An exotic star is a hypothetical compact star
composed of something other than electrons,
protons, neutrons, or muons, and balanced
against gravitational collapse by degeneracy
pressure or other quantum properties. Exotic
stars include quark stars (composed of quarks)
and perhaps strange stars (composed of strange
quark matter, a condensate of up, down and
strange quarks), as well as speculative preon
stars (composed of preons, which are hypothetical
particles and "building blocks" of quarks,
should quarks be decomposable into component
sub-particles). Of the various types of exotic
star proposed, the most well evidenced and
understood is the quark star.
Exotic stars are largely theoretical – partly
because it is difficult to test in detail
how such forms of matter may behave, and partly
because prior to the fledgling technology
of gravitational-wave astronomy, we lacked
a satisfactory means of detecting cosmic objects
that do not radiate electromagnetically or
through known particles. So it is not yet
possible to verify novel cosmic objects of
this nature by distinguishing them from known
objects. Candidates for such objects are occasionally
identified based on indirect evidence gained
from properties we can observe.
== Quark stars and strange stars ==
A quark star is a hypothesized object that
results from the decomposition of neutrons
into their constituent up and down quarks
under gravitational pressure. It is expected
to be smaller and denser than a neutron star,
and may survive in this new state indefinitely
if no extra mass is added. Effectively, it
is a very large nucleon. Quark stars that
contain strange matter are called strange
stars.
Based on observations released by the Chandra
X-Ray Observatory on 10 April 2002, two objects,
designated RX J1856.5-3754 and 3C58, were
suggested as quark star candidates. The former
appeared to be much smaller and the latter
much colder than expected for a neutron star,
suggesting that they were composed of material
denser than neutronium. However, these observations
were met with skepticism by researchers who
said the results were not conclusive. After
further analysis, RX J1856.5-3754 was excluded
from the list of quark star candidates.
== Electroweak stars ==
An electroweak star is a theoretical type
of exotic star in which the gravitational
collapse of the star is prevented by radiation
pressure resulting from electroweak burning;
that is, the energy released by the conversion
of quarks into leptons through the electroweak
force. This process occurs in a volume at
the star's core approximately the size of
an apple and containing about two Earth masses.The
stage of life of a star that produces an electroweak
star is theorized to occur after a supernova
collapse. Electroweak stars are denser than
quark stars, and may form when quark degeneracy
pressure is no longer able to withstand gravitational
attraction, but can still be withstood by
electroweak-burning radiation pressure. This
phase of a star's life may last upwards of
10 million years.
== Preon stars ==
A preon star is a proposed type of compact
star made of preons, a group of hypothetical
subatomic particles. Preon stars would be
expected to have huge densities, exceeding
1023 kg/m3. They may have greater densities
than quark stars and neutron stars, although
they would be smaller but heavier than white
dwarfs and neutron stars. Preon stars could
originate from supernova explosions or the
big bang. Such objects could be detected in
principle through gravitational lensing of
gamma rays. Preon stars are a potential candidate
for dark matter. However, current observations
from particle accelerators speak against the
existence of preons, or at least do not prioritize
their investigation, since the only particle
detector presently able to explore very high
energies (the Large Hadron Collider) is not
designed specifically for this and its research
program is directed towards other areas, such
as studying the Higgs boson, quark-gluon plasma
and evidence related to physics beyond the
Standard Model.
In general relativity, if a star collapses
to a size smaller than its Schwarzschild radius,
an event horizon will exist at that radius
and the star will become a black hole. Thus,
the size of a preon star may vary from around
1 metre with an absolute mass of 100 Earths
to the size of a pea with a mass roughly equal
to that of the Moon.
== Boson stars ==
A boson star is a hypothetical astronomical
object that is formed out of particles called
bosons (conventional stars are formed from
mostly protons, which are fermions, but also
consist of Helium-4 nuclei, which are bosons).
