A quark star or strange star is a hypothetical type of exotic star composed of quark matter, or strange matter. These are ultra-dense phases of degenerate matter theorized to form inside particularly massive neutron stars.
It is theorized that when the neutron-degenerate matter which makes up a neutron star is put under sufficient pressure due to the star's gravity, the individual neutrons break down into their constituent quarks – up quarks and down quarks. Some of these quarks may then become strange quarks and form strange matter. The star then becomes known as a "quark star" or "strange star", similar to a single gigantic hadron (but bound by gravity rather than the strong force). Quark matter/strange matter is one candidate for the theoretical dark matter that is a feature of several cosmological theories.
For a large star, death is a bit of a squeeze. Once its nuclear fuel is spent, its core collapses, sparking a dramatic supernova explosion that blasts away the outer layers. The body left is a cold, tightly packed sphere called a neutron star, which, if massive enough, makes the ultimate collapse to a black hole.
The huge pressures inside neutron stars mean that all electrons and protons have joined so only neutrons remain. Near the centre, according to theory, these neutrons sometimes decompose into a sea of quarks, or so-called strange quark matter. A recent theory implies that this matter could form a stable ground state of nuclear matter – suggesting the existence of standalone "quark stars".
Evidence for quark stars is in short supply, with only a handful of observed candidates. Yet new calculations by an international group of theorists paint a better picture of the nature of quark stars, and suggest that they might be easier to spot than previously thought. "The main conclusion of our work is that there is a clear signature for the possible detection of quark stars – and thus stable strange quark matter," says author Aleksi Vuorinen of the University of Bielefeld in Germany.
IF THE universe has weird extra-spatial dimensions in parallel to the 3D world we see around us, then billion-dollar particle accelerators may not be the only place to find them.
So say Gergely Gabor Barnaföldi and colleagues at the Research Institute for Particle and Nuclear Physics in Budapest, Hungary, who propose that extra dimensions may show their face in areas of extreme gravity around dense stars. The concept could also solve a 25-year-old puzzle about the origin of mysterious particles emanating from a distant star system.
Some string theories predict that there are many more dimensions than the four we experience: the 3D world plus time. From next year, particle physicists hope to spot these dimensions at the Large Hadron Collider near Geneva, Switzerland.
Instead, Barnaföldi's team looked to outer space for evidence of extra dimensions interacting with matter. They analysed the Cygnus X-3 binary system, in which a normal star orbits a second object, generally thought to be a neutron star.
Objects in Cygnus X-3 are under extreme gravity, which the researchers say would provide the necessary conditions for extra dimensions to affect matter. Moreover, it spews out ultra-high-energy particles as far as Earth, which the team say could have been tweaked by an extra dimension inside the system. Astronomers believe these high-energy particles, dubbed "cygnets", strike our atmosphere and decay into muons. Since 1981, underground detectors on Earth have recorded sporadic showers of muon particles coming from the direction of Cygnus X-3. The cygnets are a puzzle because no known particles could last the 37,000-light-year journey from Cygnus X-3 to Earth without decaying.
The muon (play /ˈmjuːɒn/; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with unitary negative electric charge (−1) and a spin of 1⁄2. Together with the electron, the tau, and the three neutrinos, it is classified as a lepton. As is the case with other leptons, the muon is not believed to have any sub-structure at all (i.e., is not thought to be composed of any simpler particles). The muon is an unstable subatomic particle with a mean lifetime of 2.2 µs. This comparatively long decay lifetime (the second longest known) is due to being mediated by the weak interaction. The only longer lifetime for an unstable subatomic particle is that for the free neutron, a baryon particle composed of quarks, which also decays via the weak force. All muons decay to three particles (an electron plus two neutrinos of different types), but the daughter particles are believed to originate newly in the decay.
Strange matter is a particular form of quark matter, usually thought of as a "liquid" of up, down, and strange quarks. It is to be contrasted with nuclear matter, which is a liquid of neutrons and protons (which themselves are built out of up and down quarks), and with non-strange quark matter, which is a quark liquid containing only up and down quarks. At high enough density, strange matter is expected to be color superconducting. Strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers (quark stars)
Think black holes are strange? Understandable, considering these powerhouses of the universe (many times heavier than our sun) are collapsed stars with gravity so strong that even light cannot escape their grasp.
But maybe they're not "strange" enough, some astrophysicists suggest. "Stellar" black holes, ones only a few times heavier than the sun, may actually be something even weirder called a quark star, or "strange" star.
In particle physics and astrophysics, the term is used in two ways, one broader and the other more specific 
1.The broader meaning is just quark matter that contains three flavors of quarks: up, down, and strange. In this definition, there is a critical pressure and an associated critical density, and when nuclear matter (made of protons and neutrons) is compressed beyond this density, the protons and neutrons dissociate into quarks, yielding quark matter (probably strange matter).
2.The narrower meaning is quark matter that is more stable than nuclear matter. The idea that this could happen is the "strange matter hypothesis" of Bodmer  and Witten. In this definition, the critical pressure is zero: the true ground state of matter is always quark matter. The nuclei that we see in the matter around us, which are droplets of nuclear matter, are actually metastable, and given enough time (or the right external stimulus) would decay into droplets of strange matter, i.e. strangelets.
Located 37, 000 light years away in the constellation Cygnus, which straddles the galactic plane, is a powerful x-ray source named Cygnus X-3. Although it is only the third brightest x-ray source in the constellation after the famous Cygnus X-1, it is much further away on the far side of the galaxy and is obscured by intervening interstellar gas and dust near the galactic plane. When this is factored in, it appears to be one of the two or three most luminous objects in the galaxy in intrinsic brightness. It has received attention because it is one of the few sources of ultra-high energy cosmic rays with energies in the 100 - 1000 TeV range. But its most unique aspect is the production of anomalous cosmic ray events in a proton decay detector deep in Minnesota's Soudran iron mine. These events have defied analysis and have led to questions about whether Cygnus X-3 is a standard neutron star or perhaps something more exotic, like a star made of quarks. Cygnus X-3 is a compact object in a binary system which is pulling in a stream of gas from an ordinary star companion.
Cygnus X-3 has distinguished itself by its intense X-ray emissions and by ultrahigh energy cosmic rays. It also made astronomical headlines by a radio frequency outburst in September 1972 which increased its radio frequency emissions a thousandfold. Since then it has had periodic radio outbursts with a regular period of 367 days. These flares are of unknown origin, but they are exceedingly violent events. Naval Research Laboratory observations in October 1982 using the Very Large Array detected the shock wave from a flare; it was expanding at roughly one-third the speed of light.
Cygnus X-3 has an orbital period about its companion of only 4.79 hours. Intriquing underground events in the Soudron iron mines in October 1985 included 60 anomalous muon events in a 3¡ cone around Cygnus X-3 with a precise period of 4.79 hours, so they clearly came from that source. But that requires a neutral particle traveling at almost precisely the speed of light, and there are no reasonable candidates for such a particle.