Imagine taking the whole Sun and compacting it until it is the same size as a city. Is that radical thinking? It might be, but nature keeps doing the very same experiment when it creates the so-called neutron stars, which are among the smallest and densest objects in the Universe. Astronomers know more or less how this happens, but there are few who admit that a great deal still needs to be done before science can explain what is seen out there. One of the mysteries to be cleared up is how neutron stars with a higher mass than that forecast by the theory of stellar formation and evolution appear. A group of researchers operating in Brazil is trying to throw some light on the subject by reviving a controversial hypothesis. In general terms, they are suggesting that there must be more than one way of creating neutron stars.
The fact they appear has to do with the death of stars that have a very high mass, at least eight times greater than the Sun’s mass. To understand what happens, it is first necessary to talk a little about what astronomers know about how stars live and die. Constituted from concentrated gas (mostly hydrogen) and dust, stars begin to shine when the concentration of material is such that the atoms in the most central region of these heavenly bodies begin to join together, a process known as nuclear fusion (see text here). The transformation of two hydrogen nuclei, each with one proton, into a helium nucleus, with two protons, is accompanied by a subtle reduction in the total mass. Part of the mass is converted into energy and escapes from the star; this is where all the power of these stars for bathing an entire planetary system in radiation comes from. This energy, which is generated inside the star, offsets the gravitational force, which acts in the opposite direction. Because of this equilibrium, the star remains approximately the same size throughout most of its life.
However, over millions of years the fuel available for nuclear fusion gradually runs out. Without hydrogen, heavier elements are used, like helium, carbon, oxygen until a limit is reached: iron. This is the final frontier for one simple reason: the fusion of iron nuclei consumes more energy than is released at the end of the process. At this stage, the production of energy in the central region is interrupted and gravity starts working unimpeded, without any force to offset it.
The star collapses and triggers a complicated sequence of events. The final result is the explosion of the outermost layers of the star, when 90% of its mass is launched into space. What remains after this violent episode, known as a supernova, is a very compact stellar core. If the mass of the core is relatively small, this compression gives rise to what is normally called a neutron star – if the mass is larger and the compression continues a black hole is formed, an object so dense that nothing escapes its attraction, not even light.
According to the currently accepted theory, neutron stars, so called because they have high proportions of uncharged particles (neutrons) inside them, should have all the same dimensions: a mass almost 40% greater than that of the Sun, compressed into a sphere less than 20 km in diameter.
“But no one knows exactly what mass a star needs to have during its life to die and leave a neutron star or a black hole,” says astronomer Jorge Horvath, from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) at the University of São Paulo, coordinator of a group that is investigating the characteristics of neutron stars.
“Until recently it was believed that all neutron stars had this pattern,” says João Steiner, another astronomer form the IAG. “But last year a case was discovered that is clearly bigger.”
The name of the object? PSR J1614-223, a neutron star located 3,000 light years away from Earth, discovered by a group from the National Radioastronomy Observatory (NRAO), in the United States. Presented in an article published in Nature, this star appears to have two solar masses – mammoth, in terms of objects of this type.
This finding obliged the astronomical community to accept the fact that there is a significant variation in the size of neutron stars, and this fits in very well with the forecasts recently made by Horvath’s group, which were published in the June issue of the journal Monthly Notices of the Royal Astronomical Society. In this work, Horvath, Eraldo Rangel and Rodolfo Valentim conducted a statistical analysis of the mass of 55 heavily-studied neutron stars and showed that there are 2 more common patterns: one formed by the stars with a smaller mass (around 1.37 times that of the Sun) and with little variation, as expected, and the other, with a bigger mass of around 1.73 times the solar mass, and more variable.
Why do these two different groups exist? “The results point to more than one mechanism for neutron star formation,” says Horvath.
This idea seems compatible with the distribution of neutron stars in places like globular clusters, inhabited principally by very old stars with a smaller mass than the mass that would be necessary to give rise to neutron stars, according to the theory. Recent observations made by astronomers from various countries have shown that in these regions there are many more neutron stars than would be expected if they were the exclusive product of the explosion of large mass stars.
