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
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Courier 42, #5. Wish Upon a Quark Star, Paul Beck,
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Astrophysical Journal 572: 996–1001. arXiv:astro-ph/0204159.
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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,
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Anderson, Wired News, April 19, 2002. Strange Quark Stars, Ask an
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evidence of a neutron star collapsing into a quark star is announced,
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Fiction & Fact Magazine