1: A black hole is formed when a large star starts running out of fuel and begins to collapse under its own gravity. Such a star may become a white dwarf or a neutron star, but if the star is sufficiently massive then it may continue shrinking eventually to the size of a tiny atom, known as a singularity or black hole.
2: The mass of that shrunken star is so dense and the gravity of that singularity so strong that, in accordance with Einstein‘s theory of general relativity, it actually deforms the space-time around it and not even light can escape. The boundary beyond which light cannot escape the black hole is known as the event horizon, while its radius is called the Schwarzschild radius.
3: Once particles and light-rays go past the event horizon their light cones “tip over” and point to the singularity, which now represents all future-directed paths with no escape possible.
4: To an outside observer with a telescope, an object passing the event horizon will appear to slow down then freeze without ever seeming to pass through the horizon. This is because the light takes longer to escape its gravitational pull and light signals won’t reach the viewer for an infinitely long time. As time elapses, the light becomes red shifted and dimmer as its wavelength becomes longer, eventually disappearing from the sight of the observer as it becomes infrared radiation, then radio waves.
5: If a person was able to survive long enough to describe falling into a black hole, he would at first experience weightless as he goes into free fall, but then feel intense “tidal” gravitational forces as he got closer to the center of the black hole. In other words, if his feet were closer to the centre than his head, then they would feel a stronger pull until he eventually is stretched and then ripped apart. As he falls in he may observe distorted images as the light bends around him and he will also still be able to see beyond the black hole as light continues to reach him from the outside.
6: It is important to realize that a black hole’s gravitational field is the same as that of any other object in space of the same mass. In other words, it won’t “suck” objects in any more than any other normal star, with things being more likely to just fall into them if they got too close to the event horizon.
7: A wormhole, known alternatively as a Lorentzian wormhole, Schwarzschild wormhole or Einstein-Rosen bridge, is a theoretical opening in space-time allowing a “shortcut” through intervening space to another location in the Universe. However, from the outside wormholes may exhibit many of the characteristics usually associated with a black hole and be virtually impossible to tell apart.
8: John Michell (1783) and Pierre-Simon Laplace (1796) were the first people to propose the concept of “dark stars” or object which, if compressed into a small enough radius, would have an escape velocity which exceeded even the speed of light. Later, the term “frozen star” was used to describe the last phase of a star’s gravitational collapse, when light unable to escape from its surface would make the star appear frozen in time to an observer. In the 20th century, John Wheeler eventually coined the phrase “black hole” as the object would absorbs all the light that hits it while reflecting nothing back.
9: Physicists now believe that black holes actually radiate small numbers of mainly photon particles and so can lose mass, shrink then ultimately vanish over time. This unverified evaporation process is known as “Hawking Radiation”, after Professor Stephen Hawking who theorized its existence in 1974. However, it is a staggeringly slow process and only the smallest black holes would have had time to evaporate significantly during the 14 billion years the Universe has existed.
10: It is now thought that most galaxies are held together by supermassive black holes at their centers, which cluster hundreds of solar systems around them.
credit to astronomytrek
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