ScienceWeek

ASTROPHYSICS: ON BINARY NEUTRON STARS

The following points are made by E.P. Van Den Heuvel (Nature 2003 426:504):

1) The emission of gravitational waves by accelerated masses --such as two compact stars orbiting each other -- is predicted by Einstein's theory of general relativity. In 1993 Taylor and Hulse earned the Nobel prize for their precise measurement of the rate of orbital decay of the binary pulsar PSR B1913+16 by the emission of gravitational waves(1). The measured orbital decay of this binary pulsar, and of two further double neutron stars with somewhat wider orbits, is exactly in accordance with the prediction of Einstein's theory and provides very strong --albeit indirect -- evidence for the existence of gravitational waves.

2) Gravitational waves represent one of the great challenges of present-day fundamental physics. No one has ever detected them directly, but the chances of doing so have recently improved. Burgay et al(2) have described their discovery of a remarkable system of two neutron stars that are orbiting each other, elliptically in only 2.4 hours. This orbital period is three times shorter than that of the Hulse-Taylor binary pulsar, hitherto the closest double neutron star known.

3) Peculiar as it may seem, double neutron star systems are of crucial importance for the detection of gravitational waves: at the end of their inward-spiralling motion, the two neutron stars collide and merge, and during the last minute of their lives, there is an enormous release of gravitational radiation that is likely to be detectable on Earth. Several instruments capable of picking up such gravitational waves have been built, including VIRGO in Italy(3), GEO600 in Germany(4), TAMA in Japan(5), and LIGO, the Laser Interferometer Gravitational Wave Observatory, based on sites in Washington and Louisiana, USA.

4) The Hulse-Taylor binary pulsar is expected to merge 320 million years from now; for the new system found by Burgay et al.(2), the merger is "only" 85 million years away. But there must also be systems, born long ago, that are merging today. Near the end of the spiral-in process, one minute before the stars merge, their orbit has shrunk to a size of only a few hundred kilometers, and the two neutron stars move around each other some 30 times each second, producing strong gravitational waves with that same frequency (30 hertz). In the last minute before the merger, the orbital frequency increases rapidly, from 30 to 1000 times per second, and the strength of the gravitational wave emission increases simultaneously. Converted into an audio signal, this is the characteristic "death chirp" of a double "neutron star. The chirp signal, as well as the final giant pulse of gravitational waves as the stars merge, is so strong that it should be detectable by gravitational wave antennas such as LIGO, out to a distance of about 60 million light years.

5) What makes the discovery of the new neutron star system so exciting is that it indicates that these detectable "death chirps" of double neutron stars occur much more frequently than the previous disappointing estimate of one event in 10-20 years. The most favorable new estimate for the rate of death chirps detectable with existing detectors on Earth is around one in every one to two years.

References (abridged):

1. Taylor, J. H. Rev. Mod. Phys. 66, 711-719 (1994)

2. Burgay, M. et al. Nature 426, 531-533 (2003)

3. http://www.virgo.infn.it

4. http://www.geo600.uni-hannover.de

5. http://tamago.mtk.nao.ac.jp

Nature http://www.nature.com/nature

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ON MASSIVE STARS, NEUTRON STARS, AND PULSARS.

The following points are made by J.F. Hawley and K.A. Holcomb (citation below):

1) The fate of lower-mass stars is to cease fusion after using their available supply of helium, then to eject their outer layers quietly, with the cores left to cool slowly as white dwarfs. Stars more massive than a few solar masses experience more phases at the ends of their lives, going through one nuclear fuel after another to battle the crush of gravity. After the star's helium is exhausted, the core contracts and heats again, and the outer layers expand. In very massive stars, carbon may first ignite; for sufficiently massive stars, increasingly heavy elements are subsequently burned, fusing all the way to iron. The star becomes a gigantic cosmic onion, consisting of concentric shells in which increasingly heavier elements are fused. The final fusion product is iron...

2) A star with an iron core must seek an equilibrium with gravity that does not require further expenditure of energy. Smaller stars could find their final equilibrium in electron degeneracy. However, for any object with a mass greater than 1.4 times the solar mass, the pressure from even electron degeneracy is not sufficient to support the star against its own weight. In 1930, Subramaynyan Chandrasekhar realized that in order to provide the incredible pressures required to maintain more massive stars, the electrons supplying that support would have to move at greater than the speed of light, which was known from the special theory of relativity to be an impossibility. Thus special relativity demands an upper mass limit, today called the "Chandrasekhar limit". If the dying star fails to eject enough of its matter to allow its collapsing core to drop below this limit, the electrons cannot supply the necessary pressure. But if electron degeneracy pressure falls short, the star does not just slowly contract. It collapses catastrophically, sending a shock wave into its outer layers and blowing them off in a single cataclysmic explosion called a "supernova".

3) A supernova which arises from the collapse of a massive star is designated by astronomers as Type II. (This name suggests that there must be another type of supernova, the Type I supernova. The Type I supernova originates from the explosion of a white dwarf in a binary system.).

4) Although a great deal of the star is blown out into interstellar space by a Type II supernova, some fraction is probably left behind in a core remnant. If the mass of the remnant still exceeds the Chandrasekhar limit, what can the star do? It cannot settle down as a white dwarf star; so what remains? As the star collapses to greater and greater compaction, the electrons are squeezed into the atomic nuclei themselves, where they are forced to merge into the protons, forming neutrons. The neutrons, which are much more massive than electrons, can themselves exert a degeneracy pressure known as "neutron degeneracy pressure". The entire star is compressed essentially to the density of an atomic nucleus, but composed only of neutrons. This massive neutron nucleus is known as a "neutron star".

5) A neutron star is astonishingly compact; if an object with the mass of the Sun were to collapse completely to a neutron star, its radius would be only about 10 km, roughly the size of a typical large city on Earth. The neutron star is a remarkable object. Its existence was predicted as early as 1934 by Fritz Zwicky and Walter Baade, although their suggestion was ignored for decades; a neutron star seemed too bizarre to consider. This attitude changed in 1967 when the first pulsar was detected. A pulsar emits highly regular, energetic bursts of electromagnetic radiation, generally as radio waves.

6) The pulses from the first pulsar were so regular that the discoverers, Jocelyn Bell and Anthony Hewish at Cambridge University, first thought they had received signals from another civilization! No familiar astronomical process was known at the time that could produce electromagnetic bursts of such sharpness and regularity, at such a rapid rate. Ordinary oscillations would be inadequate to explain the signal. As more and more pulsars were observed, however, the mystery slowly yielded. Thomas Gold first suggested that pulsars might be associated with the exotic neutron star. Subsequent observations have borne this idea out; no other mechanism is remotely plausible to explain the properties of pulsars.

Adapted from: J.F. Hawley and K.A. Holcomb: Foundations of Modern Cosmology. Oxford University Press 1998, p.132. More information at: http://www.amazon.com/exec/obidos/ASIN/0195104978/scienceweek

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