kong