--- title: Black hole summary: A region of spacetime where gravity is so strong that nothing, light included, can climb back out; the collapsed remains of massive stars, and the giants at the centres of galaxies. science_status: [observed, model, sim] categories: [Black holes, Stellar remnants, Compact objects, General relativity] aliases: [Black holes, Stellar black hole, Stellar-mass black hole, Intermediate-mass black hole, Supermassive black hole, Kerr black hole, Schwarzschild black hole, BHS, BHI, BHM, black_hole_stellar, black_hole_intermediate, black_hole_supermassive] infobox: type: black_hole image: File:Black_hole_10_Msun_in_game.png image_caption: "Sim render: a 10 solar-mass hole with a disk, seen 55 degrees above the disk plane" classes: [stellar, intermediate, supermassive] mass_msun: stellar: "about 3 to about 150 (Milky Way dynamical and astrometric masses 4.4 to 32.7; merger components to about 140)" intermediate: "about 100 to 100,000 (few secure cases)" supermassive: "about 100,000 to about 10,000,000,000" spin_a_star: "0 to just under 1 (dimensionless); X-ray estimates are model dependent; not measured for Sagittarius A* or M87*" schwarzschild_radius_km_per_msun: 2.953 schwarzschild_radius_formula: "r_s = 2GM/c^2" photon_sphere_rs: "1.5 (non-spinning); prograde 0.5 to retrograde 2.0 for a maximally spinning hole" shadow_radius_rs: "2.598 (non-spinning, critical impact parameter); a spinning hole's shadow is shifted and flattened on one side" isco_prograde_rs: "3.0 (a = 0); 1.160 (a = 0.9); 0.5 (a = 1)" isco_retrograde_rs: "3.0 (a = 0); 4.5 (a = 1)" horizon_rs: "1.0 (a = 0); 0.718 (a = 0.9); 0.5 (a = 1)" radiative_efficiency_thin_disk: "5.7 percent (a = 0); 15.6 percent (a = 0.9); up to 42 percent (a = 1)" hawking_temperature_k: "6.2e-8 x (solar mass / M)" accretion_state: "dormant (most); quiescent or outbursting X-ray binaries; hot, radiatively inefficient flows in quiet galactic nuclei; bright thin disks in quasars" jet: "radio jets in many accreting holes (Cygnus X-1, V404 Cygni in outburst, M87*)" evidence: [dynamical (X-ray binaries), astrometric (Gaia), microlensing, gravitational waves, stellar orbits, horizon-scale imaging] first_identified: "1972 (Cygnus X-1)" nearest_known: "Gaia BH1, 9.62 solar masses, 480 pc (about 1,560 light years)" galaxy_population_estimate: "about 1.3e8 of stellar origin (population synthesis)" known_in_sim_catalogue: 113 sim_stops: [Cygnus X-1 (two poses), Gaia BH1, V404 Cygni, Sagittarius A*] sim: entity: stellar_class.black_hole catalogue: known_black_holes.json refs: - id: kerr1963 type: article-journal author: [{family: Kerr, given: Roy P.}] title: "Gravitational Field of a Spinning Mass as an Example of Algebraically Special Metrics" container-title: Physical Review Letters issued: 1963 volume: 11 page: "237-238" DOI: 10.1103/PhysRevLett.11.237 - id: bardeen1972 type: article-journal author: [{family: Bardeen, given: James M.}, {family: Press, given: William H.}, {family: Teukolsky, given: Saul A.}] title: "Rotating Black Holes: Locally Nonrotating Frames, Energy Extraction, and Scalar Synchrotron Radiation" container-title: The Astrophysical Journal issued: 1972 volume: 178 page: "347" DOI: 10.1086/151796 - id: pagethorne1974 type: article-journal author: [{family: Page, given: Don N.}, {family: Thorne, given: Kip S.}] title: "Disk-Accretion onto a Black Hole. Time-Averaged Structure of Accretion Disk" container-title: The Astrophysical Journal issued: 1974 volume: 191 page: "499" DOI: 10.1086/152990 - id: hawking1974 type: article-journal author: [{family: Hawking, given: S. W.}] title: "Black hole explosions?" container-title: Nature issued: 1974 volume: 248 page: "30-31" DOI: 10.1038/248030a0 - id: rees1988 type: article-journal author: [{family: Rees, given: Martin J.