Thursday, 8th March 2018
8 March 2018
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Black holes

Black holes

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A black hole is a region of spacetime from which nothing can escape, even light. Black hole is not a empty space. Black holes are some of the strangest and most fascinating objects found in outer space. They are objects of extreme density, with such strong gravitational attraction that even light cannot escape from their grasp if it comes near enough

Because no light can get out, people can’t see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.

Even our home galaxy, the Milky Way, has a four million solar mass black hole located at its center, about 27,000 light years from Earth.

Albert Einstein first predicted black holes in 1916 with his general theory of relativity. The term “black hole” was coined in 1967 by American astronomer John Wheeler, and the first one was discovered in 1971.

A massive star that has run out of fuel can produce the kind of extreme density needed to create such a mangled bit of world. As it buckles under its own weight and collapses inward, space-time caves in with it. The gravitational field becomes so strong that not even light can escape, rendering the region where the star used to be profoundly dark: a black hole.

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Imagine matter packed so densely that nothing can escape. Not a moon, not a planet and not even light. That’s what black holes are — a spot where gravity’s pull is huge, ending up being dangerous for anything that accidentally strays by.

From the perspective of the collapsing star, the core compacts into a mathematical point with virtually zero volume, where it is said to have infinite density. This is called a singularity.

Where this happens, it would require a velocity greater than the speed of light to escape the object’s gravity. Since no object can reach a speed faster than light, no matter or radiation can escape. Anything, including light, that passes within the boundary of the black hole — called the “event horizon” — is trapped forever.

Thus it’s now believed that black holes are not only common throughout the Cosmos but they play a fundamental role in the formation and evolution of the Universe we inhabit today.

Types of black holes

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or “stuff,” in an object.

There are three types: stellar black holes, supermassive black holes and intermediate black holes.

Stellar black holes

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One kind of black hole is called “stellar.” Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar mass black holes in Earth’s galaxy. Earth’s galaxy is called the Milky Way.

Black holes formed by the collapse of individual stars are (relatively) small, but incredibly dense. Such an object packs three times or more the mass of the sun into a city-size range. This leads to a crazy amount of gravitational force pulling on objects around it. Black holes consume the dust and gas from the galaxy around them, growing in size.

Supermassive black holes

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The largest supermassive black hole ever found contains up to 21 billion times the mass of the sun, and resides in a more expected location: the incredibly dense Coma Cluster, which includes more than 1,000 identified galaxies.

Supermassive black holes may be the result of hundreds or thousands of tiny black holes that merge together. Large gas clouds could also be responsible, collapsing together and rapidly accreting mass. A third option is the collapse of a stellar cluster, a group of stars all falling together.

These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would fit inside a very large ball that could hold a few million Earths.

Intermediate black holes

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Researchers have spotted hints of much rarer medium-size black holes, which harbor between 100 and several hundred thousand solar masses. But it’s tough to weigh these objects definitively — so tough that their existence has been a matter of debate.

Many theories, therefore, have suggested that the initial seed black holes had to have been a few 100 -1,000 times our sun, But we did not have firm evidence for such intermediate-mass black holes.

Stellar-mass black holes also often feature paired X-ray oscillations that occur in a 3:2 frequency ratio. Therefore, the new observations suggest that medium-size black holes may behave like scaled-up versions of stellar-mass black hole systems.

In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy.

Size  of a black hole

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the black hole horizon as a sphere, and its diameter is directly proportional to the mass of the black hole. So the more mass falls into the black hole, the larger the black hole becomes. Compared to stellar objects though, black holes are tiny because the mass has been compressed into a very small volume by enormous gravitational pressure. The radius of a black hole with the approximate mass of planet Earth, for example, is only a few millimeters. Compare that to the actual radius of Earth, which is about 10,000,000,000 times larger!

The radius of a black hole is called the Schwarzschild radius, after Karl Schwarzschild who first derived black holes as a solution to Einstein’s General Relativity.

Formation of a black hole

Small black holes can form when massive stars collapse. The enormous ones at the centers of galaxies likely grow so large by taking in a lot of dust and debris early on in their lives and by colliding and merging with other black holes, which occurs when two galaxies combine.

Our star, the sun, will die a quiet death. The sun’s of only average mass, starwise, and after burning through the last of its hydrogen fuel in about five billion years, its outer layers will drift away, and the core will eventually compact to become what’s known as a white dwarf, an Earth-size ember of the cosmos.

Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space.

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Huge black holes are also thought to be formed by the collison of galaxies. When two galaxies collide they form one large galaxy and their black holes also merge together to form a larger black hole.

