Everything to know about black holes

Eaters of worlds and stars, black holes have fascinated experts and the public alike for decades. But, what exactly are they? Let's find out.
Christopher McFadden
You cannot escape the power of black holes.
You cannot escape the power of black holes.


Black holes, the very term, conjures up images of the end of life, the universe, and everything we have ever known.

But is this true? Are black holes something to be feared? Or, indeed, something to be celebrated and admired?

Let's take a closer look at one of the most misunderstood natural phenomena of the universe.

What, exactly, is a black hole?

The first critical point to note when discussing black holes is not to be misled by the name. Black holes are anything but empty spaces or nothingness.

Instead, black holes are a lot of matter crammed into a very tiny space. For a typical black hole, imagine a star 10 times as massive as our Sun being crammed into a sphere the size of New York City! That is a lot of stuff in a relatively small "container".

Compressing this much mass into a small space significantly impacts the surrounding stuff around it. For example, as a result, it generates a gravitational force that is so powerful that even light cannot escape.

For a long time, physicists had speculated about the possibility of an object in space so dense that not even light could not escape. This is where they get their name.

Einstein's theory of general relativity, which demonstrates that when a big star of a certain size dies, it leaves behind a small, dense remnant core, is most famous for theorizing about the existence of black holes.

These equations demonstrate that if the star's mass is greater than roughly three times that of the Sun, the force of gravity will outweigh all other forces and create a black hole.

Everything to know about black holes
Artistic representation of a black hole.

Black holes are invisible to telescopes looking for light, x-rays, or other types of electromagnetic radiation -- for the very reasons explained above. But, by observing how they affect nearby matter, we may deduce the existence of black holes and study them. A black hole will accrete matter, or pull it inward, for instance, if it passes through a cloud of interstellar matter.

A regular star may experience similar behavior as it approaches a black hole. In this situation, when the star is being drawn toward the black hole, it may rip apart. The resulting heated and accelerated matter attracts matter by radiating x-rays into space as it accelerates and warms up. Recent findings provide fascinating evidence that black holes can profoundly impact the areas around them.

They may release intense gamma-ray bursts, devour neighboring stars, and stimulate or hinder the birth of new stars depending on where they are located.

How many types of black holes are there?

As far as scientists have been able to ascertain, there appear to be three main types of black holes.

The first and most enigmatic are called stellar black holes. These are the smallest, but that doesn't mean they are benign.

Everything to know about black holes
Artist's impression of a stellar-mass black hole.

As the name suggests, these form when a star collapses or falls into itself after it has used up all of its fuel. The new core of smaller stars (those with masses up to around three times that of the Sun) will eventually transform into a neutron star or white dwarf. But as a larger star disintegrates, it will keep getting smaller and smaller (denser and denser) until it forms a stellar black hole.

Black holes created when individual stars collapse are comparatively tiny but immensely dense. One of these objects may have a diameter roughly the size of a city yet has more than three times the mass of the Sun. As a result, the gravitational attraction on things nearby is extremely strong. The gas and dust from the surrounding galaxies are subsequently ingested by stellar black holes, which stop them from contracting in size.

The next kind, probably the most famous, is supermassive black holes.

Everything to know about black holes
Artist's impression of a supermassive black hole.

Although many little black holes exist in the cosmos, supermassive black holes predominate. They are millions or even billions of times more massive than the Sun, the diameter of these giant black holes is around the same. It is hypothesized that such black holes reside at the heart of almost every galaxy, including the Milky Way.

The origin of such massive black holes remains largely a mystery to scientists. Once these giants have formed, they continue to increase in size by absorbing mass from the gas and dust surrounding them, which is abundant at the core of galaxies.

It's possible that hundreds or thousands of microscopic black holes can combine to form supermassive black holes. Large gas clouds that collide and quickly accrete mass might possibly be to blame.

A collapsing stellar cluster, or a collection of stars collapsing at once, is a third possibility. Lastly, vast collections of dark matter may give rise to supermassive black holes. We can directly detect this substance by its gravitational pull on other things, but we cannot observe dark matter directly; thus, we do not know what it is made of.

The third and final primary black hole type is an intermediate or middle-sized black hole.

Research has shown that midsize or intermediate black holes (IMBHs) probably exist. Previously, scientists believed that black holes only occurred in tiny and big sizes.

