Compact object

In astronomy, the term compact object (or compact star) refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.

Compact objects are often the endpoints of

. A compact object that is not a black hole may be called a degenerate star.

In June 2020, astronomers reported narrowing down the source of Fast Radio Bursts (FRBs), which may now plausibly include "compact-object mergers and magnetars arising from normal core collapse supernovae".^[1][2]

The usual endpoint of stellar evolution is the formation of a compact star.

All active 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

. For most stars, this will result in the formation of a very dense and compact stellar remnant, also known as a compact star.

Compact objects 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.^[3]

According to the most recent understanding, compact stars could also form during the phase separations of the early Universe following the Big Bang.^[4] Primordial origins of known compact objects have not been determined with certainty.

Although compact objects 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

in a very distant future.

A somewhat wider definition of compact objects may include smaller solid objects such as planets, asteroids, and comets, but such usage is less common. There are a remarkable variety of stars and other clumps of hot matter, but all matter in the Universe must eventually end as dispersed cold particles or some form of compact stellar or substellar object, according to thermodynamics.

The stars called

. White dwarfs were observed in the 19th century, but the extremely high densities and pressures they contain were not explained until the 1920s.

The

).

If matter were removed from the center of a white dwarf and slowly compressed, electrons would first be forced to combine with nuclei, changing their

neutron drip line

– the atomic nucleus would tend to dissolve into unbound protons and neutrons. If further compressed, eventually it would reach a point where the matter is on the order of the density of an atomic nucleus – about 2×10¹⁷ kg/m³. At that density the matter would be chiefly free neutrons, with a light scattering of protons and electrons.

Neutron stars

In certain

, 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.^[8][9][10] 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

, 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 exceeds its breaking point. Once the star's pressure is insufficient to counterbalance gravity, a catastrophic gravitational collapse occurs within milliseconds. The escape velocity at the surface, already at least 1⁄3 light speed, quickly reaches the velocity of light. At that point no energy or matter can escape and a black hole has formed. Because all light and matter is trapped within an event horizon, a black hole appears truly black, except for the possibility of very faint Hawking radiation. It is presumed that the collapse will continue inside the event horizon.

In the classical theory of

, 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.

• Fuzzball^[11]
• Gravastar^[11]
• Dark-energy star
• Black star
• Magnetospheric eternally collapsing object
• Dark star^[11]
• Primordial black holes

An

).

Exotic stars are hypothetical, but observations released by the

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.^{[citation needed}

]

Quark stars and strange stars

Main article: Quark star

If neutrons are squeezed enough at a high temperature, they will decompose into their component

3C58 has been suggested as a possible quark star. Most neutron stars are thought to hold a core of quark matter but this has proven difficult to determine observationally.^{[citation needed}

]

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.^{[citation needed]}

Q stars

Main article: Q star

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

Main article:

Electroweak star

An electroweak star is a theoretical type of exotic star, whereby the gravitational collapse of the star is prevented by

electroweak force. This process occurs in a volume at the star's core approximately the size of an apple, containing about two Earth masses.^[12]

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.^[13][14]

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.^[15] 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.

See also

• Galaxy formation and evolution

References

1. ^ Starr, Michelle (1 June 2020). "Astronomers Just Narrowed Down The Source of Those Powerful Radio Signals From Space". ScienceAlert.com. Retrieved 2 June 2020.
2. S2CID 218900539
   .
3. Bibcode:2006csxs.book..623T
   .
4. S2CID 14466173
   .
5. ^ Hashimoto, M.; Iwamoto, K.; Nomoto, K. (1993). "Type II supernovae from 8–10 solar mass asymptotic giant branch stars".
   doi:10.1086/187007
   .
6. ^ Ritossa, C.; Garcia-Berro, E.; Iben, I. Jr. (1996). "On the Evolution of Stars That Form Electron-degenerate Cores Processed by Carbon Burning. II. Isotope Abundances and Thermal Pulses in a 10 M_sun Model with an ONe Core and Applications to Long-Period Variables, Classical Novae, and Accretion-induced Collapse".
   doi:10.1086/176987
   .
7. ^ Wanajo, S.; et al. (2003). "The r-Process in Supernova Explosions from the Collapse of O-Ne-Mg Cores".
   S2CID 13456130
   .
8. Bibcode:2001AAS...199.1501O
   .
9. ^ Baade, W.; Zwicky, F. (1934). "On Super-Novae".
   PMID 16587881
   .
10. ^ Baade, W.; Zwicky, F. (1934). "Cosmic Rays from Super-Novae".
    PMID 16587882
    .
11. ^
    arXiv:0902.0346 [gr-qc
    ].
12. ^ Shiga, D. (4 January 2010). "Exotic stars may mimic big bang". New Scientist. Retrieved 2010-02-18.
13. ISBN 0-521-45506-5
    .
14. S2CID 115159490
    .
15. ^ Ahmed Farag Ali and A. Tawfik, Int. J. Mod. Phys. D22 (2013) 1350020

Sources

• Blaschke, D.; Fredriksson, S.; Grigorian, H.; Öztaş, A.; Sandin, F. (2005). "Phase diagram of three-flavor quark matter under compact star constraints".
  S2CID 119356279
  .
• Sandin, F. (2005). "Compact stars in the standard model – and beyond".
  S2CID 119495444
  .
• Sandin, F. (2005). Exotic Phases of Matter in Compact Stars (PDF) (Thesis). Luleå University of Technology.