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Understanding Black Holes
What is it?
It is a great amount of matter packed into a very small area - think of a star ten times more massive than the Sun squeezed into a sphere approximately the diameter of New York City. The result is a gravitational field so strong that nothing, not even light, can escape. In recent years, we have painted a new picture of these strange objects that are, to many, the most fascinating objects in space.
Although the term was not coined until 1967 by Princeton physicist John Wheeler, the idea of an object in space so massive and dense that light could not escape it has been around for centuries. Most famously, black holes were predicted by Einstein's theory of general relativity, which showed that when a massive star dies, it leaves behind a small, dense remnant core. If the core's mass is more than about three times the mass of the Sun, the equations showed, the force of gravity overwhelms all other forces and produces a black hole.
Detecting Black Holes
Scientists can't directly observe black holes with telescopes that detect light. We can, however, infer the presence of black holes and study them by detecting their effect on other matter nearby (if a black hole passes through a cloud of interstellar matter, for example, it will draw matter inward in a process known as accretion), or based in other forms of electromagnetic radiation.
Another interesting way to observe them is through the distortion they cause in light in a process called gravitational lensing:
Gravitational lensing effect, which produces two enlarged but highly distorted views of the interstellar background, since the gravity of a black hole is enough to curve light.
Structure of a Black Hole:
Event Horizon:
The defining feature of a black hole is the appearance of an event horizon, a boundary in space-time through which matter and light can only pass inward towards the mass of the black hole. Nothing, not even light, can escape from inside the event horizon. The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.
Ergosphere:
Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still, called the ergosphere. This is the result of a process known as frame-dragging; general relativity predicts that any rotating mass will tend to slightly "drag" along the spacetime immediately surrounding it. Any object near the rotating mass will tend to start moving in the direction of rotation. For a rotating black hole, this effect becomes so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.
Singularity:
At the center of a black hole as described by general relativity lies a gravitational singularity, a region where the space-time curvature becomes infinite. For a non-rotating black hole, this region takes the shape of a single point and for a rotating black hole, it is smeared out to form a ring singularity lying in the plane of rotation. In both cases, the singular region has zero volume. It can also be shown that the singular region contains all the mass of the black hole solution. The singular region can thus be thought of as having infinite density. It can be described as a mathematical limit: lim (d)→∞ and lim (s)→0, where (d) = density and (s) = space.
Death of a Black Hole:
In 1974, Hawking showed that black holes are not entirely black but emit small amounts of thermal radiation; an effect that has become known as Hawking radiation. By applying quantum field theory to a static black hole background, he determined that a black hole should emit particles in a perfect black body spectrum. Since Hawking's publication, many others have verified the result through various approaches. If Hawking's theory of black hole radiation is correct, then black holes are expected to shrink and evaporate over time because they lose mass by the emission of photons and other particles The temperature of this thermal spectrum (Hawking temperature) is proportional to the surface gravity of the black hole. Hence, large black holes emit less radiation than small black holes, and take longer to die.
Curiosities
The closest:
V404 Cygni is a binary star system consisting of a black hole with a mass of about 12±3 solar masses, and lies at a distance of 7.800 light-years from Earth.
Monster in the centre of the Milky Way:
Supermassive blacks holes are the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most (and possibly all) galaxies, including the Milky Way are believed to contain supermassive black holes at their centers.
Sagittarius A*, located at a distance of 26.000 light-years from Earth, is a bright and very compact astronomical radio source at the center of the Milky Way Galaxy, near the border of the constellations Sagittarius and Scorpius, and it is believed to be the location of a supermassive black hole with a mass of 4.31 ± 0.38 million solar masses.
Gamma-ray Burst:
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe.
Most observed GRBs are believed to consist of a narrow beam of intense radiation released during a supernova as a rapidly rotating, high-mass star collapses to form a neutron star, quark star, or black hole.
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Originally Posted by Pretan
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