}] title: "Tidal disruption of stars by black holes of 10^6-10^8 solar masses in nearby galaxies" container-title: Nature issued: 1988 volume: 333 page: "523-528" DOI: 10.1038/333523a0 - id: webster1972 type: article-journal author: [{family: Webster, given: B. Louise}, {family: Murdin, given: Paul}] title: "Cygnus X-1: a Spectroscopic Binary with a Heavy Companion?" container-title: Nature issued: 1972 volume: 235 page: "37-38" DOI: 10.1038/235037a0 - id: bolton1972 type: article-journal author: [{family: Bolton, given: C. T.}] title: "Identification of Cygnus X-1 with HDE 226868" container-title: Nature issued: 1972 volume: 235 page: "271-273" DOI: 10.1038/235271b0 - id: millerjones2021 type: article-journal author: [{family: Miller-Jones, given: James C. A.}, {literal: "et al."}] title: "Cygnus X-1 contains a 21-solar mass black hole: Implications for massive star winds" container-title: Science issued: 2021 volume: 371 page: "1046-1049" DOI: 10.1126/science.abb3363 - id: ramachandran2025 type: article-journal author: [{family: Ramachandran, given: V.}, {literal: "et al."}] title: "Comprehensive UV and optical spectral analysis of Cygnus X-1: Stellar and wind parameters, abundances, and evolutionary implications" container-title: Astronomy & Astrophysics issued: 2025 volume: 698 page: "A37" DOI: 10.1051/0004-6361/202554184 - id: zdziarski2024 type: article-journal author: [{family: Zdziarski, given: Andrzej A.}, {literal: "et al."}] title: "What Is the Black Hole Spin in Cyg X-1?" container-title: The Astrophysical Journal Letters issued: 2024 volume: 967 page: "L9" DOI: 10.3847/2041-8213/ad43ed - id: reynolds2021 type: article-journal author: [{family: Reynolds, given: Christopher S.}] title: "Observational Constraints on Black Hole Spin" container-title: Annual Review of Astronomy and Astrophysics issued: 2021 volume: 59 page: "117-154" DOI: 10.1146/annurev-astro-112420-035022 - id: khargharia2010 type: article-journal author: [{family: Khargharia, given: Juthika}, {family: Froning, given: Cynthia S.}, {family: Robinson, given: Edward L.}] title: "Near-infrared Spectroscopy of Low-mass X-ray Binaries: Accretion Disk Contamination and Compact Object Mass Determination in V404 Cyg and Cen X-4" container-title: The Astrophysical Journal issued: 2010 volume: 716 page: "1105-1117" DOI: 10.1088/0004-637X/716/2/1105 - id: millerjones2009 type: article-journal author: [{family: Miller-Jones, given: J. C. A.}, {literal: "et al."}] title: "The First Accurate Parallax Distance to a Black Hole" container-title: The Astrophysical Journal issued: 2009 volume: 706 page: "L230-L234" DOI: 10.1088/0004-637X/706/2/L230 - id: walton2017 type: article-journal author: [{family: Walton, given: D. J.}, {literal: "et al."}] title: "Living on a Flare: Relativistic Reflection in V404 Cyg Observed by NuSTAR during Its Summer 2015 Outburst" container-title: The Astrophysical Journal issued: 2017 volume: 839 page: "110" DOI: 10.3847/1538-4357/aa67e8 - id: elbadry2023a type: article-journal author: [{family: El-Badry, given: Kareem}, {literal: "et al."}] title: "A Sun-like star orbiting a black hole" container-title: Monthly Notices of the Royal Astronomical Society issued: 2023 volume: 518 page: "1057-1085" DOI: 10.1093/mnras/stac3140 - id: elbadry2023b type: article-journal author: [{family: El-Badry, given: Kareem}, {literal: "et al."}] title: "A red giant orbiting a black hole" container-title: Monthly Notices of the Royal Astronomical Society issued: 2023 volume: 521 page: "4323-4348" DOI: 10.1093/mnras/stad799 - id: panuzzo2024 type: article-journal author: [{literal: "Gaia Collaboration"}, {family: Panuzzo, given: P.