A large enough mass in a compact volume inevitably forms a black hole.
Both inside and outside the event horizon of a Schwarzschild black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape. (ANDREW HAMILTON / JILA / UNIVERSITY OF COLORADO)
In 1964, we observationally detected our first one:
Cygnus X-1. The X-ray emitter Cygnus X-1, in the constellation of Cygnus, as imaged by a balloon-borne telescope. The balloon was launched for the High Energy Replicated Optics (HERO) project on May 23, 2001, reaching an altitude of 39 km. (NASA / MARSHALL SPACE FLIGHT CENTER)
Black holes emit no light, but numerous physical processes can still reveal them.
Cygnus X-1, at left, is an X-ray emitting black hole orbiting another star. Located ~6,000 light-years away in the constellation of Cygnus, it was the first black hole candidate, later confirmed to be a black hole, observed in the Universe: in 1964. (OPTICAL: DSS; ILLUSTRATION: NASA)
Matter infalling into a black hole’s vicinity forms accretion disks.
A black hole feeding off of an accretion disk. It’s friction, heating, and the interplay of charged particles in motion creating electromagnetic forces that can funnel mass inside the event horizon. But at no point does a black hole exert a sucking force; just a standard, run-of-the-mill gravitational one, while much of the external matter gets accelerated and ejected. (MARK GARLICK (UNIVERSITY OF WARWICK))
Once sufficiently heated, that matter emits X-ray light.
When a black hole accretes matter, it grows an accretion disk and will increase its mass as matter gets funneled into the event horizon. The matter outside of the event horizon won’t all fall in; much of it will be accelerated and eventually ejected, emitting radiation of various wavelengths in the process. (NASA/ESA HUBBLE SPACE TELESCOPE COLLABORATION)
X-ray binaries” revealed humanity’s first black holes. The first black holes were detected electromagnetically: as X-ray binaries. The purple points show X-ray black hole binaries; the yellow show X-ray emitting neutron stars. The black hole and neutron star mergers detected from gravitational waves, since 2015 only, are shown in blue and orange, respectively. (LIGO/VIRGO/NORTHWESTERN UNIV./FRANK ELAVSKY)
Supermassive black holes also produce X-rays.
The supermassive black hole at the center of our galaxy, Sagittarius A*, flares brightly in X-rays whenever matter is devoured. In longer wavelengths of light, from infrared to radio, we can see the individual stars in this innermost portion of the galaxy. Gas emissions indicated a supermassive black hole of ~2.7 million solar masses, but improved observations of stars at the galactic center revealed a mass of ~4 million solar masses instead. (X-RAY: NASA/UMASS/D.WANG ET AL., IR: NASA/STSCI)
discovered thousands in its ultra-deep images. A map of the 7 million second exposure of the Chandra Deep Field-South. This region shows hundreds of supermassive black holes, each one in a galaxy far beyond our own. The GOODS-South field, a Hubble project, was chosen to be centered on this original image. Its view of supermassive black holes is only one incredible application of the NASA’s Chandra X-ray observatory. (NASA/CXC/B. LUO ET AL., 2017, APJS, 228, 2)
Energetic black hole outflows create positrons: the electron’s antimatter counterpart.
On either side of the plane of the Milky Way, enormous gamma-ray bubbles are being blown. The energy spectrum seen indicates that positrons had been generated recently in great amounts, creating bubbles some 50,000 light-years in total extent. Both gamma-rays and X-rays are generated, powered by the 4 million solar mass engine at the center of the Milky Way. (NASA/GODDARD SPACE FLIGHT CENTER)
These ejecta generate “
Fermi bubbles” around galactic centers, including our own. In the main image, our galaxy’s antimatter jets are illustrated, blowing ‘Fermi bubbles’ in the halo of gas surrounding our galaxy. In the small, inset image, actual Fermi data shows the gamma-ray emissions resulting from this process, with the red-and-blueshifts indicating that one jet is more pointed towards us and the other an equivalent amount away from us. (DAVID A. AGUILAR (MAIN); NASA/GSFC/FERMI (INSET))
Additionally, gravitational waves reveal inspiraling and merging black holes.
