Black Holes:
- Black holes are the regions of spacetime where gravity is so strong that nothing, including light and other electromagnetic waves, has enough energy to escape. The boundary of no escape is called the event horizon.
- Formation: A black hole forms when a massive star (at least three times the mass of our Sun), exhausts its fuel, explodes in a supernova, and collapses under gravity into an incredibly dense core called a singularity.
- Types:
- Stellar Black Hole: Formed by the collapse of a single massive star.
- Intermediate Black Hole: Formed by the collapse of a star having mass between 100 and 1,00,000 times that of our sun.
- Supermassive Black Hole: Masses ranging from millions to billions of times that of the sun, found at the centres of most galaxies.
- Black holes are not directly observable with telescopes that detect X-rays, light, or other forms of electromagnetic radiation. However, their presence can be inferred through their effects on surrounding matter and the gravitational waves they produce.
- E.g., If a black hole passes through a cloud of interstellar matter or if a star passes close to a black hole, it will draw matter inward in a process known as accretion. As the attracted matter accelerates and heats up, it emits X-rays and powerful gamma ray bursts that radiate into space. This reflects the presence of black holes.
- Merger of two blackholes produces powerful gravitational waves. The detection of these gravitational waves (through LIGO - Laser Interferometer Gravitational-Wave Observatory) can confirm the existence/ location of the black holes.
Terms related to black hole:
- Supernova: Supernovae are incredibly powerful explosions that occur when a massive supergiant star reaches the end of its life (exhausts its fuel). These explosions release an astonishing amount of energy, up to 10^44 joules.
- Singularity: The centre of a black hole is a gravitational singularity, a point where the general theory of relativity breaks down, i.e. where its predictions do not apply. A black hole’s great gravitational pull emerges as if from the singularity.
- Event Horizon (a point of no return): The event horizon is like a boundary around a black hole (around the singularity). Once anything (matter, energy, light) crosses this boundary, it can not escape unless it travels faster than the speed of light (which is impossible). This means nothing, not even light, can escape the black hole's strong gravity because the speed needed to escape at the event horizon should be greater than the speed of light.
- Ergosphere: The Ergosphere is a bigger sphere, outside the event horizon of a black hole, where matter and even light can enter and then return (escape the black hole's gravitational pull), if they are moving fast enough (with speeds comparable to the speed of light).
- Both rotating (Kerr) black holes and non-rotating (Schwarzschild) black holes have an ergosphere. In the case of rotating black holes, the ergosphere is larger and more elongated compared to non-rotating black holes.
- In the Ergosphere, spacetime is dragged along with the rotation of the black hole. It is theoretically possible to extract energy and angular momentum from the ergosphere via the Penrose process. (The Penrose process suggests a way to take energy and spin from the ergosphere of a rotating black hole.)
- Some scientists have suggested using this possibility to send an object into the ergosphere and allow it to accelerate there along the black hole’s direction of rotation, so that it comes out of the black hole moving faster.
- This energy ‘gain’ of the object will translate to the black hole losing some angular momentum.
- Accretion disc: An accretion disc is a flat, rotating structure of matter (such as gas, dust, or other material) that forms around a black hole. The material in the accretion disc spirals inward due to gravitational attraction of the black hole. As it spirals inward, the material often heats up due to friction and gravitational forces, emitting various forms of electromagnetic radiation, including visible light, X-rays, gamma rays and radio waves.
- Spaghettification: Spaghettification refers to the effect of extreme gravitational pressure on any particle or body of matter, in particular, when exposed to the extreme forces of the black hole.
When a particle draws too close to the event horizon, it is stretched into long thin shapes. E.g., If an astronaut falls into the event horizon, as the gravity is inversely proportional to distance, the pull on the falling astronaut’s legs will be substantially greater than the pull on his or her upper torso. Subsequently, stretching him like spaghetti (pasta).
