When stars of sufficiently large mass reach the end of their lifespans, they die in an awesome explosion termed a supernova. The explosion ejects much of the mass of the star, however some of the mass still remains. Depending on the original stellar mass, the star may become a neutron star, or even a black hole. With black holes representing the ultimate form of gravitational collapse, it was often assumed that the neutron star, with its core resembling that of a sub-atomic nucleus, was the next most extreme gravitational phenomenon[i]. However, new evidence points to a potentially more extreme state of matter density – the so-called ‘strange stars’. This report will examine the phenomenon of strange stars, and evidence for their existence.
Making neutron stars
Neutron stars result from Type II and Type Ib supernovae related to the death of massive stars[ii]. These progenitor stars have evolved iron cores, which form the basis of the neutron star. Outside of an atomic nucleus, neutrons decay into electrons and protons[iii], a routinely observed reaction. At high densities and energy levels, the reverse may occur, and electrons may combine with protons to become neutrons[iv]. It is this process, termed reverse beta-decay, which triggers the formation of a neutron star. Density levels on atomic scales are governed by what is known as the Pauli Exclusion Principle (PEP). This principal states that particles of the same energy cannot occupy the same space. So there are a limited number of electrons that can occupy a given space, and this sets a limit on atomic densities. Matter that has reached this limit (such as white dwarves) is said to have reached electron degeneracy[v]. When reverse beta-decay occurs, matter moves from an atomic state (protons and electrons) to a sub-atomic state (neutrons). The PEP still applies, but the force at play is not longer the electro-magnetic force, but the strong force. Because the strong force operates over much smaller distances, much greater densities are feasible. This allows neutrons stars to achieve much higher densities (4x1017 kg/m3)[vi]. This state is known as neutron degeneracy. A one solar mass neutron star my have a 10km radius. As well as being incredibly dense, another important property of neutron stars is their rapid rotation. All stars rotate, but because of the vastly reduced size, neutron stars spin very rapidly[vii] (with rotation periods ranging from several seconds down to sub-milliseconds). It is this rapid rotation that first allowed neutron stars to be detected, in the form of energy pulses with frequencies corresponding to their rotational periods (known as pulsars).
Neutron and quarks
Although protons, neutrons and electrons are the building blocks of matter, only electrons are believed to be truly fundamental particles. Protons and neutrons are made up of sub-particles called quarks (which are themselves believed to be elementary particles). Two of these quarks, the up and the down quark, are the building blocks of protons (two up one down) and neutrons (one up, two down). Quarks (and by extension, protons and neutrons) interact via the strong force, which is only effective at sub-atomic distance scales. Forces are mediated via carrier particles, and in the case of quarks, the carrier particles are called gluons. In short, the exchange of virtual mediation particles is what triggers the force between these particles. As the distance between quarks increases, the strength of the strong force actually increases (a reason that free quarks have never been detected). However, quarks are believed to undergo a phase transition (the MIT Bag Model being the leading theory in this field[viii]) at high densities and/or temperatures (akin to water freezing to ice). This transition is referred to as quark deconfinement, and results in a free quark gas, referred to as quark gluon plasma (QGP). As well as the up and down quark (the two lightest quarks), four other ‘flavours’ exist. The next lightest is the strange quark, which is not normally seen in ordinary matter, but has been produced in laboratories. Although the constituents of a neutron star (primarily neutrons) contain only up and down quarks, the energy levels that cause quark deconfinement may be in the range of 400 MeV. This exceeds the mass of the strange quark, and at high energy levels, weak interactions are predicted to cause up and down quarks to decay into strange quarks[ix]. As an extension of the PEP, strange quarks should exist in such a plasma, because whilst the up and down quarks have achieved their highest possible density, strange quarks with a different energy state can still exist[x].
Strange Stars
Strange stars are neutron stars in which quark deconfinement has been achieved (a process first proposed by Edward Witten in 1984[xi]), reducing the star (or parts of it – primarily the core) to a quark gluon plasma. For deconfinement to occur at low pressure, a temperature of over 2 x 1012 K is required[xii]. However, the large pressures of neutrons stars allow deconfinement to be achieved at lower temperatures. Several mechanisms for the formation of strange stars have been proposed. It is believed that strange stars begin as neutron stars, but acquire additional mass, increasing density and trigger deconfinement. Two possible sources of additional mass are:
- supernova ejecta falling back into the star several days after the explosion (this theory requires the neutron star to have slowed in its rotation in this period, to allow matter to accrete).
- a companion star providing accretion material
More esoteric proposals, such as groups of strange quarks (called ‘strangelets’) interacting with neutrons to break them down into quarks[xiii], have been put forward, but work in this area is still highly speculative. Two types of strange stars are postulated – crusted and bare. Crusted strange stars are believed to have a neutron or atomic crust (just as some neutron stars are believed to have an atomic crust). By comparison, bare strange stars contain quark matter on their surface. Because strange stars start out as neutrons stars, this transformation can take many degrees of completeness, indicating that many strange stars may be a hybrid of neutron and quark matter. Like neutron stars, the core of a strange star is predicted to be in the form of a superconducting liquid[xiv]. Superconductivity is a property whereby an electric current is able to perpetually continue its motion[xv]. The core would also exhibit superfluidity, allowing motion without friction. This property has implications for the rotational speed of strange stars (allowing faster rotational periods than neutron stars). Strange stars are believed to emit gamma radiation due to high magnetic field and electrons being released from the core (despite being called neutrons stars, the core will still contain some protons and neutrons – this is what prevents the magnetic field from dissipating[xvi]). The released electrons will create an electric field at the surface, and because of the high magnetic field strength (1012 Gauss[xvii]), this field will have sufficient energy to create electron-positron pairs, which because they are anti-particles, will annihilate, releasing gamma rays[xviii].