}, {literal: "et al."}] title: "Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry" container-title: Astronomy & Astrophysics issued: 2024 volume: 686 page: "L2" DOI: 10.1051/0004-6361/202449763 - id: sahu2025 type: article-journal author: [{family: Sahu, given: Kailash C.}, {literal: "et al."}] title: "OGLE-2011-BLG-0462: An Isolated Stellar-mass Black Hole Confirmed Using New HST Astrometry and Updated Photometry" container-title: The Astrophysical Journal issued: 2025 volume: 983 page: "104" DOI: 10.3847/1538-4357/adbe6e - id: haberle2024 type: article-journal author: [{family: Häberle, given: Maximilian}, {literal: "et al."}] title: "Fast-moving stars around an intermediate-mass black hole in ω Centauri" container-title: Nature issued: 2024 volume: 631 page: "285-288" DOI: 10.1038/s41586-024-07511-z - id: abbott2016 type: article-journal author: [{family: Abbott, given: B. P.}, {literal: "et al. (LIGO Scientific Collaboration and Virgo Collaboration)"}] title: "Observation of Gravitational Waves from a Binary Black Hole Merger" container-title: Physical Review Letters issued: 2016 volume: 116 page: "061102" DOI: 10.1103/PhysRevLett.116.061102 - id: abbott2020 type: article-journal author: [{family: Abbott, given: R.}, {literal: "et al. (LIGO Scientific Collaboration and Virgo Collaboration)"}] title: "GW190521: A Binary Black Hole Merger with a Total Mass of 150 solar masses" container-title: Physical Review Letters issued: 2020 volume: 125 page: "101102" DOI: 10.1103/PhysRevLett.125.101102 - id: gw231123 type: article-journal author: [{literal: "LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration"}] title: "GW231123: A Binary Black Hole Merger with Total Mass 190-265 solar masses" container-title: The Astrophysical Journal Letters issued: 2025 volume: 993 page: "L25" DOI: 10.3847/2041-8213/ae0c9c - id: eht2019a type: article-journal author: [{literal: "Event Horizon Telescope Collaboration"}] title: "First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole" container-title: The Astrophysical Journal Letters issued: 2019 volume: 875 page: "L1" DOI: 10.3847/2041-8213/ab0ec7 - id: eht2019f type: article-journal author: [{literal: "Event Horizon Telescope Collaboration"}] title: "First M87 Event Horizon Telescope Results. VI. The Shadow and Mass of the Central Black Hole" container-title: The Astrophysical Journal Letters issued: 2019 volume: 875 page: "L6" DOI: 10.3847/2041-8213/ab1141 - id: olejak2020 type: article-journal author: [{family: Olejak, given: A.}, {literal: "et al."}] title: "Synthetic catalog of black holes in the Milky Way" container-title: Astronomy & Astrophysics issued: 2020 volume: 638 page: "A94" DOI: 10.1051/0004-6361/201936557 - id: gralla2020 type: article-journal author: [{family: Gralla, given: Samuel E.}, {family: Lupsasca, given: Alexandru}] title: "Null geodesics of the Kerr exterior" container-title: Physical Review D issued: 2020 volume: 101 page: "044032" DOI: 10.1103/PhysRevD.101.044032 images_wanted: - file: File:Black_hole_10_Msun_in_game.png subject: "In-game frame: a 10 solar-mass hole at 30 Schwarzschild radii, seen 55 degrees above its disk; black shadow, lensed star field" source: sim source_ref: "QA:black_hole_final_before_dof_with_sky_translucency_fix/bh_orbit_el55.png" credit: "Pax Abyssi (in-game render)" note: "prefix legend in content/wiki/_notes/writer-c.md; a test-stage capture from the development build" - file: File:Kerr_black_hole_edge_on_a09_sim.png subject: "Offline reference render: a hole spinning at a = 0.9, seen 88 degrees from its axis, thin disk to 12 Schwarzschild radii; the far side of the disk lensed over and under the