For a star ten times as big as the sun, death is far more dramatic. The outer layers are blasted into space in a supernova explosion that, for a couple of weeks, is one of the brightest objects in the universe. The core, meanwhile, is squeezed by gravity into a neutron star, a spinning ball bearing a dozen miles in diameter.These stars are hot enough to burn not just hydrogen and helium as fuel, but also carbon, oxygen and silicon. Eventually, the fusion in these stars forms the element iron (which is the most stable of all nuclei, and will not easily fuse into heavier elements), which effectively ends the nuclear fusion process within the star. Lacking fuel for fusion, the temperature of the star decreases and the rate of collapse due to gravity increases, until it collapse completely on itself, blowing out material in a massive supernova explosion

In the normal life of a star there is a constant tug of war between gravity pulling in and pressure pushing out. Nuclear reactions in the core of the star produce enough energy and pressure to push outward. For most of a star’s life, gravity and pressure balance each other exactly, and so the star is stable. However, when a star runs out of nuclear fuel, gravity gets the upper hand and the material in the core is compressed even further. The more massive the core of the star, the greater the force of gravity that compresses the material, collapsing it under its own weight.

For small stars, when the nuclear fuel is exhausted and there are no more nuclear reactions to fight gravity, the repulsive forces among electrons within the star eventually create enough pressure to halt further gravitational collapse. The star then cools and dies peacefully. This type of star is called a “white dwarf.”

Like most objects in the universe (planets, stars, asteroids, and other compact objects), black holes rotate. We refer to this property as “spin” or, more formally, its “angular momentum” about its axis of rotation. This concept can be difficult to visualize because a black hole is not a solid body, but it helps if you start by imagining a black hole as the end product of a star’s evolution. If a star begins its life with more than some critical mass (usually estimated at about 10 to 20 times the Sun’s mass), it will ultimately end up as a black hole. The spin is of three types – retrograde, prograde and no spin.

How to see a black hole

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how the strong gravity affects the stars and gas around the black hole. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

Such a strong pull creates an observational problem when it comes to black holes — scientists can’t “see” them the way they can see stars and other objects in space. Instead, scientists must rely on the radiation that is emitted as dust and gas are drawn into the dense creatures. Supermassive black holes, lying in the center of a galaxy, may find themselves shrouded by the dust and gas thick around them, which can block the tell-tale emissions.

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Sometimes as matter is drawn toward a black hole, it ricochets off of the event horizon and is hurled outward, rather than being tugged into the maw. Bright jets of material traveling at near-relativistic speeds are created. Although the black hole itself remains unseen, these powerful jets can be viewed from great distances.

Cygnus X-1 was first found during balloon flights in the 1960s, but wasn’t identified as a black hole for about another decade. According to NASA, the black hole is 10 times more massive to the Sun.

NASA is using satellites and telescopes that are traveling in space to learn more about black holes. These spacecraft help scientists answer questions about the universe.

What happens if we go near a black hole

When a person goes close to a black hole at first he would appear to be going slower than the observer. Also any sound or light flashed by the person close to the black hole will reach the outsider very slolwly. As he goes still nearer he freezes and then finally burns to ash even before he is fully engulfed by the black hole

Black holes have three “layers” — the outer and inner event horizon and the singularity.

The event horizon of a black hole is the boundary around the mouth of the black hole where light loses its ability to escape. Once a particle crosses the event horizon, it cannot leave. Gravity is constant across the event horizon.

The inner region of a black hole, where its mass lies, is known as its singularity, the single point in space-time where the mass of the black hole is concentrated.

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Contrary to popular belief, a black hole does not just “suck up” everything around it in an uncontrolled orgy of destruction: it actually exerts no more gravitational pull on the objects around it than the original star from which it was formed, and any objects orbiting the original star (and which survived the supernova blast) would now orbit a black hole instead (an object would need to approach quite close to a black holebefore being sucked in). The very largest blue stars may skip even the supernova stage, so that even their outer shells become incorporated into the singularity.

Sure, if the black hole were smaller you’d have a problem. The force of gravity would be much stronger at your feet than at your head, stretching you out like a piece of spaghetti. But lucky for you this is a big one, millions of times more massive than our Sun, so the forces that might spaghettify you are feeble enough to be ignored.

In fact, in a big enough black hole, you could live out the rest of your life pretty normally before dying at the singularity.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole. Hence the earth and the other planets revolving around the sun has never got the danger of being engulfed by the sun’s black hole but will continue revolving around the dead sun









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