The stars that originally have a mass that is less than eight times that of the Sun do not generate neutron stars when they collapse but another class of object: white dwarves, with the mass of a sun compressed a the volume such as that of the Earth – this is how the Sun is likely to end its days. In some binary systems, the white dwarf, through the action of gravity, robs mass from its companion star until it reaches a limit that induces a new collapse. This event is explosive and produces a specific type of supernova, called Ia, in which the total mass of the star is violently launched into space.
However, some astronomers suggest that this can happen in a different way. Instead of resulting in a supernova, the rapid growth in mass might mean that the white dwarf becomes a neutron star. “This is an idea that has been around for 20 years and there are those who hate it,” says Horvath. “But there are also those who say that it works. It’s difficult to imagine a better alternative to explain how certain neutron stars end up where they do.”
Recent data complicate the situation by indicating that there are neutron stars with a mass smaller than that of the Sun and that were not formed by collapse.
The definitive answer is yet to emerge, but it is almost certain that the future of research will undergo reformulations in the theories of how neutron stars arise and behave.
Inside and out
If there is a mystery about the size and mass, it is no simpler when the subject is the composition of neutron stars. The level of compacting of these objects is so high – the density of a neutron star is greater than that of the nucleus of atoms and 100 trillion times that of water – that the matter may appear in forms that are not found in any other place in the Universe.
At densities greater than that of the atomic nucleus, particles such as protons and neutrons break down into their fundamental units: quarks, which as a general rule are never seen alone. It is difficult to reconcile these forecasts with the observations, but it is believed that these conditions exist in certain neutron stars, which presumably harbor a soup of quarks at their core.
At the Federal University of ABC in Santo André, in the Greater São Paulo Metropolitan Region, Germán Lugones’ group has been doing calculations and simulations of how different internal compositions of these stars would affect its mass, radius, evolution and other properties. One of the results the team arrived at is that certain phenomena that arise when matter is found in the form of quarks – like the transition to a superconductor state – naturally explain the existence of stars with masses much greater than the classic 1.4 times the solar mass. That is why the discovery of PSR J1614-223 was an important sign that they may be on the right path. Lugones believes that a more radical version of the quark stars – the strange star or autolinked quark star, in which the entire star is composed of these particles – should be considered as a candidate if stars with a mass even bigger than that of PSR J1614-223 are observed.
“According to theoretical studies carried out over the last few years by our group, the density necessary for particles of matter to break down into quarks is 5 to10 times greater than the density of the inside of an atomic nucleus,” says Lugones, emphasizing that such density may quite easily be reached in the center of neutron stars with a bigger mass.
No one knows if this occurs. There are also gaps, both in the understanding of the physics behind these processes as well as in the understanding of the observable properties of neutron stars. Manuel Malheiro, a researcher at the Aerospace Technological Institute and a collaborator of Horvath’s and Lugones’, has been at the University of Rome since 2010, where he is investigating the composition and other characteristics of another special type of neutron star: magnetars, which have a high magnetic field.
Advances in the theory and observations will still be necessary to finally arrive at a more cohesive picture. The only certainty is that there are interesting problems with regard to these stars that, by accident, are ideal laboratories for studying the most extreme properties of matter.
1. Hadronic matter and QCD in astrophysics: supernovas, grbs and compact stars (nº 2007/03633-3); Type Thematic Project; Coordinator Jorge Horvath – IAG/USP; Investment R$ 54,250.00 (FAPESP).
2. Investigation of high energy and high density astrophysical phenomena (nº 2008/09136-4); Type Young Investigators Awards Program; Coordinator German Lugones – UFABC; Investment R$ 91,207.65 (FAPESP).
VALENTIM, R. et al. On the mass distribution of neutron stars. Monthly Notices of the Royal Astronomical Society. v. 414 (2), p. 1.427-31. June 2011.