What is a black hole

What is a black hole?

Black holes are the dark remnants of collapsed stars, regions of space cut off from the rest of the universe. If something falls into a black hole, it can never come back out. Not even light can escape, meaning black holes are invisible even with powerful telescopes. Yet physicists know black holes exist because they’re consistent with time-tested theories, and because astronomers have observed how matter behaves just outside a black hole.

Naturally, science fiction loves such an enigmatic entity. Black holes have played starring roles in popular books, movies and television shows, from «Star Trek» and «Doctor Who» to the 2014 blockbuster «Interstellar.»

But black holes aren’t quite as menacing as they are commonly portrayed. “They definitely do not suck,” says Daryl Haggard, an astrophysicist at McGill University in Montreal. “A black hole just sits there, passively. Things can fall onto it, just as meteors can fall to Earth, but it doesn’t pull stuff in.”

How do black holes form?

The force of gravity governs the motion of planets, stars and galaxies, and it’s responsible for creating black holes, too.

Stars shine because of the nuclear fusion reactions taking place in their cores. The reactions create an outward pressure that counters the inward pull of gravity. As a result, the star neither expands nor contracts. But when a star’s fuel supply is exhausted and the outward pressure stops, gravity causes the star to shrink.

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What happens next depends on the size of the star. If it’s about the mass of our sun or a bit bigger, it will collapse until it’s a roughly Earth-size body known as a white dwarf. Stars that are significantly larger will collapse into an ultra-dense object known as a neutron star. If it’s really big, the collapse cannot be stopped — and you get a black hole.

Why can’t anything escape from a black hole? The key is something called escape velocity: the speed needed to overcome the gravitational tug of a particular star or planet and move out into space.

Earth’s escape velocity is about seven miles per second, or about 25,000 miles an hour. Throw a baseball into the air and it falls back down because its speed is lower than Earth’s escape velocity; if your fastball exceeded 25,000 miles an hour, it would never come down.

Escape velocity is highest for objects that are massive but small in size. In the case of a black hole, the escape velocity is greater than the speed of light. Since nothing can travel faster than light (something Einstein showed with his theory of relativity), the star disappears. With light unable to escape, it appears black.

A black hole is bounded by its event horizon, the imaginary sphere that represents the region where the escape velocity is exactly equal to the speed of light.

Black holes vary in size, with masses ranging from a handful of suns (and a diameter of a few miles) up to millions of solar masses (and a diameter of several million miles). The largest of these so-called supermassive black holes are believed to lie at the center of most galaxies, including our own Milky Way.

Astronomers are still debating how these colossal black holes form. One possibility is that they’re the result of mergers between star-size black holes.

What’s inside a black hole?

No one knows exactly what lies within a black hole’s event horizon. Some physicists hypothesize that matter inside is so compressed that it forms a point of infinite density known as a singularity. In this view, a black hole can be thought of as empty space, with its mass concentrated at an infinitesimal point in the center.

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Anyone unlucky enough to fall into a black hole would be torn apart by the intense gravity — stretched like spaghetti, as Stephen Hawking famously put it — with his or her mass added to the black hole’s.

Other physicists question this view of the interior of a black hole, arguing that a more comprehensive physical theory — one that combines Einstein’s theory of gravity with quantum theory — might do away with singularities.

Black holes may be dark, but that doesn’t mean we can’t study them

Astronomers have learned a great deal about black holes by watching what happens to gas and dust that fall into them. Such material can reach very high temperatures, causing it to emit light at various wavelengths.

The Event Horizon Telescope, a globe-spanning array of radio telescopes, is giving astronomers their closest look yet at the region immediately outside a black hole. Astronomers have also used the Very Large Telescope in Chile to study the motion of stars near the Milky Way’s supermassive black hole, known as Sagittarius A* (pronounced “Sagittarius A-star”). From these motions it’s possible to infer some of the properties of the black hole.

The 2015 discovery of gravitational waves means that scientists can use these waves to study collisions between black holes in deep space.

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What is a black hole

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An artist’s drawing shows a large black hole pulling gas away from a nearby star. Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet
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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.

An artist’s drawing shows the current view of the Milky Way galaxy. Scientific evidence shows that in the middle of the Milky Way is a supermassive black hole. Image Credit: NASA/JPL-Caltech
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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.

This image of the center of the Milky Way galaxy was taken by the Chandra X-ray Observatory. Image Credit: NASA/CXC/MIT/F.K. Baganoff et al.
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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.

This artist’s drawing shows a supermassive black hole in the center of a galaxy. The black hole is surrounded by a cloud of material that is spiraling into it. Image Credit: NASA E/PO, Sonoma State University, Aurore Simonnet
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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.

