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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Black hole | 6/13 | https://en.wikipedia.org/wiki/Black_hole | reference | science, encyclopedia | 2026-05-05T13:31:52.244381+00:00 | kb-cron |
Black holes are classified by the theory of their formation and by their mass (expressed in terms of M☉, the mass of the Sun), but these criteria are intertwined. Stellar black holes are formed by stellar collapse. The minimum mass of a black hole formed by stellar gravitational collapse is governed by the maximum mass of a neutron star and is believed to be 2-4 M☉. Hypothetical primordial black holes, believed to have formed soon after the Big Bang, could be far smaller, with masses as little as 10−5 grams at formation. These very small black holes are sometimes called micro black holes. Stellar black holes can have a wide range of masses. Estimates of their maximum mass at formation vary, but generally range from 10-100 M☉, with higher estimates for black holes progenated by low-metallicity stars. Stellar black holes can gain mass via accretion of nearby matter, often from a companion object such as a star or by merger with another black hole. A small number of candidate black holes with masses in the range 102-104 M☉ have been reported. These are larger than stellar black holes but smaller than supermassive black holes, are rarer than either extreme, and are called intermediate-mass black holes. Physicists have speculated that such black holes may form from collisions in globular and star clusters or at the center of low-mass galaxies. They may also form as the result of mergers of smaller black holes, with several gravitational wave measurements consistent with merged black holes within 110–350 M☉. The black holes with the largest masses are called supermassive black holes, with masses more than 106 M☉. These black holes are believed to exist at the centers of almost every large galaxy, including the Milky Way. Some scientists have proposed a subcategory of even larger black holes, called ultramassive black holes, with masses greater than 109-1010 M☉. Theoretical models predict that the accretion disc that feeds black holes will be unstable once a black hole reaches 50×109–100×109 M☉, setting a rough upper limit to black hole mass.
== Structure == While black holes are conceptually invisible sinks of all matter and light, in astronomical settings, their enormous gravity alters the motion of surrounding objects and pulls nearby gas inwards at near-light speed, making the area around black holes the brightest objects in the universe.
=== External geometry ===
==== Relativistic jets ====
Some black holes have relativistic jets—thin streams of plasma travelling away from the black hole at more than 90% of the speed of light. A small fraction of the matter falling towards the black hole gets accelerated away along the hole rotation axis. These jets can extend as far as millions of light-years from the black hole itself. Black holes of any mass can have jets. However, they are typically observed around spinning black holes with strongly-magnetized accretion disks. Relativistic jets were more common in the early universe, when galaxies and their corresponding supermassive black holes were rapidly gaining mass. All black holes with jets also have an accretion disk, but the jets are usually brighter than the disk. Quasars, typically found in other galaxies, are believed to be supermassive black holes with jets; microquasars are believed to be stellar-mass objects with jets, typically observed in the Milky Way. The jets can be powered by either the accretion disk or the rotating black hole spin. While many details of the jets have been studied, no complete model has emerged. One method proposed to fuel these jets is the Blandford-Znajek process, which suggests that the dragging of magnetic field lines by a black hole's rotation could launch jets of matter into space. The Penrose process, which involves extraction of a black hole's rotational energy, has also been proposed as a potential mechanism of jet propulsion. There is evidence that jets at all scales, from microquasars to gamma ray bursts may be produced by a single mechanism.
==== Accretion disk ====
Due to conservation of angular momentum, gas falling into the gravitational well created by a massive object will typically form a disk-like structure around the object. As the disk's angular momentum is transferred outward due to processes such as turbulence in the disk, its matter falls farther inward, converting its gravitational energy into heat and releasing a large amount of radiation; absent an explosive event, the radiation pressure limits the accretion rate. The temperature of these disks can range from thousands to millions of kelvins, and temperatures differ throughout a single accretion disk. Accretion disks radiate across the entire electromagnetic spectrum, depending on the disk's turbulence and magnetisation and the black hole's mass and angular momentum. Accretion disks can be defined as geometrically thin or geometrically thick. Geometrically thin disks are mostly confined to the black hole's equatorial plane and have a well-defined edge at the innermost stable circular orbit (ISCO), while geometrically thick disks are supported by internal pressure and temperature and can extend inside the ISCO. Disks with high rates of electron scattering and absorption, appearing bright and opaque, are called optically thick; optically thin disks are more translucent and produce fainter images when viewed from afar. Accretion disks of black holes accreting beyond the Eddington limit are often referred to as polish donuts due to their thick, toroidal shape that resembles that of a donut. Quasar accretion disks are expected to have a "blue spectral shape", meaning that the flux per frequency
F
ν
{\displaystyle F_{\nu }}
is proportional to
ν
1
/
3
{\displaystyle \nu ^{1/3}}
; this was not originally observed due to emission from dust surrounding the objects. The disk for a stellar black hole, on the other hand, would likely look orange, yellow, or red, with its inner regions being the brightest. Accretion disk colours may also be altered by the Doppler effect, with the part of the disk travelling towards an observer appearing bluer and brighter and the part of the disk travelling away from the observer appearing redder and dimmer.