A LIBRARY OF DISCOVERIES
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Results 11
Black Holes, Accretion Disks, and Quasars
A black hole itself cannot be observed directly: light cannot escape from inside its event horizon. But gas and dust falling toward one can form an accretion disk. Gravity and friction heat the material, and the disk shines brightly. When a supermassive black hole is actively feeding, a galaxy’s cen…
A black hole is not a cosmic vacuum cleaner
At Earth’s distance, a black hole with the Sun’s mass would pull on Earth almost the same way the Sun does: the orbit depends on mass, not on how tightly that mass is packed. So in the imagined scenario where the Sun is instantly replaced by a same-mass black hole, Earth would keep orbiting on nearl…
Spaghettification near a black hole
A black hole pulls more strongly on the near side of an object than on the far side, and this difference creates tidal forces that can stretch and tear it apart. This is called spaghettification. It does not have to begin at the event horizon: near a stellar-mass black hole, disruption can happen be…
Is a black hole a sphere or a funnel?
A funnel-shaped black hole is a diagram of curved space, not its actual shape. For a nonrotating black hole, the boundary beyond which light cannot escape is a spherical event horizon; it is not a solid surface. Real black holes usually rotate, so their geometry is more complex than an ordinary ball…
Gravity, time, and spacetime near a black hole
According to general relativity, a black hole’s mass curves spacetime, and a clock closer to it accumulates less time than a clock far away. An observer beside the clock does not notice it falling behind; the difference appears only when it is compared with a distant clock or its signal. Near the ev…
What happens near a black hole
The supermassive black hole Sagittarius A lies at the center of the Milky Way. Beyond its event horizon, neither light nor a signal can escape. But saying that any life must die instantly at the boundary is too categorical: near the horizon of a very massive black hole, tidal forces can be weaker, s…
A “teaspoon” of neutron-star matter is only a thought experiment
…e star’s pressure, its state would change. The analogy does not work for a black hole. A black hole is not described as a lump of ordinary matter with a uniform density that could be measured by the spoonful. The event horizon is the black hole’s boundary; classical general relativity predicts a sin…
Singularity: a limit of classical theory
In classical general relativity, a black-hole singularity is where calculations predict infinite spacetime curvature. This is a mathematical prediction, not a confirmed description of what physically happens at the center: it may mean classical theory can no longer be used without quantum gravity. T…
Hawking radiation: a theoretical prediction
Quantum theory in curved spacetime predicts that black holes emit energy, a process called Hawking radiation. As a black hole loses energy, it also loses mass, but for ordinary astrophysical black holes the process should be extraordinarily slow. This radiation has not yet been directly detected fro…
Sagittarius A*
At the center of the Milky Way is the supermassive black hole Sagittarius A, or Sgr A. The orbits of nearby stars show that the unseen compact object has a mass of about four million Suns. In 2022, the Event Horizon Telescope released an image of Sgr A’s shadow against hot gas; this radio observatio…
What remnant will a star leave?
… can end in a supernova: their cores collapse, leaving a neutron star or a black hole. The key is not just the star’s initial mass, but how much material remains in its core after its evolution and loss of outer layers.