What is a Quark Star? A quark star is a hypothetical neutron star with a core of superdense strange quark matter. The basic principle is that the matter within a quark star is more densely packed, even than nuclear matter. This ‘squashed’ area is thought to be composed of free quarks, or crystallized [material arranged into a lattice] subnuclear particles, rather than neutrons. To understand a quark star, we must consider a neutron star. The most compact solid objects in the universe known to exist are neutron stars; 16 miles across and about 1.5 times as massive as our Sun. The density is perhaps a thousand million million tons per cubic metre,1000 million million times that of water.(Moore 0, p.269) (Spacedaily.com 2008) A neutron star is the result of an area of space, with extremely high mass, and small volume. They are usually formed by the gravitational collapse of a giant star at the end of a supernova. This collapse is caused when the star can no longer sustain tqhe processes that counteract the crushing effects of gravity on the mass of the star. (Odenwald n.d.) The theory is that within this collapsing star, gravitational pressure is high enough to overcome the nuclear bonds that repel normal matter, creating neutron degenerate matter; where electrons fuse with protons, creating a substance consisting of nothing but neutrons. This is a ‘typical’ neutron star. A hypothetical upper limit to this mass is the TolmanOppenheimer-Volkoff limit, and mass more concentrated than this is thought to degenerate into quark matter, or collapse into a black hole. Many scientists believe that there would be no neutron to quark phase change, and that such objects would collapse immediately into black holes(Seggewiss & de Boer 2009, p.233) Anyone who has seen ice melt has seen matter change phases, and when electrons, atoms and other specs of matter change quantum phases, they behave just as differently as do ice and water in a glass. “ - (Hulet 2006) Quark stars are the product of the ‘strange matter’ hypothesis. The protons and neutrons of normal matter in the everyday world are made of two types of quark –‘up’ and ‘down’. This baryonic
matter is the most predominant form of matter we experience, and it is matter than includes atoms of any sort. A quark star can be seen as a single gigantic hadron (a proton or neutron), but consisting of many quadrillions of quarks rather than the typical three, which comprise a ‘normal’ hadron. (Annisimov n.d.) Strange matter, however, is composed of up, down, and strange quarks. Its properties are thought to be different from normal matter. Strange matter is expected to be colour superconducting. This means that it’s constituent quarks form ‘cooper pairs’. These pairs only interact with quarks from their flavour (type); so ‘up’ pair with ‘up’ and so on(Mark Alford et al. 1998). Like the matter in a neutron star this is a degenerative state, and can only be overcome by additional gravitational pressure. This pressure causes a collapse into the next (and hypothesized final) state of matter, a black hole. The problem with strange matter is that it has not yet been accepted hypothetically, let alone physically observed. "Strange matter may exist or it may not…It's not proven theoretically - it's an open issue." (Shiga 2007) How would a Quark Star form? Certain neutron stars more massive than the TolmanOppenheimer-Volkoff limit break down their neutron degenerate matter into its constituent parts; up quarks, and down quarks. Some of these quarks possibly become strange quarks (a different flavour), and create strange matter. “Calculations suggest that at sufficiently high temperatures or pressures.. the baryons and mesons either smash into each other so hard, or get so severely squashed, that they burst open. The result should be a soup of free quarks and gluons..(Baez 1998) This strange matter behaves differently to neutron degenerate matter; In a neutron star, it is the degeneracy pressure between neutrons that keeps the body from collapsing into a black hole - in a quark star, it is the pressure between quarks.(Annisimov n.d.)