Black holes: Everything you need to know

These gluttonous beasts are some of the most fascinating objects in space.

Black holes are some of the strangest and most fascinating objects in space. They’re extremely dense, with such strong gravitational attraction that not even light can escape their grasp.

The first image of a black hole was captured in 2019 by the Event Horizon Telescope (EHT) collaboration. The striking photo of the black hole at the center of the M87 galaxy 55 million light-years from Earth thrilled scientists around the world.

Black hole discovery

Albert Einstein first predicted the existence of black holes in 1916, with his general theory of relativity. The term «black hole» was coined many years later in 1967 by American astronomer John Wheeler. After decades of black holes being known only as theoretical objects.

How many black holes are there?

According to the Space Telescope Science Institute (opens in new tab) (STScI) approximately one out of every thousand stars is massive enough to become a black hole. Since the Milky Way contains over 100 billion stats, our home galaxy must harbor some 100 million black holes.

Though detecting black holes is a difficult task and estimates from NASA (opens in new tab) suggest there could be as many as 10 million to a billion stellar black holes in the Milky Way.

The closest black hole to Earth is dubbed «The Unicorn» and is situated approximately 1,500 light-years away. The nickname has a double meaning. Not only does the black hole candidate reside in the constellation Monoceros («the unicorn»), its incredibly low mass — about three times that of the sun — makes it nearly one of a kind.

Black hole images

In 2019 the Event Horizon Telescope (EHT) collaboration released the first image ever recorded of a black hole. The EHT saw the black hole in the center of galaxy M87 while the telescope was examining the event horizon or the area past which nothing can escape from a black hole. The image maps the sudden loss of photons (particles of light). It also opens up a whole new area of research in black holes, now that astronomers know what a black hole looks like.

In 2021, astronomers revealed a new view of the giant black hole at the center of M87, showing what the colossal structure looks like in polarized light. As polarized light waves have a different orientation and brightness compared to unpolarized light, the new image shows the black hole in even more detail. Polarization is a signature of magnetic fields and the image makes it clear that the black hole’s ring is magnetized.

What do black holes look like?

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, past which light cannot 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 the object’s mass lies, is known as its singularity, the single point in space-time where the mass of the black hole is concentrated.

Scientists can’t see black holes the way they can see stars and other objects in space. Instead, astronomers must rely on detecting the radiation black holes emit as dust and gas are drawn into the dense creatures. But supermassive black holes, lying in the center of a galaxy, may become shrouded by the thick dust and gas around them, which can block the telltale emissions.

Sometimes, as matter is drawn toward a black hole, it ricochets off 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 remains unseen, these powerful jets can be viewed from great distances.

The EHT’s image of a black hole in M87 (released in 2019) was an extraordinary effort, requiring two years of research even after the images were taken. That’s because the collaboration of telescopes, which stretches across many observatories worldwide, produces an astounding amount of data that is too large to transfer via the internet.

With time, researchers expect to image other black holes and build up a repository of what the objects look like. The next target is likely Sagittarius A*, which is the black hole in the center of our own Milky Way galaxy. Sagittarius A* is intriguing because it is quieter than expected, which may be due to magnetic fields smothering its activity, a 2019 study reported. Another study that year showed that a cool gas halo surrounds Sagittarius A*, which gives unprecedented insight into what the environment around a black hole looks like.

Types of black holes

So far, astronomers have identified three types of black holes: 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, the object may collapse, or fall into itself. For smaller stars (those up to about three times the sun‘s mass), the new core will become a neutron star or a white dwarf. But when a larger star collapses, it continues to compress and creates a stellar black hole.

Black holes formed by the collapse of individual stars are relatively small but incredibly dense. One of these objects packs more than three times the mass of the sun into the diameter of a city. This leads to a crazy amount of gravitational force pulling on objects around the object. Stellar black holes then consume the dust and gas from their surrounding galaxies, which keeps them growing in size.

Supermassive black holes — the birth of giants

Small black holes populate the universe, but their cousins, supermassive black holes, dominate. These enormous black holes are millions or even billions of times as massive as the sun but are about the same size in diameter. 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 these giants have formed, they gather mass from the dust and gas around them, material that is plentiful in the center of galaxies, allowing them to grow to even more enormous sizes.

Supermassive black holes may be the result of hundreds or thousands of tiny black holes that merge. 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. Fourth, supermassive black holes could arise from large clusters of dark matter. This is a substance that we can observe through its gravitational effect on other objects; however, we don’t know what dark matter is composed of because it does not emit light and cannot be directly observed.

