Black holes sound like they’re straight out of a science fiction story: objects so dense that nothing in the universe can escape from their gravitational pull. But over the past few decades astronomers have been steadily building up evidence that black holes are not only real, but, in fact, quite prevalent in the universe.
It is now thought that almost all galaxies contain gigantic black holes in their centers, millions or even billions of times more massive than the Sun. Some of these beasts are among the most violent and energetic objects in the universe – active galactic nuclei and quasars, which shoot off jets even as they suck in surrounding gas – while others, often older ones like the black hole at the center of the Milky Way, are considerably more quiet feeders.
Galaxies are also thought to contain many examples of small black holes, with masses only a few times greater than that of the Sun. Astronomers have detected a handful of these in our galaxy, by observing the light emitted when they shred apart their companion star in a binary system. Several of these small black holes have been dubbed “microquasars” because they produce miniature jets akin to those of their larger cousins.
Theory of Black Holes
Though the concept of a black hole was first proposed in 1783, it was Albert Einstein’s 1915 theory of general relativity which put the idea on a firm theoretical footing. Einstein showed that gravity can bend the path of light just as it bends the path of any other moving object – the only reason we don’t observe this effect in our daily lives is that light moves fast and gravity pulls weak. When this was confirmed by observations, the idea of a black hole became obvious. If you pack enough material together, its gravitational pull should be strong enough to not only bend light’s path but also keep it from escaping, just as the Earth is strong enough to pull back much slower objects (like baseballs) to its surface.
Formation of Black Holes
Regular black holes are thought to form from heavy stars (perhaps those which start off with masses more than 20 or 25 times that of the Sun, but this is still an area of active research). When these stars end their lives in a supernova explosion, their cores collapse and gravity wins out over any other force that might be able to hold the star up.
Eventually, the star collapses so much that it is contained within its Schwarzschild radius, or event horizon, the boundary within which light cannot escape. At this point, the black hole is extremely tiny; a black hole with the mass of the Sun would fit in a small town, while one with the mass of the Earth would fit in the palm of your hand! The material inside the Schwarzschild radius will continue to collapse indefinitely, reaching the point where our understanding of the laws of physics breaks down. But no information from inside the Schwarzschild radius can escape to the outside world.
Supermassive black holes, meanwhile, form differently – perhaps from the merger of many smaller black holes early in the universe’s history – and grow over the years as they suck in gas from their surroundings. The formation of these objects and their relationship to the galaxy that harbors them is still an area of active research.
In the past couple of decades scientists concluded that black holes are materials that can have infinite density if they reached singularity, unlike the older belief which suggest that black holes are just emptiness !
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