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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Black hole | 11/13 | https://en.wikipedia.org/wiki/Black_hole | reference | science, encyclopedia | 2026-05-05T13:31:52.244381+00:00 | kb-cron |
== Observational evidence == Millions of black holes derived from stellar collapse are expected to exist in the Milky Way. Even a dwarf galaxy like Draco should have hundreds. Only a few of these have been detected. By nature, black holes do not themselves emit any electromagnetic radiation other than the hypothetical, typically extremely weak Hawking radiation, so astrophysicists searching for black holes must rely on indirect observations. The defining characteristic of a black hole is its event horizon. The horizon itself cannot be imaged, so all other possible explanations for these indirect observations must be considered and eliminated before concluding that a black hole has been observed.
=== Direct interferometry ===
The Event Horizon Telescope (EHT) is a global system of radio telescopes capable of directly observing a black hole shadow. The angular resolution of a telescope is based on its aperture and the wavelengths it is observing. Because the angular diameters of Sagittarius A* and Messier 87* in the sky are very small, a single telescope would need to be about the size of the Earth to clearly distinguish their horizons using radio wavelengths. By combining data from several different radio telescopes around the world, the Event Horizon Telescope creates an effective aperture the diameter size of the Earth. The EHT team used imaging algorithms to compute the most probable image from the data in its observations of Sagittarius A* and M87*.
=== Gravitational waves === Gravitational-wave interferometry can be used to detect merging black holes and other compact objects. In this method, a laser beam is split, sent down two long arms of a tunnel, then reflected at the far end of the tunnels to reconverge at the intersection of the arms, precisely cancelling each other. However, when a gravitational wave passes, it warps spacetime, changing the relative lengths of the arms themselves. Since each laser beam is now travelling a slightly different distance, they do not cancel out and produce a recognisable signal. Analysis of the signal can give scientists information about what caused the gravitational waves. Since gravitational waves are very weak, gravitational-wave observatories such as LIGO must have arms several kilometres long and carefully control for noise from Earth to be able to detect these gravitational waves. Since the first measurements in 2016, multiple gravitational waves from black holes have been detected and analysed.
=== Stars orbiting Sagittarius A* ===
The proper motions of stars near the centre of the Milky Way provide strong observational evidence that these stars are orbiting a supermassive black hole. Astronomers have tracked the motions of over 100 stars orbiting an invisible object coincident with the radio source Sagittarius A*. One of the stars—called S2—completed a full orbit. By fitting the motions of stars to Keplerian orbits, the astronomers were able to infer that the invisible object assumed to be Sagittarius A* must have a mass of 4.3×106 M☉, with a radius of less than 0.002 light-years. This upper limit radius is larger than the Schwarzschild radius for the estimated mass, so the combination does not prove Sagittarius A* is a black hole. Nevertheless, these observations strongly suggest that the central object is a supermassive black hole as there are no other plausible scenarios for confining so much invisible mass into such a small volume. Additionally, luminosity data from this object implies it must possess an event horizon, a defining feature of black holes. By tracking the motion of the center it has been shown that the central object is motionless at the center of the galaxy. The Event Horizon Telescope image of Sagittarius A*, released in 2022, provided further confirmation that it is indeed a black hole.
=== Binaries ===
X-ray binaries are binary systems that emit significant amounts of X-ray radiation. These X-ray emissions result when a compact object accretes matter from an ordinary star. The presence of an ordinary star in such a system provides an opportunity for studying the central object and to determine if it might be a black hole. By measuring the orbital period of the binary, the distance to the binary from Earth, and the mass of the companion star, scientists can estimate the mass of the compact object. The Tolman-Oppenheimer-Volkoff limit (TOV limit) dictates the largest mass a nonrotating neutron star can be, and is estimated to be about two solar masses. While a rotating neutron star can be slightly more massive, if the compact object is much more massive than the TOV limit, it cannot be a neutron star and is generally expected to be a black hole. X-ray binaries can be categorised as either low-mass or high-mass; This classification is based on the mass of the companion star, not the compact object itself. In a class of X-ray binaries called soft X-ray transients, the companion star is of relatively low mass, allowing for more accurate estimates of the black hole mass. These systems actively emit X-rays for only several months once every 10–50 years. During the period of low X-ray emission, called quiescence, the accretion disk is extremely faint, allowing detailed observation of the companion star. Numerous black hole candidates have been measured by this method. Black holes are also sometimes found in binaries with other compact objects, such as white dwarfs, neutron stars, and other black holes.
=== Galactic nuclei === The centre of nearly every large galaxy contains a supermassive black hole. The close observational correlation between the mass of this hole and the velocity dispersion of the host galaxy's bulge, known as the M–sigma relation, strongly suggests a connection between the formation of the black hole and that of the galaxy itself. In some galaxies, the black hole forms a powerful source of radiation called an active galactic nucleus.
==== Active galactic nucleus ====