shadow" source: sim source_ref: "BHIMG:kerr_edge_a09.png" credit: "Pax Abyssi (sim reference render)" note: "prefix legend in content/wiki/_notes/writer-c.md; desk oracle image, 800 px wide; label as a reference render, not an in-game frame" - file: File:M87_black_hole_EHT_2019.jpg subject: "The first image of a black hole: the ring around M87*, Event Horizon Telescope, 2019" source: eso page_url: "https://www.eso.org/public/images/eso1907a/" image_url: "https://cdn.eso.org/images/large/eso1907a.jpg" credit: "EHT Collaboration" licence: "CC BY 4.0" --- A **black hole** is a region of spacetime where gravity is so strong that nothing that crosses its boundary, light included, can come back out. That boundary, the **event horizon**, is a feature of the geometry of space and time, a point of no return with nothing solid there. Black holes form when the cores of very massive stars collapse, and giant ones, millions to billions of times the mass of the Sun, sit at the centres of most large galaxies, including our own ([[Sagittarius A*]]). Once theoretical curiosities, they are now weighed by the stars that orbit them and detected by the gravitational waves they send out when they merge. Two have been photographed against the glowing gas around them. ::figure{src="File:Black_hole_10_Msun_in_game.png" size=wide alt="A black disc ringed by a thin bright line, set in a swirling pale-blue disk of gas, with small stars scattered on a black sky." caption="Sim render: a 10 solar-mass black hole seen from 30 Schwarzschild radii, 55 degrees above its disk. The shadow is black; the stars near it are bent by its gravity."} ## The geometry of a black hole ### Size: the Schwarzschild radius The size of a non-spinning black hole is set by its mass alone. Karl Schwarzschild found the solution of Einstein's equations for a point mass in 1916; its horizon lies at the **Schwarzschild radius** $$ r_\mathrm{s} = \frac{2GM}{c^2} \approx 2.95\ \mathrm{km}\times\frac{M}{M_\odot}, $$ where $G$ is the gravitational constant, $c$ the speed of light and $M_\odot$ the mass of the Sun. Squeeze the Sun into a ball 5.9 km across and it would be a black hole; Earth would need to fit inside a marble 18 mm wide. A typical stellar black hole of 10 solar masses has a horizon about 59 km across, the length of a long commute. Three more radii, all fixed multiples of $r_\mathrm{s}$ for a non-spinning hole, decide what a visitor would see :cite[bardeen1972]: - The **photon sphere**, at $1.5\,r_\mathrm{s}$, is where light itself can orbit, unstably. A photon nudged inward falls in; nudged outward, it escapes. - The **shadow** is the dark patch a distant observer sees. Rays that pass the hole with an impact parameter below $\tfrac{3\sqrt{3}}{2}\,r_\mathrm{s} \approx 2.598\,r_\mathrm{s}$ are captured, so the shadow looks about 2.6 times wider than the horizon: gravity magnifies its own hole. - The **innermost stable circular orbit** (ISCO), at $3\,r_\mathrm{s}$ ($6GM/c^2$), is the closest a particle can circle without spiralling in. It marks the inner edge of a thin accretion disk. :::callout{type=science title="How long is an orbit at the photon sphere?"