A quark star is a hypothetical type of
compact exotic star composed of quark
matter. These are ultra-dense phases of
degenerate matter theorized to form
inside particularly massive neutron
stars.
The existence of quark stars has not
been confirmed theoretically or
astronomically. The equation of state of
quark matter is uncertain, as is the
transition point between
neutron-degenerate matter and quark
matter. Theoretical uncertainties have
precluded making predictions from first
principles. Experimentally, the
behaviour of quark matter is currently
being actively studied with particle
colliders, although this can only
produce hot quark-gluon plasma blobs the
size of an atomic nucleus, which decay
immediately after formation. There are
no known artificial methods to produce
or store cold quark matter as it would
be found in quark stars.
Creation 
It is theorized that when the
neutron-degenerate matter, which makes
up neutron stars, is put under
sufficient pressure from the star's own
gravity or the initial supernova
creating it, the individual neutrons
break down into their constituent
quarks, forming what is known as quark
matter. This conversion might be
confined to the neutron star's center or
it might transform the entire star,
depending on the physical circumstances.
Such a star is known as a quark star.
= Stability and strange quark matter =
Ordinary quark matter consisting of up
and down quarks has a very high Fermi
energy compared to ordinary atomic
matter and is only stable under extreme
temperatures and/or pressures. This
suggests that only quark stars
comprising neutron stars with a quark
matter core will be stable, while quark
stars consisting entirely of ordinary
quark matter will be highly unstable and
dissolve spontaneously.
It has been shown that the high Fermi
energy making ordinary quark matter
unstable at low temperatures and
pressures can be lowered substantially
by the transformation of a sufficient
number of u and d quarks into strange
quarks, as strange quarks are,
relatively speaking, a very heavy type
of quark particle. This kind of quark
matter is known specifically as strange
quark matter and it is speculated and
subject to current scientific
investigation whether it might in fact
be stable under the conditions of
interstellar space. If this is the case,
quark stars made entirely of quark
matter would be stable if they quickly
transform into strange quark matter.
= Strange stars =
Quark stars made of strange quark matter
are known as strange stars, and they
form a subgroup under the quark star
category.
Strange stars might exist without regard
of the Bodmer-Witten assumption of
stability at near-zero temperatures and
pressures, as strange quark matter might
form and remain stable at the core of
neutron stars, in the same way as
ordinary quark matter could. Such
strange stars will naturally have a
crust layer of neutron star material.
The depth of the crust layer will depend
on the physical conditions and
circumstances of the entire star and on
the properties of strange quark matter
in general. Stars partially made up of
quark matter are also referred to as
hybrid stars.
Theoretical investigations have revealed
that quark stars might not only be
produced from neutron stars and powerful
supernovas, but they could also be
created in the early cosmic phase
separations following the Big Bang. If
these primordial quark stars transform
into strange quark matter before the
external temperature and pressure
conditions of the early Universe makes
them unstable, they might turn out
stable, if the Bodmer–Witten assumption
holds true. Such primordial strange
stars could survive to this day.
Characteristics 
If the conversion of neutron-degenerate
matter to quark matter is total, a quark
star can to some extent be imagined as a
single gigantic hadron. But this
"hadron" will be bound by gravity,
rather than the strong force that binds
ordinary hadrons.
= Strange stars =
Recent theoretical research has found
mechanisms by which quark stars with
"strange quark nuggets" may decrease the
objects' electric fields and densities
from previous theoretical expectations,
causing such stars to appear very much
like — nearly indistinguishable from —
ordinary neutron stars. This suggests
that many, or even all, known neutron
stars might in fact be strange stars.
However, the investigating team of
Prashanth Jaikumar, Sanjay Reddy, and
Andrew W. Steiner made some fundamental
assumptions, that led to uncertainties
in their results large enough that the
case is not finally settled. More
research, both observational and
theoretical, remains to be done on
strange stars in the future.
Other theoretical work contends that, "A
sharp interface between quark matter and
the vacuum would have very different
properties from the surface of a neutron
star"; and, addressing key parameters
like surface tension and electrical
forces that were neglected in the
original study, the results show that as
long as the surface tension is below a
low critical value, the large
strangelets are indeed unstable to
fragmentation and strange stars
naturally come with complex strangelet
crusts, analogous to those of neutron
stars.
Other theorized quark formations 
Jaffe 1977, suggested a four-quark state
with strangeness.
Jaffe 1977 suggested the H dibaryon, a
six-quark state with equal numbers of
up-, down-, and strange quarks.
Bound multi-quark systems with heavy
quarks.
In 1987, a pentaquark state was first
proposed with a charm anti-quark.