Intermediate black holes

Scientists once thought that black holes came in only small and large sizes, but research has revealed the possibility that midsize, or intermediate, black holes (IMBHs) could exist. Such bodies could form when stars in a cluster collide in a chain reaction. Several of these IMBHs forming in the same region could then 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. And in 2021 astronomers took advantage of an ancient gamma-ray burst to detect one.

Research, from 2018, suggested that these IMBHs may exist in the heart of dwarf galaxies (or very small galaxies). Observations of 10 such galaxies (five of which were previously unknown to science before this latest survey) revealed X-ray activity — common in black holes — suggesting the presence of black holes of from 36,000 to 316,000 solar masses. The information came from the Sloan Digital Sky Survey, which examines about 1 million galaxies and can detect the kind of light often observed coming from black holes that are picking up nearby debris.

Binary black holes: double trouble

In 2015, astronomers using the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from merging stellar black holes.

There are two theories on how binary black holes form. The first suggests that the two black holes in a binary form at about the same time, from two stars that were born together and died explosively at about the same time. The companion stars would have had the same spin orientation as one another, so the two black holes left behind would as well.

Under the second model, black holes in a stellar cluster sink to the center of the cluster and pair up. These companions would have random spin orientations compared to one another according to LIGO Scientific Collaboration. LIGO’s observations of companion black holes with different spin orientations provide stronger evidence for this formation theory.

«We’re starting to gather real statistics on binary black hole systems,» said LIGO scientist Keita Kawabe of Caltech, who is based at the LIGO Hanford Observatory. «That’s interesting because some models of black hole binary formation are somewhat favored over the others even now, and in the future, we can further narrow this down.»

Black hole facts

Additional resources

Dive deeper into the mystery of black holes (opens in new tab) with NASA Science. Watch videos and read more about black holes (opens in new tab) from NASA’s Hubblesite. Discover more about black holes (opens in new tab) with the National Science Foundation.

Bibliography

Hubblesite: Black holes: Gravity’s relentless pull interactive (opens in new tab) : Encyclopedia. STScI Home. Retrieved May 6, 2022.

NASA. Imagine the universe! (opens in new tab) NASA. Retrieved May 6, 2022.

Boen, B. (2013, August 29 (opens in new tab) ). Supermassive black hole Sagittarius A*. NASA. Retrieved May 6, 2022.

NASA’s Chandra Finds Intriguing Member of Black Hole Family Tree. (opens in new tab) Chandra X-ray Observatory. (2015, February 25). Retrieved May 6, 2022.

Black Holes Are the Terrifying Behemoths of Space. Here’s How They Tick

Much like water gushing down a drain, the very fabric of space (and time) also appears to be draining away within some of the most enigmatic things in the universe — black holes. But, what exactly are they?

Are they more common than we think? Should we be concerned about them? What role do they play in the universe?

These are just some of the «big picture» questions some of the greatest minds of astrophysics have mulled over for many decades.

Let’s see what, if anything, they’ve managed to learn about the «Great Devourers» of the cosmos.

What is the definition of a black hole?

Black holes can be defined, according to NASA, as » a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space.»

How are black holes made of, and what different kinds of them are there?

How black holes form depends on their type and origin. To date, scientists have managed to define at least four different kinds of black holes:

Current theories suggest that small, or miniature, black holes (some as small as an atom) probably formed in the earliest moments of the universe. These tiny black holes are, to date, purely theoretical, and it is theorized that most of them may have already evaporated. These tiny black holes are thought to have masses of hundreds of solar masses or less.

Like miniature black holes, intermediate black holes are only really theoretical. This type of black hole would have several hundred of thousands of solar masses, rather than millions, or even billions of solar masses, like their larger cousins.

Some scientists believe that intermediate black holes form from a merging of miniature black holes. Others believe that, if they do indeed exist, they would form from the collapse of stars with masses equal to hundreds of thousands of solar masses (one solar mass is equal to the mass of our own Sun, or 1.989 × 10 30 kg ).

Needless to say, there is little consensus in the field over these types of enigmatic black holes.

Artistic impression of a black hole, Source: Cappan/iStock

Stellar black holes (about the mass of 20 of our Suns or more) are created when massive stars collapse in on themselves.

As National Geographic explains, «in their final stages, enormous stars go out with a bang in massive explosions known as supernovae. Such a burst flings star matter out into space but leaves behind the stellar core. While the star was alive, nuclear fusion created a constant outward push that balanced the inward pull of gravity from the star’s own mass. In the stellar remnants of a supernova, however, there are no longer forces to oppose that gravity, so the star core begins to collapse in on itself.»

