Quark Stars
By Gemma Tierney
RX J185635-375, one of the two possible quark stars observed in 2002 (photo by Chandra X-Ray Observatory)
The possibility of the existence of quark stars has been proposed by astronomical theorists for many years but only recently was substantial evidence of their existence found. Quark stars are extremely dense, small stars made up of what is often called strange matter but is essentially a certain type of individual quarks. They form when massive neutron stars collapse into themselves. Neutron stars, in turn, form when massive stars collapse into themselves. All of this collapsing while not losing mass leads to the ludicrous density of quark stars.
Two stars that were originally classified as neutron stars were observed in April of 2002 as being colder and smaller than any other neutron stars. Previous to this point, neutron stars were thought to be the densest matter (unless you want to count black holes) and the smallest stars in the universe. But because of their small size and lower temperature, quark stars would be even denser.
A comparison of neutron stars and quark stars (illustration from Harvard)
In theory, quark stars form from neutron stars. Neutron stars are created when massive stars undergo a supernova (most stars turn into white dwarfs instead of neutron stars) to collapse into a super dense material made up entirely of neutrons. Because of neutron stars' extreme pressure, they are very unstable. This instability may lead the more massive neutron stars to collapse further into themselves to form quark stars. Quarks are basically sub-sub-atomic particles and are the smallest particle known to man. Protons and neutrons are made up of three quarks. Neutrons and protons are just two examples of a hadron, or a larger grouping of quarks. Quarks can also briefly occur in pairs, a grouping that is known as a pions, which in turn become electrons, as well as neutrinos and photons. Quarks never occur alone naturally. The only way they can occur in this way is if atoms are smashed in particle accelerators. Even then, singular quarks only last for a brief period of time.
There are six different kinds, or flavors, of quarks, which are each paired up as follows: up and down, strange and charm, top and bottom. Different combinations of these flavors are what create the difference between neutrons, protons and other hadrons. The strange flavor is what makes up quark stars.
Quark stars are made up entirely of individual quarks, that came from the broken-down neutrons of the neutron stars. This process of breaking down hadrons into single quarks is called quark deconfinement. When neutrons break down into quarks, lots of energy is released, which would manifest as lots of light. This would explain why the brightest supernova ever, which was observed in 2006, was probably a supernova turning a neutron star into a quark star, or a quark nova. This transformation would occur a few weeks after the initial creation of the unstable neutron star. Because the energy released from this initial supernova has already blown all of the clouds surrounding the neutron star away, the quark nova will appear even brighter because it is not obscured by clouds.
An illustrated depiction of the brightest supernova to ever occur, observed on 16 September, 2006 (illustration from NASA)
While neutron stars are about 16 miles across, quark stars are only 12 miles across. Because our sun, which is an average-sized star, is 92 million miles across, the density of both a neutron star and a quark star is nearly impossible to imagine. A frequently-invoked comparison is that a teaspoon of the matter making up a quark star would weigh billions of tons. Quark stars fall between neutron stars (obviously) and black holes in terms of both their mass and their density. This means that, just like a neutron star can turn into a quark star, a quark star can turn into a black hole.
The existence of quark stars has not yet been definitively proven, although it would explain many things. There have been many stars observed that fit neither the definition for neutron star or for black hole and so the existence of a quark star would make a lot of sense. The candidates for quark stars are clearly significantly denser than neutron stars because of the difference in size and temperature. There have also been what are currently classified as neutron stars that have no radio emissions, unlike most neutron stars. These so-called radio-quiet stars are also likely candidates for quark stars. Also, the formation process for a quark star would explain abnormally bright supernovae that are sometimes seen. Yet, research has yielded no answer as of now. Further observations of any abnormal neutron stars, especially with accelerating technology, should eventually give science an answer. This answer would provide much insight into the mystery or the behavior of matter at super high densities and maybe also lead us towards understanding the phenomenon of black holes.
Works Cited:
http://www.reference.com/browse/quark+star
http://www.popsci.com/military-aviation-space/article/2002-06/wish-upon-quark-star
http://www.spacedaily.com/reports/Quark_Stars_Could_Produce_Biggest_Bang.html
http://news.bbc.co.uk/1/hi/sci/tech/1922574.stm
http://www.sciencedaily.com/releases/2008/06/080628224224.htm
http://www.space.com/scienceastronomy/080603-aas-neutron-quark.html