iv. Black holes: A simple explanation

What is a black hole?	Black holes are extremely dense points in space that create deep gravity sinks from which even light cannot escape. It can be formed by the death of a massive star. A black hole takes up zero space but does have mass, that used to be a star. And black holes get more massive as they consume matter near them. The bigger they are, the larger a zone of “no return” they have, where anything entering their territory is irrevocably lost to the black hole. This point of no return is called the event horizon. When a massive star (more than 8 times bigger than the Sun) runs out of the thermonuclear fuel in its core- signifies the end of its life and the core becomes unstable. Then its gravity caused the core to collapse upon itself. This huge weight of its constituent matter falling in compresses the dying star to a point of zero volume and infinite density- called the singularity. The concept was given by Albert Einstein in 1915 but the term ‘black hole’ was coined in the 1960s by American physicist John Archibald Wheeler.
The black holes belong to two categories	One ranges between a few solar masses and tens of solar masses. These are thought to form when massive stars die. The other is supermassive black holes. These range from hundreds of thousands to billions of times that of the sun from the Solar system to which Earth belongs. A black hole cannot be observed but only detected by the effects of its enormous gravitational fields on nearby matter. Since any matter flowing into the black hole becomes intensely heated, it radiates x-rays before entering the event horizon and disappearing forever. These x-rays are detected and the radio images define our idea of the black hole. Supermassive black holes At the centre of most galaxies including our Milky Way, there is a supermassive black hole. Sometimes these supermassive black holes collect disks of gas, dust, and stellar debris around them- when they fall into the black hole. Its gravitational energy can be converted into light. This process makes the centres of the galaxies very bright and is called Active Galactic Nuclei (AGN). At times the AGN shoots out jets of matter at the speed of light from its center called a ‘quasar’. And when a galaxy is oriented such that these jets shoot towards the direction of the earth, it’s called ‘blazar’. In simpler words, a quasar and a blazer are the same things but are pointed at different angles.
Latest observations on black holes	April 2019: The scientists at the Event Horizon Telescope project released the first-ever image of a black hole. January 2021: The Indian astronomers of ARIES institute reported one of the strongest flares from a blazer called BL Lacertae from 10 million light-years away. *NOTE:* A light-year is the distance light travels in one Earth year. One light-year is about 6 trillion miles (9 trillion km). Light travels at a speed of 186,000 miles (or 300,000 km) per second. Our Sun is the closest star to us. It is about 93 million miles away. So, the Sun’s light takes about 8.3 minutes to reach us. This means that we always see the Sun as it was about 8.3 minutes ago. August 2021: Indian scientists have discovered the merger of three supermassive black holes from as many galaxies to form a triple Active Galactic Nucleus. This suggests that such group mergers also drive the growth of black holes.
Black hole triple system	A black hole triple system is a rare and complex astronomical setup in which three black holes orbit each other in a mutual gravitational dance. These systems can form in several ways, such as through galaxy mergers, which lead to the interaction and eventual binding of the supermassive black holes at the centres of the merging galaxies. Alternatively, some black hole triples might form through dense stellar environments where gravitational interactions lead to multiple black holes coalescing into close orbits.
Characteristics and Dynamics:	Hierarchical Structure: Triple black hole systems often exhibit a hierarchical structure where two of the black holes form a close binary, and the third orbits this binary at a greater distance. This setup allows the outer black hole to exert periodic gravitational influences on the inner binary, leading to unique dynamics like the Lidov-Kozai effect, where the orbits oscillate in eccentricity and inclination. Gravitational Wave Emissions: The close interactions and gravitational pulls in a black hole triple system can accelerate mergers and increase the production of gravitational waves. These emissions offer valuable data for observatories like LIGO and Virgo, which detect such waves and help researchers understand black hole dynamics in multiple systems. Potential for High-energy Events: Interactions among black holes in these systems can generate intense energy outputs, including relativistic jets and high-energy radiation. These interactions also provide clues about galaxy formation and the conditions in dense star clusters. V404 Cygni System The V404 Cygni system is a well-known binary star system that consists of a black hole and a companion star. It lies in the constellation Cygnus, approximately 7,800 light-years from Earth. V404 Cygni gained attention because of its dramatic outbursts and high-energy emissions, which have provided scientists with important insights into black hole behaviour and accretion dynamics. Binary System with a Black Hole: The system consists of a stellar-mass black hole with a mass estimated at around 9-12 times that of the Sun and a companion star, likely a G or K-type star. The companion star orbits the black hole every 6.5 days. Accretion Disk and Outbursts: The black hole’s intense gravitational pull draws matter from its companion star, forming an accretion disk around the black hole. Occasionally, this disk undergoes periods of instability, resulting in significant outbursts of X-ray, gamma-ray, and optical radiation. These outbursts, observed in 1938, 1989, and 2015, are some of the most powerful recorded for a black hole binary, and they provide valuable data on how black holes consume matter. Relativistic Jets: The black hole occasionally ejects relativistic jets of particles during outbursts. These jets are believed to form when matter near the event horizon interacts with magnetic fields, resulting in streams of particles that are shot into space at nearly the speed of light. Significance for Black Hole Studies: V404 Cygni’s behaviour offers clues about black hole accretion mechanisms and jet formation. Unlike many black hole systems, V404 Cygni’s accretion disk shows extreme variability, which has helped scientists explore how accretion dynamics can vary in response to environmental changes. Observational Highlights and Challenges: Due to its active nature, V404 Cygni has been a focus for multi-wavelength observations. Its activity has been captured by observatories such as the Chandra X-ray Observatory, Swift, Hubble, and ground-based radio telescopes. One challenge in studying V404 Cygni, however, is predicting its outbursts, as they do not follow a regular pattern, adding complexity to long-term monitoring and observational campaigns.
Low-Supermassive blackholes	2025: An international team of researchers using NASA’s James Webb Space Telescope (JWST) and the Chandra X-ray Observatory has discovered a bizarre black hole that may provide insights into the genesis and growth of supermassive black holes. LID-568 Black Hole LID-568 is a low-mass supermassive black hole that existed approximately 1.5 billion years after the Big Bang. It was found in a galaxy with minimal star formation, likely due to the powerful outflows from the black hole. Super-Eddington Accretion: LID-568 accretes matter at a rate 40 times the Eddington limit. The Eddington Limit represents the point at which the outward radiation pressure balances gravitational pull, limiting further accretion. Exceeding this limit, as LID-568 does, suggests a unique and rapid feeding process. Galaxy Impact: The powerful outflows from the black hole prevent the accumulation of gas and matter, halting star formation in its host galaxy. Demonstrates the feedback mechanism of black holes that regulate galaxy evolution. Significance: Challenges to Existing Models: Current models suggest that supermassive black holes grow gradually through sustained accretion. LID-568’s rapid and intense growth contradicts these theories, highlighting the need to reconsider early black hole formation mechanisms. Early Universe Insights: The existence of such a rapidly growing black hole just 1.5 billion years after the Big Bang suggests that short bursts of intense feeding could play a significant role in forming large black holes in the early universe. Indicates that early black hole growth may not require extended accretion periods. Research Opportunities: Studying LID-568 offers insights into: The dynamics of super-Eddington accretion. The interaction between black holes and their host galaxies. The role of feedback in halting star formation. Future Research Directions: Exploring black hole accretion processes under extreme conditions. Studying similar low-mass supermassive black holes to determine if LID-568’s behaviour is common. Investigating the impact of black hole outflows on galaxy evolution during the early universe.