Evidence for strange stars
Because strange stars have many similar properties to neutrons stars, particularly those that may be hybrid (mix of neutron and quark matter) and/or contain a non-quark crust, differentiating them is a difficult process (further complicated by their small size in general). However, several differentiating characteristics of strange stars have been identified:
- higher rates of rotation due to higher bulk viscosity (and greater density allowing greater resistance to centrifugal forces)
- mass radius relationships being highly contrasted between strange stars (M ~ R3) and neutron stars(M ~ R-3)
- differences in polar surface conditions
- higher rates of x-rays emitted than a similarly sized neutron star
- emission of gamma rays
It is believed that the cooling rates will be different for strange stars, but possibly only in the first 30 years. If we take the scenario of a crusted strange star, its temperature may actually increase, due to the matter in the crust (possibly neutrons) moving into the quark core and being broken down into quarks, releasing thermal radiation[xix]. Generally, strange stars are thought to have an initially cooler temperature, due to greater neutrino emission during formation carrying away more energy. Akin to a black hole, with its event horizon, a neutron star has a Last Stable Orbit – the minimum distance from a neutron star that matter can maintain a stable orbit before being made unstable via the gravitational field[xx]. However, more rapidly rotating neutron stars inflate via centrifugal force, and may take in the LSO. Strange stars, due to their more compact state, and greater viscosity may still have a LSO, and detecting this in a rapidly rotating neutron star may point to a potential strange star. Usov[xxi] has predicted that perhaps one percent of currently identified neutron stars may be strange stars. 1E1740.7-2942 is one such candidate star, which is a strong x-ray source, rapid pulsing and has gamma ray emissions. SAX J1808.4-2658 is another high energy x-ray emitting star with rapid rotational period. Further, x-ray observations predict a diameter of between 9 and 17km, and a mass of around 2.27 solar masses[xxii]. This is much higher than standard neutron star dimensions, also indicating potential strange star status. The difficulty with strange star predictions is their small size, which makes accurately determining their radii difficult (as with most distance estimates in astronomy). Secondly, many of the phenomena predicted to be associated with strange stars can be described by equally or more plausible theories, particularly in areas such as cooling rates[xxiii].
Conclusion
Whilst the physics underlying the prediction and description of strange stars appears sound, identifying potential candidates is far more difficult. Their small size, relative rarity and significant similarity to their less dense cousins could make conclusive identification of a strange star a long way off. Nonetheless, they provide an example of the universe being able to offer insights into physics that cannot be achieved under earth based conditions. By filling the gap between neutron stars and black holes they may in future offer further insights into this most extreme of gravitational phenomenon, with possible contributions to many other areas of physics and astronomy.
Bibliography
Kaufmann, W., Freedman, R. “Universe”, Fifth edition, W.H. Freeman and Company Price, A. “Strange Mystery: Strange Stars”, JAAVSO, Volume 30, 2002 Xu, R. “Strange quark stars – A review”, Conference Paper - High Energy Processes and Phenomena in Astrophysics, 2003 Schaab, C., Hermann, B., Weber, F., Weigel, M. “Differences In The Cooling Behavior Of Strange Quark Matter Stars And Neutron Stars”, The Astrophysical Journal, 480 :L111–L114, 1997 Gourgoulhon, E. “Have strange quark stars been discovered?”, http://polywww.in2p3.fr/gdr/pche02/doc/gourgoulhon_2.pdf “Are pulsars and x-binaries strange stars?” - http://www.obspm.fr/actual/nouvelle/etrange.en.shtml
[i] Conditions in the early universe excepted [ii] Type Ia supernova are the result of white dwarfs exploding, whilst Type Ic are stars that have lost significant hydrogen and helium. Both those types lack sufficient mass to achieve neutron star densities. [iii] and anti-neutrinos… [iv] and neutrinos… [v] http://hyperphysics.phy-astr.gsu.edu/hbase/astro/whdwar.html [vi] Kaufmann et al, p569 [vii] A product of conservation of momentum [viii] Schaab, p1 [ix] Xu, p. 5 [x] Price, p113 [xi] Schaab et al, p1 [xii] http://physicsweb.org/article/world/13/10/9 [xiii] Price, p114 [xiv] Price, p115 [xv] Kaufmann et al, p578 [xvi] Kaufmann et al, p578 [xvii] Kaufmann et al, p579 [xviii] Price, p115 [xix] Price, p114 [xx] http://www.obspm.fr/actual/nouvelle/etrange.en.shtml [xxi] Usov, V., Astrophysics Journal Letters, 1997 [xxii] Price, p116 [xxiii] Gourgoulhon, p.31
