A quark star is a hypothetical type of compact
exotic star, where extremely high temperature
and pressure has forced nuclear particles
to form quark matter, a continuous state of
matter consisting of free quarks.
It is well known, both theoretically and observationally,
that some massive stars collapse to form neutron
stars at the end of their life cycle. Under
the extreme temperatures and pressures inside
neutron stars, the neutrons are normally kept
apart by a degeneracy pressure, stabilizing
the star and hindering further gravitational
collapse. However, it is hypothesized that
under even more extreme temperature and pressure,
the degeneracy pressure of the neutrons is
overcome, and the neutrons are forced to merge
and dissolve into their constituent quarks,
creating an ultra-dense phase of quark matter
based on densely packed quarks. In this state,
a new equilibrium is supposed to emerge, as
a new degeneracy pressure between the quarks,
as well as repulsive electromagnetic forces,
will occur and hinder gravitational collapse.
If these ideas are correct, quark stars might
occur, and be observable, somewhere in the
universe. Theoretically, such a scenario is
seen as scientifically plausible, but it has
been impossible to prove both observationally
and experimentally, because the very extreme
conditions needed for stabilizing quark matter
can not be created in any laboratory nor observed
directly in nature. The stability of quark
matter, and hence the existence of quark stars,
is for these reasons among the unsolved problems
in physics.
If quark stars can form, then the most likely
place to find quark star matter would be inside
neutron stars that exceed the internal pressure
needed for quark degeneracy - the point at
which neutrons break down into a form of dense
quark matter. They could also form if a massive
star collapses at the end of its life, provided
that it is possible for a star to be large
enough to collapse beyond a neutron star but
not large enough to form a black hole. However,
as scientists are unable so far to explore
most properties of quark matter, the exact
conditions and nature of quark stars, and
their existence, remain hypothetical and unproven.
The question whether such stars exist and
their exact structure and behavior is actively
studied within astrophysics and particle physics.
If they exist, quark stars would resemble
and be easily mistaken for neutron stars:
they would form in the death of a massive
star in a Type II supernova, be extremely
dense and small, and possess a very high gravitational
field. They would also lack some features
of neutron stars, unless they also contained
a shell of neutron matter, because free quarks
are not expected to have properties matching
degenerate neutron matter. For example, they
might be radio-silent, or not have typical
sizes, electromagnetic fields, or surface
temperatures, compared to neutron stars.
The hypothesis about quark stars was first
proposed in 1965 by Soviet physicists D. D.
Ivanenko and D. F. Kurdgelaidze. Their existence
has not been confirmed. 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 being actively studied with particle colliders,
but this can only produce very hot (above
1012 K) quark-gluon plasma blobs the size
of atomic nuclei, which decay immediately
after formation. The conditions inside compact
stars with extremely high densities and temperatures
well below 1012 K can not be recreated artificially,
as there are no known methods to produce,
store or study "cold" quark matter directly
as it would be found inside quark stars. The
theory predicts quark matter to possess some
peculiar characteristics under these conditions.
== 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 (up quarks and down
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 (also referred to as u and d 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 the only stable quark stars will be neutron
stars with a quark matter core, 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 up and down 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 (i.e. near zero external
pressure and temperature). If this is the
case (known as the Bodmer–Witten assumption),
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.Theoretical
investigations have revealed that quark stars
might not only be produced from neutron stars
and powerful supernovas, 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 ==
Quark stars have some special characteristics
that separate them from ordinary neutron stars.
Under the physical conditions found inside
neutron stars, with extremely high densities
but temperatures well below 1012 K, quark
matter is predicted to exhibit some peculiar
characteristics. It is expected to behave
as a Fermi liquid and enter a so-called color-flavor-locked
(CFL) phase of color superconductivity, where
"color" refers to the six "charges" exhibited
in the strong interaction, instead of the
positive and the negative charges in electromagnetism.
At slightly lower densities, corresponding
to higher layers closer to the surface of
the compact star, the quark matter will behave
as a non-CFL quark liquid, a phase that is
even more mysterious than CFL and might include
color conductivity and/or several additional
yet undiscovered phases. None of these extreme
conditions can currently be recreated in laboratories
so nothing can be inferred about these phases
from direct experiments.If the conversion
of neutron-degenerate matter to (strange)
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.
== Observed overdense neutron stars ==
At least under the assumptions mentioned above,
the probability of a given 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 observers
would be looking for the wrong type of star.Quark
stars and strange stars are entirely hypothetical
as of 2018, but there are several candidates.
Observations released by the Chandra X-ray
Observatory on April 10, 2002 detected two
possible quark stars, 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 quark star
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 SN 2006gy,
SN 2005gj and SN 2005ap also suggest the existence
of quark stars. It has been suggested that
the collapsed core of supernova SN 1987A may
be a quark star.In 2015, Z.G. Dai et al. from
Nanjing University suggested that Supernova
ASASSN-15lh is a newborn strange quark star.
== Other theorized quark formations ==
Apart from ordinary quark matter and strange
quark matter, other types of quark-gluon plasma
might theoretically occur or be formed inside
neutron stars and quark stars. This includes
the following, some of which has been observed
and studied in laboratories:
Jaffe 1977, suggested a four-quark state with
strangeness (qsqs).
Jaffe 1977 suggested the H dibaryon, a six-quark
state with equal numbers of up-, down-, and
strange quarks (represented as uuddss or udsuds).
Bound multi-quark systems with heavy quarks
(QQqq).
In 1987, a pentaquark state was first proposed
with a charm anti-quark (qqqsc).
Pentaquark state with an antistrange quark
and four light quarks consisting of up- and
down-quarks only (qqqqs).
Light pentaquarks are grouped within an antidecuplet,
the lightest candidate, Ө+.
This can also be described by the diquark
model of Jaffe and Wilczek (QCD).
Ө++ and antiparticle Ө−−.
Doubly strange pentaquark (ssddu), member
of the light pentaquark antidecuplet.
Charmed pentaquark Өc(3100) (uuddc) 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.
== 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
== 
Sources and further reading ==
Blaschke, David and Sedrakian, David: "Superdense
QCD Matter and Compact Stars", NATO Science
Series, Springer (2003)
Blaschke, David., Glendenning, Norman K. and
Sedrakian, A.: "Physics of neutron star interiors",
Lecture Notes in Physics (Vol. 578), Springer
(2001)
Plessas, W. and Mathelitsch, L. (Leopold):
"Lectures on quark matter", Lecture Notes
in Physics (Vol. 583), Springer (2002
