Are Neutron Stars Strange?

The state of matter in the interior of neutron stars, the ultra-dense remnants of collapsed stars, is still one of the biggest unsolved riddles of astrophysics. Researchers at the Max Planck Institute for Astrophysics together with colleagues from the Universities in Frankfurt, Heidelberg, and Jena perform computer simulations of the violent collisions of such stars to determine signals that could help to unravel this mystery.

Fig. 1: Four snapshots of the collision of two initially cold neutron stars in a binary. The computer simulated evolution covers only 0.02 seconds, in which the two stars approach each other quickly due to gravitational-wave emission (top left), collide (top right), merge (bottom left), and form a dense, superheavy neutron star surrounded by an extended, more dilute halo of hot gas (bottom right). The colors represent the gas temperatures with blue encoding "cool" gas up to about 15 billion degrees, and green temperatures of 30-40 billion degrees. The dynamics of the merger is nicely visible in the accompanying movies, which differ in size and employed visualization technique:
linkPfeil.gifMOVIE (1 MB) linkPfeil.gifMOVIE (2 MB) linkPfeil.gifMOVIE (4 MB)

Fig. 2: Similar to the neutron stars in Fig.1, two stars of strange quark matter approach each other quickly during their final orbits and merge to a rapidly rotating superheavy remnant, which after a short period of violent vibrations will collapse to a black hole. Different from the neutron star case, two thin, long spiral arms form, whose structure is determined by the special properties of the strange quark matter. Some strange matter is shed off the tips of these tidal tails and is injected as stranglets into interstellar space. It can eventually reach the earth and be captured in experiments on the International Space Station. The accompanying movies again visualize the dynamical evolution of the merger. They are available in different sizes and employed visualization techniques:
linkPfeil.gifMOVIE (2 MB) linkPfeil.gifMOVIE (4 MB) linkPfeil.gifMOVIE (8 MB)

The atomic nuclei of all known matter are composed of positively charged protons and electrically neutral neutrons. Both are built up by more fundamental constituents, the quarks. In neutrons and protons only the two lightest of the six identified kinds of quarks are present, named "up" and "down". In order to produce the other quarks in terrestrial experiments, physicists use big accelerator machines like the Large Hadron Collider (LHC) of the European particle physics laboratory CERN near Geneva, which has become operational only recently. In the LHC, for example, protons are accelerated to nearly the speed of light and then brought to collision. For tiny moments such particle crashes produce conditions as they occurred in the early universe just fractions of a second after the big bang.

It cannot be excluded that also in the present universe quarks different from the up and down types exist, in particular the strange quarks, which are more massive than their lightest relatives and usually decay quickly to the energetically preferred nucleonic quarks. However, because of interactions between the quarks, objects could contain not only up and down quarks but also strange quarks. A lower energy state would make such objects more stable than atomic nuclei. They could exist as tiny clumps of a few hundred nucleons, called strangelets, and could propagate as cosmic rays through the interstellar space. Strange quarks could also be present in the interior of neutron stars, the densest known objects, which are the extremely compact remnants of the collapse of massive stars. Such stellar relics would then actually not be neutron stars but strange stars. These exotic objects would have 1.5 times the solar mass and a radius of 10 kilometers. With a similar mass as neutron stars, they would be more compact than the latter, which could be a characteristic property for their identification. Therefore, in contrast to neutron stars, their matter would not only be bound by gravitational forces but by the strong interaction of quarks, which would also cause their surface to be sharper than that of neutron stars.

Astronomers conduct intense searches for these strange stars. Their discovery would be a sensation. It would mean that the stable, exotic ground state of matter composed of strange quarks would really exist. Unfortunately, identifying strange stars is extremely difficult: so far the masses of only a few neutron stars in binary systems have been measured accurately, but their radii are either unknown or can be estimated only with large uncertainties. The known data do not yield a good constraint of the compactness of the stars.

A team of astrophysicists of the Max Planck Institute for Astrophysics and the University of Jena together with nuclear physicists of the Universities of Frankfurt and Heidelberg has now suggested an alternative way to uncover the existence of strange stars. To this end the team performed computer simulations to determine the observable signals that are produced by collisions of two strange matter stars in a binary system. Binary systems of stars do not exist forever. As moving masses the system radiates gravitational waves, perturbations of spacetime, which are predicted by Einstein's theory of General Relativity and propagate like waves through the fabric of space and time. Radiating gravitational waves, the circling stars lose energy and thus gradually approach each other. They orbit around each other faster and faster, thus radiating more and more gravitational waves, until they collide in a final catastropy (Figs.1 and 2). When this happens, a powerful outburst of gravitational waves is produced, which afterwards subsides when the violent vibrations of the merger remnant gradually wane and the superheavy compact object eventually collapses to a black hole. The computer models of the team show that a variety of properties of the gravitational wave signal of such a merger event are suitable to discriminate whether the colliding binaries are neutron stars or strange stars.

Estimates say that these cosmic collisions of two stars composed of neutron matter or strange matter might happen at most once every 10,000 years in a galaxy like the Milky Way. This is far to rare to wait for such an event in our cosmic vicinity. However, huge gravitational-wave antennas like the linkPfeilExtern.gifGEO600 instrument near Hannover, which is operated by the Max Planck Institute for Gravitational Physics, or the linkPfeilExtern.gifLIGO facility in the USA will be upgraded in a few years to a sensitivity that will allow them to capture the weak signals even from the linkPfeilExtern.gifVirgo galaxy cluster, a concentration of several thousand galaxies at a distance of 65 million light years.

Even then the measurement of such gravitational-wave signals will require quite a bit of luck. It is therefore good that the team of researchers has pointed out yet another interesting possibility. The cosmic star crashes also lead to mass ejection into the stellar environment. If the colliding strange stars are not too compact, several earth masses can be added as tiny nuggets of strange matter to the cosmic particle flux pervading the interstellar space (Fig.2). With the linkPfeilExtern.gif"Alpha Magnetic Spectrometer" (AMS-02) experiment, which will be installed on the International Space Station (ISS) next year, strangelets reaching the Earth within this flow of cosmic rays will be searched for. It would be a real sensation if AMS-02 could find some hints of their existence. But even if this does not happen, the models of the astro- and nuclear physicists will allow them to extract from the measurements important constraints on the possible properties of strange stars. In any case, the researchers await with great excitement the moment when the new experiment is sent on its way to the ISS.

A. Bauswein und H.-Thomas Janka


A. Bauswein, H.-T. Janka, R. Oechslin, G. Pagliara, I. Sagert, J. Schaffner-Bielich, M.M. Hohle and R Neuhäuser, Physical Review Letters, 103, 011101 (2009)

A. Bauswein, R. Oechslin and H.-T. Janka, Physical Review D, 2009, submitted linkPfeilExtern.gifarXiv:0910.5169


The computations were performed at the Rechenzentrum Garching (RZG) and the Leibniz-Rechenzentrum Garching (LRZ). We thank Markus Rampp (RZG) for visualization of our simulation results. The use of the Splotch visualization package by K. Dolag, M. Reinecke, C. Gheller and S. Imboden is acknowledged.