A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
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.
How Big Are 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.
Another 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.
The largest black holes are called "supermassive." 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.
How Do Black Holes Form?
Scientists think the smallest black holes formed when the universe began.
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.
Scientists think supermassive black holes were made at the same time as the galaxy they are in.
If Black Holes Are "Black," How Do Scientists Know They Are There?
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.
When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.
Could a Black Hole Destroy Earth?
Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.
Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.
The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.
How Is NASA Studying Black Holes?
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.
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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.
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.
There are three types: stellar black holes, supermassive black holes and intermediate black holes.
Stellar black holes — small but deadly
When a star burns through the last of its fuel, it may find itself collapsing. For smaller stars, up to about three times the sun's mass, the new core will be a neutron star or a white dwarf. But when a larger star collapses, it continues to fall in on itself to create a stellar black hole.
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.
According the Harvard-Smithsonian Center for Astrophysics, "the Milky Way contains a few hundred million" stellar black holes.
Supermassive black holes — the birth of giants
Small black holes populate the universe, but their cousins, supermassive black holes, dominate. Supermassive black holes are millions or even billions of times as massive as the sun, but have a radius similar to that of Earth's closest star. Such black holes are thought to lie at the center of pretty much every galaxy, including the Milky Way.
Scientists aren't certain how such large black holes spawn. Once they've formed, they can easily gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to enormous sizes.
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.
Intermediate black holes – stuck in the middle
Scientists once thought black holes came in only small and large sizes, but recent research has revealed the possibility for the existence of midsize, or intermediate, black holes (IMBHs). Such bodies could form when stars in a cluster collide in a chain reaction. Several of these forming in the same region could eventually fall together in the center of a galaxy and create a supermassive black hole.
In 2014, astronomers found what appeared to be an intermediate-mass black hole in the arm of a spiral galaxy.
"Astronomers have been looking very hard for these medium-sized black holes," co-author Tim Roberts, of the University of Durham in the United Kingdom, said in a statement.
"There have been hints that they exist, but IMBH's have been acting like a long-lost relative that isn't interested in being found."
Black hole theory — how they tick
Black holes are incredibly massive, but cover only a small region. Because of the relationship between mass and gravity, this means they have an extremely powerful gravitational force. Virtually nothing can escape from them — under classical physics, even light is trapped by 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.
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.
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.
Under the classical mechanics of physics, nothing can escape from a black hole. However, things shift slightly when quantum mechanics are added to the equation. Under quantum mechanics, for every particle, there is an antiparticle, a particle with the same mass and opposite electric charge. When they meet, particle-antiparticle pairs can annihilate one another.
If a particle-antiparticle pair is created just beyond the reach of the event horizon of a black hole, it is possible to have one drawn into the black hole itself while the other is ejected. The result is that the event horizon of the black hole has been reduced and black holes can decay, a process that is rejected under classical mechanics.
Scientists are still working to understand the equations by which black holes function.
Interesting facts about black holes
If you fell into a black hole, theory has long suggested that gravity would stretch you out like spaghetti, though your death would come before you reached singularity. But a 2012 study in Nature suggests that quantum effects would cause the event horizon to act much like a wall of fire, instantly burning anyone to death.
Black holes do not "suck." Suction is caused by pulling something into a vacuum, which the massive black hole definitely is not. Instead, objects fall into them.
The first object considered to be a black hole is Cygnus X-1. Rockets carrying Geiger counters discovered eight new X-ray sources. In 1971, scientists detected radio emissions coming from Cygnus X-1, and a massive hidden companion was found and identified as a black hole.
Cygnus X-1 was the subject of a 1974 friendly wager between Stephen Hawking and a fellow physicist Kip Thorne, with Hawking betting that the source was not a black hole. In 1990, he conceded defeat. [VIDEO: Final Nail in Stephen Hawking's Cygnus X-1 Bet?]
Miniature black holes may have formed immediately after the Big Bang. Rapidly expanding space may have squeezed some regions into tiny, dense black holes less massive than the sun.
If a star passes too close to a black hole, it can be torn apart.
Astronomers estimate there are anywhere from 10 million to a billion stellar black holes, with masses roughly thrice that of the sun, in the Milky Way.
The interesting relationship between string theory and black holes gives rise to more types of massive giants than found under conventional classical mechanics.
Black holes remain terrific fodder for science fiction books and movies. Check out the science behind the movie "Interstellar," which relied heavily on theoretical physicist Kip Thorne to bring real science to the Hollywood feature. In fact, work with the special effects of the blockbuster lead to an improvement in the scientific understanding of how distant stars might appear when seen near a fast-spinning black hole.
HEARTS OF DARKNESS
Most astronomers now concede that a black hole heavyweight lurks in the centre of our own Milky Way galaxy. Latest estimates are that it weighs in at a whopping 2 million times the mass of the sun - a dwarf in comparison to some of the truly supermassive black holes that may lurk in the cosmos.
By the 1950s, astronomers began turning optical telescopes towards some of the strongest signals that the new radio telescopes were picking up. Source number 3C 273 was found to be a bright star-like object with a 'jet' of intense radiation sticking out of it. It was the first of a number of similar objects given the name of ‘quasar’ or 'quasi-stellar radio source', but their real identity remained hidden for decades.
Quasars have now been revealed to be the energetic hearts of very active galaxies: brilliant discs of superheated gas and ruptured stars swirling at nearly the speed of light. Great jets of charged particles are blasted thousands of light years into space from above & below - like an axle through a wheel. The central engine that is driving all this activity, though, is hidden deep inside. It has to be small and it must be extraordinarily dense. The mathematics demand that the only beast that can drive such a display of raw power is a supermassive black hole. The heavier the hole, the faster the gases whirl in orbit. Astronomers have observed speeds which tally with black holes weighing up to five thousand million suns.
The theory goes like this: a galaxy evolves from a vast rotating cloud of gas that begins to clump and condense under its own weight into billions of stars arranged like an enormous Catherine wheel, a Mexican hat or a bee swarm. In the centre, where the gas is concentrated, enough matter to make millions or even billions stars has undergone titanic gravitational collapse to make a supermassive black hole. While the hole is still actively feeding on the inner part of the new galaxy it manifests itself as a quasar. Later, when all nearby food has been consumed, the black hole becomes quiescent, leaving a relatively quiet galactic core like the one in the Milky Way. If this theory is correct, then supermassive black holes are present in all but the smallest galaxies.
A black hole is an astronomical contradiction - a dark star, an invisible nothing, a prison of light. Its boundary is marked by the so-called Event Horizon, a sphere of darkness that shrouds the inside and defines the point of no return. There is no solid surface beyond, just a bottomless gravitational whirlpool so strong that it sucks everything - even light - relentlessly inward. Oblivion waits at the centre in the form of the Singularity, Gravity’s fatal attractor.
Hidden eternally from view, the Singularity marks the spot where an immense gravitational force has been concentrated. All the mass, light and energy that has ever fallen into the black hole is compressed by its own overwhelming gravity into a point that is infinitely small and infinitely dense. The more a black hole swallows, the heavier it gets, yet the singularity never changes. Space has been squelched out of existence and Time has been squeezed to a stop. Step over the event horizon and for all intents and purposes you’ve fallen off the edge of the universe.