} In the time kept by a distant clock, light at the photon sphere completes a lap in $2\pi\sqrt{27}\,GM/c^3$, about 161 microseconds per solar mass. Around a 10 solar-mass hole that is 1.6 thousandths of a second; around Sagittarius A*, at 4.3 million solar masses, it is about 11.5 minutes. ::: ### Spin: the Kerr solution Real black holes spin, because the stars that made them did. In 1963 Roy Kerr found the exact solution for a rotating mass :cite[kerr1963], and it is the one astronomers expect every astrophysical black hole to follow. Spin is written as a dimensionless number $a_* = cJ/GM^2$, where $J$ is the angular momentum; it runs from 0 (not spinning) to just below 1 (the fastest rotation a horizon allows). A spinning hole drags spacetime round with it, and the radii that matter move: | Spin $a_*$ | Horizon | Prograde ISCO | Photon orbits (with / against the spin) | Disk efficiency | |---|---|---|---|---| | 0 | $1.000\,r_\mathrm{s}$ | $3.000\,r_\mathrm{s}$ | 1.5 / 1.5 $r_\mathrm{s}$ | 5.7 per cent | | 0.5 | $0.933\,r_\mathrm{s}$ | $2.117\,r_\mathrm{s}$ | | 8.2 per cent | | 0.9 | $0.718\,r_\mathrm{s}$ | $1.160\,r_\mathrm{s}$ | 0.779 / 1.955 $r_\mathrm{s}$ | 15.6 per cent | | 1 (limit) | $0.500\,r_\mathrm{s}$ | $0.500\,r_\mathrm{s}$ | 0.5 / 2.0 $r_\mathrm{s}$ | 42 per cent | The ISCO values follow the formula of Bardeen, Press and Teukolsky :cite[bardeen1972]; the efficiency is the fraction of the rest-mass energy of infalling gas that a thin disk radiates before the gas reaches the ISCO :cite[pagethorne1974]. That last column is why accreting black holes are the most efficient engines in nature: hydrogen fusion in the Sun releases 0.7 per cent of the fuel's mass as energy, while gas spiralling into a fast-spinning hole can release more than twenty times as much. Mass, spin and electric charge are the only properties a black hole keeps. Charge is quickly neutralised by surrounding plasma, so in practice two numbers, $M$ and $a_*$, describe any black hole in the sky. ### Tides and spaghettification Near a black hole, gravity pulls harder on the near side of an object than on the far side. A star of radius $R_\star$ and mass $M_\star$ is torn apart inside the **tidal disruption radius** $$ r_\mathrm{t} \approx R_\star \left(\frac{M_\mathrm{BH}}{M_\star}\right)^{1/3}. $$ For a Sun-like star and a 10 solar-mass hole, $r_\mathrm{t}$ is about 2.2 solar radii (1.5 million km), some 50,000 times the horizon's radius. For a hole of 4.3 million solar masses it is about 160 solar radii (0.76 AU), roughly nine times the horizon. Because $r_\mathrm{t}$ grows only as the cube root of the hole's mass while $r_\mathrm{s}$ grows in direct proportion, the two meet near $10^8$ solar masses: a hole heavier than that swallows a Sun-like star whole, and one lighter shreds it first, producing a months-long flare called a tidal disruption event :cite[rees1988]. ### Hawking radiation Quantum mechanics lets a black hole radiate faintly, at a temperature $T = \hbar c^3 / 8\pi G M k_\mathrm{B}$ :cite[hawking1974], about 60 billionths of a kelvin for one solar mass. Every known black hole is far colder than the 2.7 K cosmic microwave background, so all of them are absorbing more energy than they emit. Hawking radiation has never been observed. ## Kinds of black hole **Stellar black holes** form when the core of a very massive star, typically one born with more than about twenty times the Sun's mass, collapses at the end of its life. Those weighed in the Milky Way range from about 4 to 33 solar masses. Population models put about 130 million of them in the Galaxy, some 7 per cent in binary systems :cite[olejak2020], but almost all are invisible: a black hole shows itself only when something falls in or orbits it. **Intermediate-mass black holes**, from about a hundred to a hundred thousand solar masses, have been hard to find. The strongest case in our Galaxy is in the globular cluster Omega Centauri, where seven stars moving faster than the cluster's escape speed require a central mass of at least 8,200 solar masses :cite[haberle2024]. Gravitational-wave detectors have caught mergers that build them: GW190521 left a remnant of about 142 solar masses :cite[abbott2020], and GW231123 merged holes of about 137 