Such entities could develop as a result of repeated collisions between stars in a cluster, in a type of chain reaction. If too many of these IMBHs develop in the same area, they may eventually collide to generate a supermassive black hole in the galaxy's center.

In the arm of a spiral galaxy in 2014, scientists discovered what seemed to be an intermediate-mass black hole. And in 2021, scientists used an old gamma-ray burst to their advantage to find one.

2018 research found a possible location for these IMBHs, the center of dwarf galaxies (or very small galaxies). X-ray activity, which is typical of black holes, was detected in observations of 10 such galaxies, five of which were previously unknown to science before this most recent study.

Everything to know about black holes
Globular cluster Mayall II (M31 G1) is a possible candidate for hosting an intermediate-mass black hole at its center.

This suggested the existence of black holes with masses ranging from 36,000 to 316,000 solar masses. The Sloan Digital Sky Survey, which studies around a million galaxies, provided the data. It can detect the type of light frequently seen emanating from black holes capturing surrounding material.

What is a black hole made of?

In short, and in most circumstances, "failed" stars.

Most black holes are created from the remains of massive stars that explode in events called supernovae. Smaller stars eventually develop into dense neutron stars, which lack the mass necessary to confine light.

Theoretically, it may be demonstrated that no force can prevent the star from collapsing under the pull of gravity if the overall mass of the star is great enough (generally, more than three times the mass of the Sun). However, an odd thing happens when a black hole is formed.

Everything to know about black holes

Time, that is, coordinate time, slows down as the star's surface approaches the "event horizon" — the threshold around the black hole where the escape velocity surpasses the speed of light.

This is because the curvature of spacetime at the event horizon is so great that a photon emitted 'outward' at the horizon simply remains on the horizon. Within the event horizon, the curvature is so great that the "future" is in the direction of the singularity.

Stellar collisions may also produce much larger black holes.

Swift, a telescope operated by NASA, started observing gamma-ray bursts shortly after its launch in December 2004. Astronomers deduced from Chandra and NASA's Hubble Space Telescope observations that tremendous explosions could happen when a black hole and a neutron star meet, creating another black hole. Chandra and Hubble later acquired data from the event's "afterglow."

Black holes' basic formation mechanism is well understood. Still, black holes' apparent existence on two dramatically different scales has long been a conundrum in black hole science.

Everything to know about black holes
Fantastic black hole in outer space.

What is the closest black hole to Earth?

There are a few candidates, but one of the most recently discovered is an odd little object known as "The Unicorn." Hidden barely 1,500 light-years from Earth, this appears to be the closest black hole to Earth that is currently known to exist.

The moniker has two distinct meanings. Admittedly still a black hole candidate, it is considered unique due to its extremely low mass, roughly three times that of the Sun, and the fact that it is located in the constellation Monoceros (aka "The Unicorn").

A bloated, red giant star that is close to dying is "The Unicorn's" partner. Many instruments, like the All Sky Automated Survey and NASA's Transiting Exoplanet Survey Satellite, have observed that companion over the years.

After studying the vast amount of data, the team behind the "Unicorn's" discovery also found something intriguing: the red giant's light occasionally changes in intensity, indicating that something is likely "pulling" on the star and altering its shape.

Based on information about the star's velocity and the light distortion, the scientists concluded that the object doing the pulling is most likely a black hole, although one of only around three solar masses. To put that into context, consider that our Milky Way galaxy's supermassive black hole masses are approximately 4.3 million solar masses.

"Just as the moon's gravity distorts the Earth's oceans, causing the seas to bulge toward and away from the moon, producing high tides, so does the black hole distort the star into a football-like shape with one axis longer than the other," study co-author Todd Thompson, chair of Ohio State's astronomy department, said in a statement.

In the team's view, this means that the new object is most likely a very small black hole of some kind. Despite how feasible that explanation could be, "The Unicorn" is still only a candidate black hole.

Interesting facts about black holes

We've covered a lot of ground already, but there are many more things to learn about these strange objects. While we cannot possibly cover everything there is to know about them in one place, here is a handpicked selection of the most notable facts about black holes.

1. Black holes were first theorized in 1916 but wouldn't be found until the 1960s

Everything to know about black holes
Chandra X-ray Observatory image of Cygnus X-1.