Pentaquark state with an antistrange
quark and four light quarks consisting
of up- and down-quarks only.
Light pentaquarks are grouped within an
antidecuplet, the lightest candidate,
Ө+.
This can also be described by the
diquark model of Jaffe and Wilczek.
Ө++ and antiparticle Ө−−.
Doubly strange pentaquark, member of the
light pentaquark antidecuplet.
Charmed pentaquark Өc(3100) state was
detected by the H1 collaboration.
Tetra quark particles might form inside
neutron stars and under other extreme
conditions. In 2008, 2013 and 2014 the
tetra quark particle of Z(4430), was
discovered and investigated in
laboratories on Earth.
Observed overdense neutron stars 
Statistically, the probability of a
neutron star being a quark star is low,
so in the Milky Way there would only be
a small population of quark stars. If it
is correct however, that overdense
neutron stars can turn into quark stars,
that makes the possible number of quark
stars higher than was originally
thought, as we would be looking for the
wrong type of star.
Quark stars and strange stars are
entirely hypothetical as of 2011, but
observations released by the Chandra
X-ray Observatory on April 10, 2002
detected two candidates, designated RX
J1856.5-3754 and 3C58, which had
previously been thought to be neutron
stars. Based on the known laws of
physics, the former appeared much
smaller and the latter much colder than
it should be, suggesting that they are
composed of material denser than
neutron-degenerate matter. However,
these observations are met with
skepticism by researchers who say the
results were not conclusive; and since
the late 2000s, the possibility that RX
J1856 is a quark star has been excluded.
Another star, XTE J1739-285, has been
observed by a team led by Philip Kaaret
of the University of Iowa and reported
as a possible candidate.
In 2006, Y. L. Yue et al., from Peking
University, suggested that PSR B0943+10
may in fact be a low-mass quark star.
It was reported in 2008 that
observations of supernovae SN2006gy,
SN2005gj and SN2005ap also suggest the
existence of quark stars. It has been
suggested that the collapsed core of
supernova SN1987A may be a quark star.
See also 
Quark-nova
Quantum chromodynamics
Neutron stars – neutron matter –
neutron-degenerate matter – neutron
Deconfinement
Tolman–Oppenheimer–Volkoff limit on the
mass of a neutron star.
Compact star
Exotic star
Neutron star
Pulsar
Magnetar
White dwarf
Stellar black hole
Degenerate matter
QCD matter
Quark–gluon plasma
Strangelet
Quark matter
Neutronium
Preon matter
References 
Quark star on arxiv.org
External links 
Jaffe, R.. "Perhaps a Stable Dihyperon".
Physical Review Letters 38: 195–198.
Bibcode:1977PhRvL..38..195J.
doi:10.1103/PhysRevLett.38.195. 
Neutron Star/Quark Star Interior
Quark star glimmers, Nature, April 11,
2002.
Debate sparked on quark stars, CERN
Courier 42, #5.
Wish Upon a Quark Star, Paul Beck,
Popular Science, June 2002.
Drake; Marshall; Dreizler; Freeman;
Fruscione; Juda; Kashyap; Nicastro; et
al.. "Is RX J185635-375 a Quark Star?".
Astrophysical Journal 572: 996–1001.
arXiv:astro-ph/0204159.
Bibcode:2002ApJ...572..996D.
doi:10.1086/340368. 
Perhaps a 1,700-year-old quark star in
SNR MSH 15-52
Curious About Astronomy: What process
would bring about a quark star?
RX J185635-375: Candidate Quark Star,
Astronomy Picture of the Day, April 14,
2002.
Quarks or Quirky Neutron Stars?, Mark K.
Anderson, Wired News, April 19, 2002.
Strange Quark Stars, Ask an
Astrophysicist, question submitted April
12, 2002.
Seeing "Strange" Stars, physorg.com,
February 8, 2006.
Quark Stars Could Produce Biggest Bang,
spacedaily.com, June 7, 2006.
Meissner Effect in Strange Quark Stars,
Brian Niebergal, web page, University of
Calgary.
Irina Sagert; Mirjam Wietoska; Jurgen
Schaffner-Bielich. "Strange Exotic
States and Compact Stars". Journal of
Physics G 32: S241–S249.
arXiv:astro-ph/0608317.
Bibcode:2006JPhG...32S.241S.
doi:10.108832S30. 
Quark Stars Involved in New Theory of
Brightest Supernovae – The first-ever
evidence of a neutron star collapsing
into a quark star is announced,
Space.com, 3 June 2008
Quark Stars, Alternate View Column
AV-114, John G. Cramer, Published in the
November-2002 issue of Analog Science
Fiction & Fact Magazine