and 101 solar masses, both spinning fast :cite[gw231123]. Those merger masses matter because stellar theory predicts a **pair-instability gap**: stars whose cores would make holes of roughly 60 to 130 solar masses are expected to blow themselves apart instead, leaving nothing. GW231123's heavier component sits in or above that gap, which suggests it grew from earlier mergers :cite[gw231123]. **Supermassive black holes**, from about $10^5$ to $10^{10}$ solar masses, sit in galactic nuclei. How they grew so large so early in cosmic history is an open question. The two imaged so far are [[Sagittarius A*]] and M87*, the 6.5 billion solar-mass hole at the heart of the galaxy Messier 87 :cite[eht2019f]. ## How we know **X-ray binaries.** In 1972 two teams showed that the X-ray source Cygnus X-1 orbits a blue supergiant every 5.6 days, and that the unseen partner was too heavy to be a neutron star :cite[webster1972] :cite[bolton1972]. The method still anchors the field: measure the visible star's orbital speed, and Kepler's laws give a minimum mass for its companion. A compact object above about 3 solar masses, the most a neutron star can hold (see [[Neutron star]]), is taken to be a black hole. **Dormant binaries.** Most black holes in binaries are not feeding. The Gaia spacecraft finds them by the wobble they give their companion star on the sky. Gaia BH1, a 9.62 solar-mass hole circled by a Sun-like star every 185.6 days, is 480 parsecs away, the nearest black hole known :cite[elbadry2023a]; Gaia BH2 has a red giant partner in a 1,277-day orbit :cite[elbadry2023b]; Gaia BH3, at 32.70 solar masses, is the heaviest stellar black hole known in the Galaxy :cite[panuzzo2024]. **Microlensing.** A black hole passing in front of a distant star bends and brightens its light, and shifts its apparent position. The lens of the event OGLE-2011-BLG-0462 emits no light and weighs 7.15 ± 0.83 solar masses: the first isolated stellar black hole, drifting alone 1.52 kiloparsecs away :cite[sahu2025]. **Gravitational waves.** On 14 September 2015 the LIGO detectors recorded the merger of two holes of about 36 and 29 solar masses, which radiated about three solar masses of energy as ripples in spacetime :cite[abbott2016]. Merging black holes are now observed routinely, and their masses and spins form the largest black hole sample we have. **Images of the shadow.** The Event Horizon Telescope links radio dishes across the Earth into a telescope the size of the planet. In 2019 it resolved M87* as a bright ring 42 ± 3 microarcseconds across around a dark centre, as general relativity predicts :cite[eht2019a]. ::figure{src="File:M87_black_hole_EHT_2019.jpg" size=medium alt="A blurred orange ring, brighter at the bottom, around a dark centre on a black background." caption="Observation: the ring of glowing gas around the shadow of M87*, 55 million light years away, imaged by the Event Horizon Telescope at 1.3 mm wavelength. Credit: EHT Collaboration."} **Spin** is the hardest property to measure. Astronomers fit the X-ray spectrum of the inner disk, or the shape of an iron emission line smeared by relativity, and both depend on models of the disk :cite[reynolds2021]. Cygnus X-1 shows the problem: the same data give a spin of about 0.9 with one plausible disk model and 0.1 or less with another :cite[zdziarski2024]. ## Notable black holes | Name | Class | Mass (solar masses) | Distance | Found by | Note | |---|---|---|---|---|---| | Gaia BH1 | stellar | 9.62 ± 0.18 | 480 pc | astrometry | nearest known; no X-rays :cite[elbadry2023a] | | Gaia BH2 | stellar | 8.9 ± 0.3 | 1.16 kpc | astrometry | red giant companion :cite[elbadry2023b] | | Gaia BH3 | stellar | 32.70 ± 0.82 | about 