In 1916, Albert Einstein's general theory of relativity made the first prediction about the existence of black holes. However, he never actually coined the name. That would come many years later; in 1967, American astronomer John Wheeler first used the term "black hole."

But, until recently, black holes were solely understood as hypothetical entities.

That was until Cygnus X-1, in the Milky Way's constellation of Cygnus, or the Swan, became the first black hole ever found. According to NASA, the first indications of the black hole were discovered in 1964 when a sounding rocket discovered astronomical X-ray sources.

Astronomers discovered in 1971 that the source of the X-rays was a brilliant blue star circling a mysterious black object. It was hypothesized that the discovered X-rays were caused by stellar material being "gobbled" up by the dark object—an all-consuming black hole—after being peeled away from the blazing star.

2. There are likely billions of black holes out there, and counting

Everything to know about black holes
There are likely billions of black holes out there.

While black holes have only recently been discovered, scientists have since found that the universe is filled with them.

The Space Telescope Science Institute (STScI) estimates that one out of every thousand stars probably has enough mass to develop into a black hole. Since there are more than 100 billion stars in the Milky Way, there must, by that estimation, be 100 million black holes in our galaxy alone!

Though it can be challenging to find black holes, NASA estimates that the Milky Way may contain up to billion-stellar black holes.

The "The Unicorn" black hole, as we previously discussed, which is 1,500 light-years from Earth, is the nearest one to our planet to be found.

3. The only actual image of a black hole was captured/rendered in 2019

Everything to know about black holes
First ever image of a black hole.

If you are a fan of space science, you'll probably remember the fiery donut image of a black hole doing the rounds a few years ago, created by the Event Horizon Telescope (EHT) team, who published the first black hole picture ever taken in 2019.

While the telescope was studying the event horizon, or the region past which nothing can escape from a black hole, the EHT discovered the black hole at the heart of galaxy M87.

The image depicts the abrupt loss of photons (particles of light). Additionally, since that image demonstrated to astronomers what a black hole looks like when imaged, it opened up an entirely new field of study into them.

A fresh image of the massive structure in the heart of M87, as seen in polarized light, was released by scientists in 2021. The new picture reveals significantly more information about the black hole because polarized light waves differ in direction and brightness from unpolarized light waves. The picture makes it evident that the ring of the black hole is magnetized since polarization indicates magnetic fields.

4. Black holes "spaghettify" everything they encounter

If you were ever to come into close quarters with a black hole, you'd soon find that you would become stretched into a long spaghetti-like shape.

The reason for this is related to how gravity operates across long distances. Your feet are currently more strongly attracted than your head since they are closer to Earth's center. That difference in this attraction will only really start acting against you in situations of tremendous gravity, like, close to a black hole.

Your feet will become more and more drawn into the black hole's center as they get closer due to gravity's intense pull. The stretching becomes more extreme the closer they get. The top half of your body, however, is farther away and thus not moving as quickly toward the center, hence the term spaghettification.

5. Some believe that black holes could create new universes

Everything to know about black holes
Artist's impression of the "Big Bang"

Believe it or not, but some scientists believe that black holes could create other universes (including our own). Leaving aside the can of worms about the actual existence of other universes, there is an entire field of science dedicated to the study of this actual possibility.

An extremely condensed explanation of how this works is that, when you look at the calculations, our universe today has certain incredibly convenient circumstances that came together to generate life. We wouldn't be here if you even slightly altered these circumstances.

It is theoretically possible that the singularity at the center of a black hole may shift these circumstances and create a new, slightly modified universe. Needless to say, this is hotly contested.

6. Black holes literally warp space around themselves

Like any large and massive object in existence, black holes can distort the very fabric of spacetime. Sound funky? Then let's briefly explain how it works.

Imagine space as a stretched rubber sheet with grid lines running through it. An item that is placed on the sheet sinks a little.

When you place an object on the sheet, its sinking depth increases with its mass. The grid lines become distorted and bent due to the sinking effect.

Space twists and distorts more the deeper the "well" you form in it. Black holes, being incredibly massive objects in a relatively small area, create localized and very deep "space wells", resulting in a highly distorted portion of space.

The effect is so significant that black holes can carve up a hole in space so deep that not even light has the energy to escape! Amazing.