If this mass collapses into an infinitely small point, a black hole is born—many times the mass of our own sun. There may be thousands of these stellar-mass black holes within our own galaxy.

Supermassive black holes (millions or even billions of solar masses in size) are thought to form at the same time as the galaxy they inhabit is formed and are predicted by Einstein’s General Theory of Relativity. The Milky Way has a supermassive black hole at its center, Sagittarius A* (pronounced “a star”), that may be more than four million times as massive as our sun. Scientists aren’t sure how such large black holes come into being, although there are a number of theories.

Who first discovered black holes?

While everyone has heard of black holes nowadays, have you ever wondered who first discovered them?

Technically speaking, we haven’t really «found» a black hole yet, but we can infer their existence through a variety of techniques (more on this later). That being said, scientists have speculated about the existence of something like them for hundreds of years.

In 1783, for example, an English cleric and amateur scientist called John Mitchell managed to demonstrate that Newton’s law of gravity could be used to show a place where gravity was so intense light cannot escape.

He went even further. Mitchell suggested that although these areas would be invisible, they should reveal their presence by interfering with things like stars that might orbit them.

His theoretical work would prove to be years ahead of his time, with the later groundbreaking work of the great Albert Einstein.

Einstein first predicted that such things should exist way back in 1916, in his «General Theory of Relativity». According to him, big enough stars should be able to collapse under their own gravity and create what we call today black holes.

For decades after this, black holes remained a purely theoretical concept, and the actual term wasn’t coined until 1967 by the American astronomer John Wheeler.

Mitchell and Einstein’s work was reinforced in 1971 when two British astronomers, Louise Webster and Paul Murdin, independently announced they had discovered one in space using indirect methods. Murdin worked out of the Royal Greenwich Observatory in London and Webster at the University of Toronto.

What they had found was an intense x-ray source, now called Cygnus X-1, orbiting a blue star around 6,000 light-years away. It would be the first of many.

As amazing as this all is, it wasn’t until very recently that scientists managed to «see» one for the first time. Back in 2019, the Event Horizon Telescope (EHT) collaboration managed to release a computerized image of what is believed to be a black hole.

First-ever image of a black hole’s event horizon and accretion disk. Source: EHT

The image itself is actually a composite rendering of a petabyte of data collected from a series of radio telescopes sited around the world.

The EHT focussed the radio telescopes on the center of the Messier 87 Galaxy (Virgo A) where a black hole was thought to lurk. This galaxy is somewhere in the region of 54 million light-years away from Earth.

It is thought that the black hole in question has a mass of about 6.5 billion suns. The team was attempting to examine and image the black hole’s event horizon and accretion disk (a large cloud of hot gas and dust trapped in orbit around the black hole).

This they did, and two years later they were able to image the shape of the magnetic fields in the hot gas swirling around the hole. The discovery of this black hole has proved to be groundbreaking, as it is hoped that it will open a whole new area of research into the nature of black holes. In 2021, astronomers took advantage of an ancient gamma-ray burst to detect an intermediate-mass black hole. Information from the Sloan Digital Sky Survey suggests IMBHs may exist in the center of most dwarf galaxies.

What Is a Black Hole?

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Click here to read a transcript of this video.

Space is a pretty dark place. Even so, some areas are darker than others. Nothing is darker than a black hole.

A black hole is an area of such immense gravity that nothing—not even light—can escape from it.

Black holes form at the end of some stars’ lives. The energy that held the star together disappears and it collapses in on itself producing a magnificent explosion.

Here’s where things get crazy. All of that material left over from the explosion, many times the mass of our Sun, falls into an infinitely small point.

Black holes can form in many ways though, and large black holes can have tens to millions of times the mass of our sun trapped in a point smaller than the tip of a pin! Some black holes trap more and more material as their mass increases.

The point where all that mass is trapped is called a singularity. It may be infinitely small, but its influence is enormous.

Imagine a circle with a singularity in the middle. The gravity on the inside of the circle is so strong that nothing can escape—it sucks in everything, even light. That’s why it’s black!

This circle is known as the event horizon. An event horizon is probably what you are thinking of when you think of a black hole.

What would happen, you might wonder, if we took a spacecraft near a black hole’s event horizon? The answer—spaghettification! That’s the technical term, at least.

As our spacecraft approaches it, the gravity will be so much stronger on the side closer to the black hole than at the other side that it will get completely stretched out like a piece of spaghetti.

Try as you may, you would be hard-pressed to find anything weirder or cooler than a black hole…