590 pc | astrometry | heaviest stellar hole in the Galaxy :cite[panuzzo2024] | | Cygnus X-1 | stellar | 21.2 ± 2.2, or 12.7 to 17.8 | 2.22 kpc | X-ray binary | first black hole identified; the mass depends on the analysis :cite[millerjones2021] :cite[ramachandran2025] | | V404 Cygni | stellar | 9.0 (+0.2 / -0.6) | 2.39 kpc | X-ray binary | spin probably above 0.92; outbursts in 1989 and 2015 :cite[khargharia2010] :cite[millerjones2009] :cite[walton2017] | | OGLE-2011-BLG-0462 | stellar | 7.15 ± 0.83 | 1.52 kpc | microlensing | isolated, no companion :cite[sahu2025] | | Omega Centauri | intermediate | at least 8,200 | 5.4 kpc | stellar motions | in a globular cluster :cite[haberle2024] | | [[Sagittarius A*]] | supermassive | 4.30 million | 8.28 kpc | stellar orbits, imaging | centre of the Milky Way | | M87* | supermassive | 6.5 billion | 16.8 Mpc | imaging, gas and stars | first black hole imaged :cite[eht2019f] | ## What a black hole looks like up close A black hole with nothing around it is still visible, because it bends the light of every star behind it. The shadow appears as a black disc, and around it the background sky is folded into rings and arcs: each distant star appears twice, once on each side, stretched along the edge. Closer to the shadow the images crowd together into an infinite series of ever-thinner rings, the **photon subrings**. If gas is falling in, it settles into a disk whose inner edge is the ISCO. Gas there moves at a large fraction of the speed of light, so the side coming towards you is brighter and bluer (Doppler beaming) and the receding side dimmer and redder, while the whole disk is reddened by climbing out of the hole's gravity. The disk's far side is lensed up over the top of the shadow and down under it, which is why a black hole seen nearly edge-on seems to wear a halo. ::figure{src="File:Kerr_black_hole_edge_on_a09_sim.png" size=wide alt="A black shadow with a thin bright disk crossing in front of it; the far side of the disk arches over the top and curls under the bottom, brighter on the left." caption="Sim reference render: a hole spinning at a = 0.9, seen almost edge-on. The disk runs in close to the shadow on the side turning towards the viewer, and the shadow is pushed sideways and flattened on that side, as the Kerr geometry predicts."} :::callout{type=sim title="In Pax Abyssi"} **Built, awaiting the lead's verdict.** Five black-hole stops can be flown to: Cygnus X-1 (two poses, one beside its supergiant), Gaia BH1, V404 Cygni and Sagittarius A*. Their masses, companions and orbital periods are read from the game's catalogue of 113 known black holes, each with a published source. A non-spinning hole is drawn with precomputed light-bending tables; a spinning one traces every pixel's light ray through the Kerr geometry in closed form, after Gralla and Lupsasca :cite[gralla2020], and the shadow edge agrees with Bardeen's analytic outline. The disk's colour and brightness come from one formula combining gravitational and Doppler shifts, and the background sky is built from the stars and the Milky Way model as seen from the hole's real position. Spins are ruled choices where the literature disagrees: Cygnus X-1 is drawn at 0.9, V404 Cygni at 0.92, Gaia BH1 at 0 and Sagittarius A* at 0.9. Near a hole the ship switches to a flight regime scaled to the hole's size, a game abstraction. Not yet built: black holes as galaxy-map destinations, generated black holes beyond the catalogue, the hole orbiting its companion, and any consequence of flying inside the horizon. ::: ## See also - [[Sagittarius A*]] - [[Neutron star]] - [[White dwarf]] - [[Supergiant]] - [[Milky Way]] - [[Orbit]] - [[Stellar classification]]