For this type of star to exist, there must
be a stable type of boson with self-repulsive
interaction; one possible candidate particle
is the still-hypothetical "axion" (which is
also a candidate for the not-yet-detected
"non-baryonic dark matter" particles, which
appear to compose roughly 25% of the mass
of the Universe). It is theorized
that unlike normal stars (which emit radiation
due to gravitational pressure and nuclear
fusion), boson stars would be transparent
and invisible. The immense gravity of a compact
boson star would bend light around the object,
creating an empty region resembling the shadow
of a black hole's event horizon. Like a black
hole, a boson star would absorb ordinary matter
from its surroundings, but the transparency
means this matter (which likely would heat
up and emit radiation) would be visible at
its center. Simulations further suggest that
rotating boson stars would be doughnut-shaped
as centrifugal forces would give the bosonic
matter that form.
As of 2018, there is no significant evidence
that such stars exist. However, it may become
possible to detect them by the gravitational
radiation emitted by a pair of co-orbiting
boson stars.Boson stars may have formed through
gravitational collapse during the primordial
stages of the big bang. At least in theory,
a supermassive boson star could exist at the
core of a galaxy, which might explain many
of the observed properties of active galactic
cores.Boson stars have also been proposed
as candidate dark matter objects,
and it has been hypothesized that the dark
matter haloes surrounding most galaxies might
be viewed as enormous "boson stars."The compact
boson stars and boson shells are often studied
involving fields like the massive (or massless)
complex scalar fields, the U(1) gauge field
and gravity with conical potential. The presence
of a positive or negative cosmological constant
in the theory facilitates a study of these
objects in de Sitter and anti-de Sitter spaces.Bratten,
Mohapatra, and Zhang have theorized that a
new type of dense axion star may exist in
which gravity is balanced by the mean-field
pressure of the axion Bose-Einstein condensate.
== Planck stars ==
In loop quantum gravity, A Planck star is
a theoretically possible astronomical object
that is created when the energy density of
a collapsing star reaches the Planck energy
density. Under these conditions, assuming
gravity and spacetime are quantized, there
arises a repulsive "force" derived from Heisenberg's
uncertainty principle. In other words, if
gravity and spacetime are quantized, the accumulation
of mass-energy inside the Planck star cannot
collapse beyond this limit because it would
violate the uncertainty principle for spacetime
itself.
== See also ==
Dark star
Exotic matter
Glueball
Q star
== References ==
Johan Hansson, A hierarchy of cosmic compact
objects – without black holes. Acta Physica
Polonica B, Vol. 38, p.91 (2007). PDF
Johan Hansson and Fredrik Sandin, The observational
legacy of preon stars – probing new physics
beyond the LHC.
J. E. Horvath, Constraints on superdense preon
stars and their formation scenarios. Astrophys.
Space Sci. 307, 419 (2007).
Fredrik Sandin, Exotic Phases of Matter in
Compact Stars (2007). PDF
Nature News article: Splitting the quark (Nov.
2007).
"A New Way To Shine, A New Kind Of Star".
SpaceDaily. 16 December 2009. Retrieved 2009-12-16.
== External links ==
Abstract: Are Q-stars a serious threat for
stellar-mass black hole candidates?. Miller,
J.C.; Shahbaz, T.; Nolan, L.A. (1997)
Abstract: No observational proof of the black-hole
event-horizon. Abramowicz, Marek A.; Kluzniak,
Wlodek; Lasota, Jean-Pierre (2002)
New Scientist issue 2643: "Could preon stars
reveal a hidden reality?" (6 February 2008)
New Scientist issue 2472: "Micro-stars may
manage to avoid black-hole fate" (6 November
2008)
Dai, De-Chang; Lue, Arthur; Starkman, Glenn;
Stojkovic, Dejan (2010). "Electroweak stars:
how nature may capitalize on the standard
model's ultimate fuel". Journal of Cosmology
and Astroparticle Physics. 12: 004. arXiv:0912.0520.
Bibcode:2010JCAP...12..004D. doi:10.1088/1475-7516/2010/12/004.