7. It can be argued that black holes are better than suns at generating energy

Believe it or not, black holes are, under the right circumstances, capable of producing energy more effectively than a star, like our Sun.

But how? Well, this has everything to do with the material disc that revolves around a black hole. The material on the inner edge of the disc closest to the event horizon will orbit considerably more quickly than the material on the outer edge of the disc. This is due to the higher gravitational force closer to the event horizon.

Due to its orbit and rapid movement, the material can warm up to billions of degrees Fahrenheit, which has the power to convert the substance's mass into energy in the form of blackbody radiation (aka "Hawking Radiation"). What's more, this energy potential is enormous.

Comparatively, nuclear fusion produces energy from mass at a rate of roughly 0.7 percent. By comparison, something up to 40 percent of the mass around a black hole is converted to energy by this process! That is, orders of magnitude more efficient.

While far beyond our technological capability yet, scientists have even postulated that we could one day exploit a similar process to power starships in the future!

8. Black holes can also distort time itself

Everything to know about black holes
Black holes actually slow down time.

Black holes not only distort space but also distort time itself.

Consider the twin experiment, which is frequently used to clarify how time and space interact in Einstein's theory of general relativity, to see why.

One twin travels to the edge of space at the speed of light, turns around, and returns to Earth while the other twin stays on Earth. The twin who traveled through space ages far more slowly because time moves more slowly for someone moving more quickly.

Due to the intense gravitational pull of the black hole, you are traveling at such high speeds as you approach the event horizon that time will slow down for you!

9. Black holes are great at creating energy, but it kills them in the end

Everything to know about black holes
Artist's impression of gravity waves around a black hole.

While black holes might be more efficient than stars at generating energy, the same process may also be their Achilles' heel.

In theory, black holes will simply "evaporate" into nothingness over time unless constantly fed with mass from stars and other heavenly bodies. Although, admittedly, it takes a great deal of time.

Stephen Hawking initially developed this startling finding in 1974, which is why the phenomenon is known as "Hawking radiation".

A black hole emits Hawking radiation over a volume of around 10-20 Schwarzschild radii (the radius to the event horizon). Black holes decay and lose mass over time because the energy emitted by Hawking radiation slowly reduces the curvature of space around the black hole. Once enough time passes (and we're talking billions of years for even the tiniest black hole), the black holes will evaporate entirely.

A sad end to one of the universe's most enigmatic objects.

10. In theory, anything could become a black hole

Everything to know about black holes
Artist's impression of a black hole and accretion disk.

The main distinction between a black hole and our Sun is that a black hole has a much stronger gravitational field because it is much denser. We cannot observe black holes because of their powerful gravitational field, which can prevent the escape of even photons of light.

So, theoretically, at least, you could create a black hole out of anything.

For instance, if you compressed all of the mass in our Sun into a region that was only 3.7 miles (6 km) across, you would have created a black hole and an exceedingly dense region.

However, there is only one known process that can create a black hole: the gravitational collapse of a star that is much more massive than the Sun. So, while theoretically possible, the energy required to do so is probably only available when a massive star collapses very quickly.

11. In the end, only black holes will remain in the universe until they too disappear

Everything to know about black holes
In the end, only black holes will remain.

As we previously mentioned, nothing can escape from inside the event horizon of a black hole. Not even light itself.

This led some, including the late great Stephen Hawking, to predict that black holes will be the only objects left in the universe in the far future. Long after all stars have died and galaxies have been ripped from our field of vision by the speeding cosmic expansion, only black holes will remain to show that anything ever existed.

But, as previously mentioned, even black holes have an expiry date. They, too, will cease to exist.

The projected time required for the most gigantic black holes to disappear is 10 to the 100th power years, or 10 with 100 zeroes after it. That is a very long time, but it is not forever in the grander scheme of things.

And so, after a dramatic start and a good run, eventually, everything in the universe will, for all intent and purposes, cease to be. Nice!

Everything to know about black holes
Alien world and black hole.

And that is your lot for today, black hole boffins.

Since they were first postulated in the early-1910s, black holes have fascinated scientists and the public in equal measure. A popular subject for science documentaries and entertainment for many decades, we still know very little about them today.

If our species survives long enough, we may, one day, harness their very power to explore the universe before it all disappears forever! Unless, of course, we are all consumed by one first.

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