Review

Black Holes, White Dwarfs and Neutron Stars

Dr Matthew Bailes

On Tuesday 13th May the SA branch of the AIP held it's 1997 Bronze Bragg Medal and Certificate presentation, in conjunction with a public lecture. The presentation was made by Prof. Ian McCarthy of Flinders University. Unfortunately, Ms Joanne Chong was unable to be present to receive the Bronze Bragg medal, awarded for highest achievement in the year-12 Physics examination, but many others were present to receive certificates awarded for gaining the maximum grade in the same examination. Following this, they, their parents and friends, together with AIP members and members of the public, were treated to a superbly illustrated tour of time and space by Dr. Matthew Bailes of the University of Melbourne. Speaking to the title of "Black Holes, White Dwarfs and Neutron Stars", Dr Bailes first gave a brief but encompassing overview of the structure of the Universe, well illustrated with an integrated computer display of pictures, movies and cartoons. This overview ranged from a "big picture" display of many galaxies, down to the dimensions of our own solar system, introducing the audience along the way to galaxies, dust, globular clusters, dark matter and galactic structure. Dr Bailes made the point that "Big stars live fast and die young", emphasized by cartoons of "good" and "bad" stars. He described the eventual demise of our own solar system as the Sun becomes first a red giant and then a white dwarf. With pictures of planetary nebula produced by the outer layers of larger (than the Sun) stars being blown away, he then described how a star of greater than six solar masses can collapse to a neutron star, blowing away much of its outer layers in the process. Dr Bailes illustrated the Physics behind this process (of how a collapse leads to an explosion) by dropping two balls, a smaller one above a heavier one, with the result that the smaller one rebounded to the ceiling. Dr Bailes then described how neutron stars produce directional radio pulses, leading to us observing some of them as pulsars. He gave a short review of pulsar characteristics, including the statistical correlation that those with larger magnetic field strengths rotate more slowly. To explain this, he related a "tale of two stars", looking at a binary system which starts with an ordinary star and a giant. The larger star initially accretes material from the smaller one until it explodes, producing a neutron star. This has a large magnetic field strength and rapidly spins down until the pulsar emission stops. The two stars then orbit each other for about ten billion years, obeying Kepler's laws (illustrated by Dr. Bailes and a hapless volunteer from the audience stepping out the orbital motion on the floor of the theatre). Then the ordinary star expands as it reaches the red giant phase, with the result that it's outer layers accrete onto the neutron star, with the gravitational energy of the infalling material being converted to rotational energy of the neutron star, causing it to spin up again and become a "resurrected pulsar". The flux of gamma radiation released in this process may be sufficient to destroy the companion, hence it is called the "Black widow scenario". It leaves a pulsar with a period of about one millisecond and a much weaker magnetic field. In some cases the companion survives, so that the millisecond pulsar is still in an orbit. Finally Dr. Bailes described his own observations of pulsars and some of the motivation for the work. He described a low radio-frequency Parkes survey, which looked at 44000 different locations in the sky and found about 100 new pulsars, including 17 millisecond pulsars. These periods are extremely stable, rivaling the Earth's best clocks, and hence allow very precise measurements of the parameters of the orbits of those pulsars which are in binary systems. Observations of these parameters over time may allow the detection of gravitational waves as they distort the orbit.

Laurence Campbell