In astronomy, the term "compact star" (or
"compact object") refers collectively to white
dwarfs, neutron stars, and black holes.
It would grow to include exotic stars if such
hypothetical dense bodies are confirmed.
Most compact stars are the endpoints of stellar
evolution, and thus often referred to as stellar
remnants, the form of the remnant depending
primarily on the mass of the star when it
formed.
All of these objects have a high mass relative
to their radius, giving them a very high density.
The term compact star is often used when the
exact nature of the star is not known, but
evidence suggests that it is very massive
and has a small radius, thus implying one
of the above-mentioned categories.
A compact star that is not a black hole may
be called a degenerate star.
== Formation ==
The usual endpoint of stellar evolution is
the formation of a compact star.
Most stars will eventually come to a point
in their evolution when the outward radiation
pressure from the nuclear fusions in its interior
can no longer resist the ever-present gravitational
forces.
When this happens, the star collapses under
its own weight and undergoes the process of
stellar death.
For most stars, this will result in the formation
of a very dense and compact stellar remnant,
also known as a compact star.
Compact stars have no internal energy production,
but will—with the exception of black holes—usually
radiate for millions of years with excess
heat left from the collapse itself.According
to the most recent understanding, compact
stars could also form during the phase separations
of the early Universe following the Big Bang.
Primordial origins of known compact objects
have not been determined with certainty.
== Lifetime ==
Although compact stars may radiate, and thus
cool off and lose energy, they do not depend
on high temperatures to maintain their structure,
as ordinary stars do.
Barring external disturbances and proton decay,
they can persist virtually forever.
Black holes are however generally believed
to finally evaporate from Hawking radiation
after trillions of years.
According to our current standard models of
physical cosmology, all stars will eventually
evolve into cool and dark compact stars, by
the time the Universe enters the so-called
degenerate era in a very distant future.
The somewhat wider definition of compact objects
often includes smaller solid objects such
as planets, asteroids, and comets.
There is a remarkable variety of stars and
other clumps of hot matter, but all matter
in the Universe must eventually end as some
form of compact stellar or substellar object,
according to the theory of thermodynamics.
== White dwarfs ==
The stars called white or degenerate dwarfs
are made up mainly of degenerate matter; typically
carbon and oxygen nuclei in a sea of degenerate
electrons.
White dwarfs arise from the cores of main-sequence
stars and are therefore very hot when they
are formed.
As they cool they will redden and dim until
they eventually become dark black dwarfs.
White dwarfs were observed in the 19th century,
but the extremely high densities and pressures
they contain were not explained until the
1920s.
The equation of state for degenerate matter
is "soft", meaning that adding more mass will
result in a smaller object.
Continuing to add mass to what is now a white
dwarf, the object shrinks and the central
density becomes even larger, with higher degenerate-electron
energies.
The star's radius has now shrunk to only a
few thousand kilometers, and the mass is approaching
the theoretical upper limit of the mass of
a white dwarf, the Chandrasekhar limit, about
1.4 times the mass of the Sun (M☉).
If we were to take matter from the center
of our white dwarf and slowly start to compress
it, we would first see electrons forced to
combine with nuclei, changing their protons
to neutrons by inverse beta decay.
The equilibrium would shift towards heavier,
neutron-richer nuclei that are not stable
at everyday densities.
As the density increases, these nuclei become
still larger and less well-bound.
At a critical density of about 4×1014 kg/m3),
called the neutron drip line, the atomic nucleus
would tend to fall apart into protons and
neutrons.
Eventually we would reach a point where the
matter is on the order of the density (c.
2×1017 kg/m3) of an atomic nucleus.
At this point the matter is chiefly free neutrons,
with a small amount of protons and electrons.
== Neutron stars ==
In certain binary stars containing a white
dwarf, mass is transferred from the companion
star onto the white dwarf, eventually pushing
it over the Chandrasekhar limit.
Electrons react with protons to form neutrons
and thus no longer supply the necessary pressure
to resist gravity, causing the star to collapse.
If the center of the star is composed mostly
of carbon and oxygen then such a gravitational
collapse will ignite runaway fusion of the
carbon and oxygen, resulting in a Type Ia
supernova that entirely blows apart the star
before the collapse can become irreversible.
If the center is composed mostly of magnesium
or heavier elements, the collapse continues.
As the density further increases, the remaining
electrons react with the protons to form more
neutrons.
The collapse continues until (at higher density)
the neutrons become degenerate.
A new equilibrium is possible after the star
shrinks by three orders of magnitude, to a
radius between 10 and 20 km.
This is a neutron star.
Although the first neutron star was not observed
until 1967 when the first radio pulsar was
discovered, neutron stars were proposed by
Baade and Zwicky in 1933, only one year after
the neutron was discovered in 1932.
They realized that because neutron stars are
so dense, the collapse of an ordinary star
to a neutron star would liberate a large amount
of gravitational potential energy, providing
a possible explanation for supernovae.
This is the explanation for supernovae of
types Ib, Ic, and II.
Such supernovae occur when the iron core of
a massive star exceeds the Chandrasekhar limit
and collapses to a neutron star.
Like electrons, neutrons are fermions.
They therefore provide neutron degeneracy
pressure to support a neutron star against
collapse.
In addition, repulsive neutron-neutron interactions
provide additional pressure.
