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Black Holes

Black Holes

What is a Black Hole ?

One of the most awe inspiring phenomenons in space is the black hole. According to the definition of Wikipedia, A black hole is a region of space-time from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform space-time to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called “black” because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Blackhole Milkyway

How big is a black hole?

There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up.

There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the Sun, or 10^{36} kilograms.

The more massive a black hole is, the more space it takes up. In fact, the Schwarzschild radius (which means the radius of the horizon) and the mass are directly proportional to one another: if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometers. So a typical 10-solar-mass black hole would have a radius of 30 kilometers, and a million-solar-mass black hole at the center of a galaxy would have a radius of 3 million kilometers. Three million kilometers may sound like a lot, but it’s actually not so big by astronomical standards. The Sun, for example, has a radius of about 700,000 kilometers, and so that supermassive black hole has a radius only about four times bigger than the Sun.

Artist's concept of a black hole from top down. Image credit: NASA

Artist’s concept of a black hole from top down. Image credit: NASA



Interesting Facts about Black Holes:

 In the strictest and most exact sense, there are currently 14 known black holes.

 The known closest black hole to Earth is Cygnus X-1, located about 8000 light years away.

 Although black holes have a strong suction force, they may only suck up what crosses their event horizons, and, therefore, are not capable of absorbing the whole universe.

 In theory, any matter can become black holes, as long as they are compressed to zero volume and thus, yielding infinite density. However, only the largest of stars have cores capable with the gravitational force to compress the star to the Schwarzschild radius. Most others stars without this gravitational force end up as neutron stars and white dwarfs.

 Although white holes are mathematically possible, there have yet to be observations to prove their existence.

 Black holes can suck up other black holes when they come in close proximity. Usually the larger one will suck up the smaller one. Depending on the size of the matter that is making up the black holes, the size of the black hole created will differ. Direct collisions between black holes are rare, as black holes are very small for their mass. Black holes may also merge.

Black Hole Fusion

Black Hole Fusion

 The center of a black hole, the singularity, is the point where the laws of physics break down. These singularities are hidden, or ‘clothed’ by the black hole, so that the effects of the breakdown cannot be observed by people outside.

 At the center of a black hole, spacetime has infinite curvature and matter is crushed to infinite density under the pull of ‘infinite’ gravity. At a singularity, space and time cease to exist as we know them. The laws of physics as we know them break down at a singularity, thus, making it impossible to envision something with zero volume and infinite density, such qualities of a black hole.

 By using the correct equations for motion, it can be predicted that near a black hole, an object on a radial path will have a velocity approaching the speed. This occurs as the object approches the event horizon.

 Stars are powered by nuclear fuel; most stars use hydrogen. The larger a star is, the faster it will use up its fuel, and thus, the sooner it “dies”. If the stars are large enough, however, then the gravitational pull will crush the star to ‘zero volume’, or in the Schwarzschild radius. This forms a black hole.

 As black holes, age, they gain more mass, as they suck in more matter

 A black hole cannot be viewed directly because light cannot escape it. However, matter swirling around a black hole, usually gas and dust, heats up and emits radiation that can be detected. However, deep in the center of a supermassive black hole, stars can also be found.

 On February 1997, the Hubble Space Telescope had a new instrument installed. Called the Space Telescope Imaging Spectrograph (STIS), this equipment is the main black hole seeker on the telescope. A spectrograph splits any incoming light using prisms and diffraction gratings into a rainbow. The STIS can measure ultraviolet, visible, and near-infrared wavelengths, allowing it to capture a wide range of places at once. The placement and intensity of the spectrum gives indispensable information to scientists. Every spectrum can be analyzed to find out the speed of which stars and gas swirl at a certain location. From this information, the mass of the object that the stars are orbiting can be found. A massive central object is found if the stars swirl quickly.


Black holes are weird.

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