How would we detect a quark star? Quark stars would be hard to distinguish from neutron stars, in part because the strange quark matter could be enveloped in a shell of normal matter(Minkel 2002) But, there are a number of ways that we could detect quark star candidates. Over-dense Neutron Stars Quark stars that appear to be too ‘small’ in size for their mass imply that some of their matter is packed tighter than nuclear matter, and although we have no direct observational evidence of a quark star yet, a number of overly dense neutron stars have been put forward as candidates. RX J1856.5-3754(Drake et al. 2002), a neutron star located in Corona Australis, and 3C 58, a pulsar located in Cassiopeia, initially appeared too heavy and hot. However later reexaminations challenged this hypothesis(Slane et al. 2004). Quark Novae / Super Luminous Supernovae Another theory suggests that the conversion from a neutron star to a quark star creates a release of energy similar to the release in a supernova event that formed the neutron star. The transformation.. blasts the neutron star's outer layers into space at close to light speed. The layers then slam into debris from the original supernova, creating an intense glow bright enough to explain the observations of SN 2006gy” - Rachid Ouyed (Shiga 2007)” This could be an explanation for ‘super luminous supernovae’; SN 2006gy is a super luminous supernova remnant, two to three times brighter than the previous record holder for the brightest supernova, it is now considered a quark star candidate because of this. (Shiga 2007) These energetic explosions also could be responsible for the mysterious ‘Gamma Ray Burst’ phenomena(Bombaci 2006, p.208) Magentars: Magnetars are a class of neutron star with an extremely powerful magnetic field. The effect of this field is a star that generates
enormous amounts of radiation, such as x rays and gamma rays. As of July 2009, 19 such stars are confirmed to exist. (McGill University 2009) Another detection theory suggests that the monstrous magnetic activity of a magnetar could be caused by the properties of the strange quark matter contained within its core; “..Colour ferromagnetism in quark matter could be a viable mechanism for generating immensely powerful magnetic flux as observed in magnetars� (O'Neill n.d.) Conclusion: Currently, Quark stars are purely hypothetical in nature, and until evidence of strange quark matter is found, other explanations for magnetars, superluminous supernovae and dense neutron stars are likely.
Annisimov, M., What is a Quark Star? wiseGEEK. Available at: http://www.wisegeek.com/what-is-a-quark-star.htm [Accessed December 9, 2009]. Baez, J., 1998. week 117. This Week's Finds In Mathematical Physics. Available at: http://math.ucr.edu/home/baez/week117.html [Accessed December 8, 2009]. Bombaci, I., 2006. Quark deconfinement in neutron stars and gamma-ray bursts. Journal of Physics: Conference Series 50, (5th International Conference on Physics and Astrophysics of Quark Gluon Plasma). Drake, J. et al., 2002. Is RX J185635-375 a Quark Star? The Astrophysical Journal. Available at: http://arxiv.org/abs/astro-ph/0204159 [Accessed December 10, 2009]. Hulet, R., 2006. Unbalanced Superfluid Could Be Akin to Exotic Matter Found in Quark Star. Physorg.com. Available at: http://www.physorg.com/news11748.html [Accessed December 8, 2009]. Mark Alford, Krishna Rajagopal & Frank Wilczek, 1998. Color-Flavor Locking and Chiral Symmetry Breaking in High Density QCD. Available at: http://web.mit.edu/physics/facultyandstaff/faculty_documents/wilczek_color_flavor_locking.pdf. [Accessed December 11, 2009]. McGill University, 2009. McGill SGR/AXP Online Catalog. Mcgill Pulsar Group. Available at: http://www.physics.mcgill.ca/~pulsar/magnetar/main.html [Accessed December 8, 2009]. Minkel, J., 2002. Casting a Strange Glow. Physical Review Focus. Available at: http://focus.aps.org/story/v10/st13 [Accessed December 8, 2009].
Moore, P., 0. The Data Book of Astronomy 1st ed., Taylor & Francis. O'Neill, I., Could Quark Stars Explain Magnetars Strong Magnetic Field? Universe Today. Available at: http://www.universetoday.com/2009/01/07/could-quark-stars-explain-magnetars-strong-magnetic-field/ [Accessed December 8, 2009]. Odenwald, D.S., What are Quark Stars? Astrnomy Cafe. Available at: http://www.astronomycafe.net/qadir/ask/a11795.html [Accessed December 8, 2009]. Seggewiss, W. & de Boer, K., 2009. Stars and Stellar Evolution, EDP Sciences. Shiga, D., 2007. Was the brightest supernova the birth of a quark star? New Scientist. Available at: http://www.newscientist.com/article/dn12514 [Accessed December 8, 2009]. Slane, P. et al., 2004. New Constraints on the Structure and Evolution of the Pulsar Wind Nebula 3C 58. The Astrophysical Journal, (Issue 1). Available at: http://www.iop.org/EJ/abstract/0004-637X/616/1/403/ [Accessed December 10, 2009]. Spacedaily.com, 2008. Neutron Star Could Sign Off As A Quark Star In Final Explosive Conversion. Spacedaily.com. Available at: http://www.spacedaily.com/reports/Neutron_Star_Could_Sign_Off_As_A_Quark_Star_In_Final_Explosive_ Conversion_999.html [Accessed December 8, 2009].