A black hole is an astronomical body with gravity so intense that even light cannot escape it. Predicted by Einstein's general relativity, it forms when a mass becomes sufficiently compact. Key features include the event horizon, the boundary beyond which escape is impossible, and the singularity, a point where spacetime curvature is infinite. Surprisingly, black holes act like ideal black bodies, absorbing and not reflecting light. Quantum theory suggests they emit Hawking radiation, making them possibly visible over astronomical timescales.

The idea of black holes dates back to the 18th century when Michell and Laplace considered objects with gravity too strong for light to escape. In 1916, Schwarzschild provided the first solution of general relativity hinting at black holes, although they weren't recognized as physical entities until the 1960s. The term 'black hole' was popularized by Wheeler in the late 1960s. Since then, comprehensive theoretical and observational studies have established black holes as a scientific reality.

Black holes are categorized by mass: Stellar black holes form from collapsing stars; Intermediate black holes may form from mergers of stellar black holes; Supermassive black holes, believed to be at the centers of galaxies, possibly originate from massive stars in the early universe. Primordial black holes could have formed soon after the Big Bang. Sizes range from a few kilometers for stellar black holes to several astronomical units for supermassive ones.

At a black hole's core is the singularity, a point where density becomes infinite and the laws of physics as currently understood cease to operate. Surrounding this is the event horizon, the 'point of no return' beyond which nothing can escape. The properties of event horizons differ based on the black hole's spin and charge. For instance, charged black holes present additional structures like the Cauchy horizon.

Hawking radiation is a theoretical prediction that black holes emit radiation due to quantum effects near the event horizon. This emission causes black holes to lose mass and could ultimately lead to their evaporation over extreme timescales. The predicted temperature of this radiation is inversely proportional to the black hole's mass. For stellar black holes, the temperature is exceedingly low, making direct observation challenging.

Black hole thermodynamics draws parallels between the laws of black holes and classical thermodynamics. It includes the four laws akin to thermodynamics, where the surface area of a black hole relates to entropy, and the surface gravity to temperature. This analogy is bolstered by Hawking's discovery of radiation emitted from the black hole, which showcases the interplay between quantum mechanics and macroscopic gravitational systems.

Matter falling into a black hole often forms an accretion disk, heated to extreme temperatures by friction and eventually emitting significant electromagnetic radiation. Some black holes, particularly those with strong magnetic fields and high spin, may also exhibit relativistic jets. These jets are streams of material expelled at nearly the speed of light along the poles of black holes, a process not yet fully understood but essential in transporting energy and affecting cosmic environments.

Supermassive black holes at galaxy centers significantly influence galactic dynamics and star formation. When a supermassive black hole is actively accreting matter, the resulting quasar can become extremely luminous, outshining its host galaxy. The correlation between black hole mass and stellar velocity dispersions in galaxies suggests a link between black holes and their host galaxies' evolution.

Black hole mergers produce gravitational waves—ripples in spacetime detected by observatories like LIGO and Virgo. The first detection in 2015 provided direct evidence of black holes, confirming predictions of general relativity. Gravitational waves allow astronomers to study black holes through mass estimations and spin measurements, expanding our understanding of these phenomena beyond electromagnetic observations.

The information paradox arises from the question of whether information that falls into a black hole is lost forever, conflicting with principles of quantum mechanics that assert information must be preserved. Hawking's radiation suggests black holes evaporate over time, seemingly destroying the information they absorbed. Resolving this paradox is believed to be crucial for progress in quantum gravity research, aiming to unify quantum mechanics and general relativity.

In 2019, the Event Horizon Telescope collaboration released the first direct image of a black hole's event horizon, located in galaxy M87. This achievement was a monumental step in astrophysics, confirming theoretical predictions and demonstrating the capabilities of earth-scale radio telescope arrays. In 2022, the EHT team released an image of Sagittarius A*, the black hole at our galaxy's center, providing new insights into supermassive black hole characteristics.

Black holes can spin, possessing angular momentum, which impacts the dynamics of accretion disks and the potential for relativistic jet formation. The measurement of a black hole's spin offers insights into its evolutionary history, since spin rates can indicate past mergers or accretions. High-resolution observations of accretion disks or gravitational wave data can help determine these spin characteristics.

The term 'black hole' was popularized by physicist John Wheeler in the 1960s for its simplicity and evocative nature. It gained acceptance even though similar concepts had been discussed for centuries, like 'dark stars' proposed by Michell and Laplace in the 18th century. The term succinctly captures the idea of a region in space from which nothing can escape.

The discovery that virtually all large galaxies host supermassive black holes suggests a close relationship between black holes and the galaxies they inhabit. The correlation between black hole mass and galactic bulge properties indicates that their growth may be intertwined with star formation and galactic mergers, shaping the evolutionary pathways of galaxies over cosmic timescales.

Several theoretical models propose alternatives to traditional black holes, such as gravastars and dark energy stars, which could mimic black holes without singularities. These models aim to address singularity issues and information paradox concerns, suggesting that our understanding of the ultimate fate of collapsed stars might need revision. The viability of these alternatives hinges on reconciling them with current observational data.