Two black holes of approximately equal mass, when they inspiral and merge, will exhibit the gravitational wave signal (in amplitude and frequency) shown at the bottom of the animation. The gravitational wave signal will spread out in all three dimensions at the speed of light, where it can be detected from billions of light-years away by a sufficient gravitational wave detector. (N. FISCHER, H. PFEIFFER, A. BUONANNO (MAX PLANCK INSTITUTE FOR GRAVITATIONAL PHYSICS), SIMULATING EXTREME SPACETIMES (SXS) COLLABORATION)
But radio studies uncover black holes most abundantly.
This X-ray/radio composite shows a supermassive black hole actively feeding within a distant galaxy. From a great distance, the X-ray emission will often be invisible, but the radio emissions can frequently be seen from active galaxies across the cosmos. (X-RAY: NASA/CXC/KIPAC/N. WERNER ET AL RADIO: NSF/NRAO/AUI/W. COTTON)
Infalling matter around black holes commonly produces radio waves.
This is an artist’s impression of a distant quasar 3C 279. The bipolar jets are a common feature, but it’s extremely uncommon for such a jet to be pointed directly at us. When that occurs, we have a Blazar, now confirmed to be a source of both high-energy cosmic rays and the ultra-high-energy neutrinos we’ve been seeing for years. (ESO/M. KORNMESSER)
This explains the origin of
quasars: QUAsi-StellAr Radio Sources. The Pictor A galaxy has a supermassive black hole at its center, and material falling onto the black hole is driving an enormous beam, or jet, of particles at nearly the speed of light into intergalactic space. This composite image contains X-ray data obtained by Chandra at various times over 15 years (blue) and radio data from the Australia Telescope Compact Array (red). By studying the details of the structure seen in both X-rays and radio waves, scientists might better understand the nature of quasars. (X-RAY: NASA/CXC/UNIV OF HERTFORD)
Supermassive, active black holes emit tremendously powerful radio signals.
When hot gas actively falls onto the central black hole within a galaxy, a quasar can be produced. The radiation can span across the electromagnetic spectrum, but the proper radio survey can reveal even X-ray quiet quasars that an X-ray survey would miss. (NASA/CXC/PENN. STATE/G. YANG ET AL AND NASA/CXC/ICE/M. MEZCUA ET AL.; OPTICAL: NASA/STSCI; ILLUSTRATION: NASA/CXC/A. JUBETT)
The 52-station LOFAR array
just synthesized an unprecedented set of radio data. Installation Manager Derek McKay checks some of the 96 radio antennae installed for the new European Low Frequency Array (LOFAR) telescope. The LOFAR array spans the entire continent of Europe, and is humanity’s most sensitive radio telescope in its particular frequency band. (Chris Ison/PA Images via Getty Images)
Spanning 740 square degrees,
they found 25,247 supermassive black holes. The survey area and the detected signals (in surface brightness) of the LOFAR telescope. Covering 740 square degrees on the sky, or 1.85% of what’s out there, the team identified 25,247 individual sources, where each one is a supermassive black hole. Note how they reveal the clustering of the Universe. (F. DE GASPERIN ET AL. (2021), ARXIV:2102.09238)
reveals the clustering of galaxies; every point is a black hole. When quasar orientation can be observed and identified, it is found that they align in a non-random way with the large-scale cosmic web that defines the Universe’s structure. The LOFAR data is the best-ever quasar data taken of such a significant region of the Universe, and has revealed clustering effects even beyond this one. (ESO/M. KORNMESSER) LOFAR will eventually survey the entire northern hemisphere, expecting ~600,000+ identifiable black holes. Thus far, LOFAR has only observed where the yellow dots are indicated: about 2% of the total sky. By the end of 2022, it will have observed everywhere the red dots are located, and its eventual goal is to survey the entire northern hemisphere. At its current sensitivity, LOFAR can expect a total yield of over 600,000 quasars. (F. DE GASPERIN ET AL. (2021), ARXIV:2102.09238)
Observationally abundant, black holes aren’t purely theoretical anymore.
This 20-year time-lapse of stars near the center of our galaxy comes from the ESO, published in 2018. Note how the resolution and sensitivity of the features sharpen and improve towards the end, and how the central stars all orbit an invisible point: our galaxy’s central black hole, matching the predictions of Einstein’s general relativity. (ESO/MPE)