Like the Chandrasekhar limit for white dwarfs,
there is a limiting mass for neutron stars:
the Tolman-Oppenheimer-Volkoff limit, where
these forces are no longer sufficient to hold
up the star.
As the forces in dense hadronic matter are
not well understood, this limit is not known
exactly but is thought to be between 2 and
3 M☉.
If more mass accretes onto a neutron star,
eventually this mass limit will be reached.
What happens next is not completely clear.
== Black holes ==
As more mass is accumulated, equilibrium against
gravitational collapse reaches its breaking
point.
The star's pressure is insufficient to counterbalance
gravity and a catastrophic gravitational collapse
occurs in milliseconds.
The escape velocity at the surface, already
at least 1/3 light speed, quickly reaches
the velocity of light.
No energy nor matter can escape: a black hole
has formed.
All light will be trapped within an event
horizon, and so a black hole appears truly
black, except for the possibility of Hawking
radiation.
It is presumed that the collapse will continue.
In the classical theory of general relativity,
a gravitational singularity occupying no more
than a point will form.
There may be a new halt of the catastrophic
gravitational collapse at a size comparable
to the Planck length, but at these lengths
there is no known theory of gravity to predict
what will happen.
Adding any extra mass to the black hole will
cause the radius of the event horizon to increase
linearly with the mass of the central singularity.
This will induce certain changes in the properties
of the black hole, such as reducing the tidal
stress near the event horizon, and reducing
the gravitational field strength at the horizon.
However, there will not be any further qualitative
changes in the structure associated with any
mass increase.
=== Alternative black hole models ===
Fuzzball
Gravastar
Dark energy star
Black star
Magnetospheric eternally collapsing object
Dark star
Primordial black holes
== 
Exotic stars ==
An exotic star is a hypothetical compact star
composed of something other than electrons,
protons, and neutrons balanced against gravitational
collapse by degeneracy pressure or other quantum
properties.
These include strange stars (composed of strange
matter) and the more speculative preon stars
(composed of preons).
Exotic stars are hypothetical, but observations
released by the Chandra X-Ray Observatory
on April 10, 2002 detected two candidate strange
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 they should, suggesting that they
are composed of material denser than neutronium.
However, these observations are met with skepticism
by researchers who say the results were not
conclusive.
=== Quark stars and strange stars ===
If neutrons are squeezed enough at a high
temperature, they will decompose into their
component quarks, forming what is known as
a quark matter.
In this case, the star will shrink further
and become denser, but instead of a total
collapse into a black hole, it is possible,
that the star may stabilize itself and survive
in this state indefinitely, as long as no
extra mass is added.
It has, to some extent, become a very large
nucleon.
A-type star in this hypothetical state is
called a quark star or more specifically a
strange star.
The pulsars RX J1856.5-3754 and 3C58 have
been suggested as possible quark stars.
Most neutron stars are thought to hold a core
of quark matter, but it has proven hard to
determine observationally.
=== 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 kilogram per cubic
meter – intermediate between quark stars
and black holes.
Preon stars could originate from supernova
explosions or the Big Bang; however, current
observations from particle accelerators speak
against the existence of preons.
=== Q stars ===
Q stars are hypothetical compact, heavier
neutron stars with an exotic state of matter
where particle numbers are preserved with
radii less than 1.5 times the corresponding
Schwarzschild radius.
Q stars are also called "gray holes".
=== Electroweak stars ===
An electroweak star is a theoretical type
of exotic star, whereby the gravitational
collapse of the star is prevented by radiation
pressure resulting from electroweak burning,
that is, the energy released by conversion
of quarks to leptons through the electroweak
force.
This process occurs in a volume at the star's
core approximately the size of an apple, containing
about two Earth masses.
=== Boson star ===
A boson star is a hypothetical astronomical
object that is formed out of particles called
bosons (conventional stars are formed out
of fermions).
For this type of star to exist, there must
be a stable type of boson with repulsive self-interaction.
As of 2016 there is no significant evidence
that such a star exists.
However, it may become possible to detect
them by the gravitational radiation emitted
by a pair of co-orbiting boson stars.
== Compact relativistic objects and the generalized
uncertainty principle ==
Based on the generalized uncertainty principle
(GUP), proposed by some approaches to quantum
gravity such as string theory and doubly special
relativity, the effect of GUP on the thermodynamic
properties of compact stars with two different
components has been studied, recently.
Tawfik et al. noted that the existence of
quantum gravity correction tends to resist
the collapse of stars if the GUP parameter
is taking values between Planck scale and
electroweak scale.
Comparing with other approaches, it was found
that the radii of compact stars should be
smaller and increasing energy decreases the
radii of the compact stars.
== References ==
== Sources ==
Blaschke, D.; Fredriksson, S.; Grigorian,
H.; Öztaş, A.; Sandin, F. (2005).
"Phase diagram of three-flavor quark matter
under compact star constraints".
Physical Review D. 72 (6).
arXiv:hep-ph/0503194.
Bibcode:2005PhRvD..72f5020B.
doi:10.1103/PhysRevD.72.065020.
Sandin, F. (2005).
"Compact stars in the standard model – and
beyond".
European Physical Journal C. 40: 15.
arXiv:astro-ph/0410407.
Bibcode:2005EPJC...40...15S. doi:10.1140/epjcd/s2005-03-003-y.
Sandin, F. (2005).
Exotic Phases of Matter in Compact Stars (PDF)
(Thesis).
Luleå University of Technology.
