What Are Black Holes?

Image of a black hole

Black holes are one of the most fascinating aspects of space. They have such an extreme gravitational pull that not even light can escape. They are so phenomenal that they are said to have the ability to alter entire galaxies.

Black holes are severely curved off that it can be considered pinched off from the rest of the universe. Within a black hole, the laws of physics as we know them no longer apply. Using Einstein’s theory of relativity, physicists such as Karl Schwarzschild theorized that any mass can become a black hole if it was compressed into a space small enough. Black holes were first predicted by Einstein in 1916, however, it was only until 1971 that the first black hole was physically discovered.

Whatever happens within the black hole will never be known since no signal comes from this area. As we know, black holes have an infinitely curved space-time and density. Therefore a black hole has three layers so to speak. The outer and inner event horizon and the singularity. The singularity is where most of the black hole’s mass is concentrated.

A common misconception about a black hole is that it sucks in everything like a vacuum cleaner. This is not the case. A black hole will only affect matter that comes across its outer edge known as the event horizon. The event horizon is a boundary surrounding the black hole and in this area, nothing can escape its gravitational pull. Anything that comes across here is ultimately there for good. How wide the event horizon is depends on the mass of the black hole.

Objects fall into black holes due to their extreme gravitational pull, just as objects fall while on earth due to the earth’s gravitational pull.

How are black holes created?

When a star with a core that is 2.8 times bigger than the sun’s mass dies and its core collapses, a stellar-mass black hole is formed. How does this happen? When the core of a star begins collapsing, its gravity progressively gets stronger, meaning its escape velocity gets higher and higher. For a neutron star, during its collapse, its velocity is half the speed of light. For a star that is 2.8 times the sun’s mass or bigger, its core keeps collapsing,  its size begins to continuously drop and when it gets to approximately 18 kilometers, its escape velocity is equal to the speed of light. As we know, nothing travels faster than the speed of light, and at this size, nothing can escape this hole. Therefore whatever comes across the path of a black hole, will never come out.

That is the origin of a standard stellar-mass black hole, however, it has not been determined how supermassive black holes such as Sagittarius A* are formed. Some theories have suggested that supermassive black holes are formed after stellar mass black holes and intermediate mass black holes collide.

Black holes are such a hot topic for astronomers and physicists today that even years after their discovery, their properties are still debated and argued over.

Can the earth and the sun become black holes?

For an object to form a black hole, it needs to have 20 times the sun’s mass or more. The object’s mass has to be large enough that its gravity is able to overcome any external forces pushing for its collapse. The earth has a mass of approximately 6×1024 kilograms, that’s too light in comparison. The sun itself does not have enough mass for it to become a black hole.

Sizes of black holes

As mentioned before, black holes have different sizes and those discussed most often are stellar black holes. These black holes have a mass of approximately thrice the sun and can go up to a dozen times the size of the sun. When stellar masses absorb more matter, they become larger. Huge black holes can be found in the center of galaxies including one in our very own milky way. Its mass is said to be 4.3 million times the sun’s mass and is known as Sagittarius A*.

The smallest black hole ever observed was found in the binary system GRO J1655-40. This black hole is said to be 5.4 times the mass of the sun and with a radius of ten miles. It is thought that unlike stellar black holes and larger black holes, small black holes are formed after two neutron stars merge instead of the collapse of a dying star. The size of stellar black holes is dependent on the mass of the original dying star. Some scientists also believe that primordial black holes, which are microscopic black holes, were created during the Big Bang.

Intermediate black holes have been harder to find. These black holes are bigger than stellar black holes and smaller than supermassive black holes. The most recent intermediate black hole to be discovered was found by the Hubble Space Telescope. Although bigger than stellar black holes, they are said to have a mass between 100 and 100,000 solar masses. In comparison, supermassive black holes can be a billion times the sun’s mass.

Within our galaxy, we have a supermassive black hole named Sagittarius A*. It weighs approximately 4 million solar masses and its radius is 17 times more than that of the sun. This may sound big, however, if compared to other black holes in other galaxies, this is small. The biggest black hole ever discovered was found within the Abell 85 galaxy cluster. Within this cluster is the galaxy Holm 15A. The black hole is found at the center of this galaxy. Its mass has been said to be astonishingly 40 billion solar masses with a diameter the size of the entire solar system.

How astronomers find black holes

Simply because nothing can escape a black hole, not even light, it is not possible to see them. These ‘objects’ do not produce photons that would allow them to be observed as an ‘object’ that is why they are called black holes. Because of their dark nature, only a few of them have been found although astronomers and physicists speculate that they’re more even in our galaxy. Astronomers have found them by observing the behavior of stars around them.

When a star falls into a black hole, material from the star heats up within the black hole and releases x-rays which can be observed by astronomers. A star that gets too close to a black hole will get torn apart by the black hole’s forceful tides. As it gets destroyed, it emits light which is blasting out energy.

In 2019, astronomers made a historic announcement. They were able to capture the first image of a black hole. To be more specific, they were able to capture an image of a black hole’s event horizon. This image was captured through a collaboration of the  Event Horizon Telescope. This black hole was discovered in the center of the galaxy M87. 

How many black holes are in the universe?

Black holes are generally shrouded in mystery. There is a general agreement by astronomers that there should be tens of millions of them in our galaxy. This has been inferred based on the number of stars we have. Because they are so difficult to find, only dozens have been found.

Some of the closest black holes are; A0620-00(V616 Monocerotis), Cygnus X-1, V404 Cygni, GRO J0422+32, Cygnus X-3, GRO J1655-40, Sagittarius A*, 47 Tuc X9, XTE J1118+480, and GS2000+25.

The closest black hole is only 1000 light years away and it is HR 6819. It is also said to be the only black hole containing system that is visible to the naked eye. The Sagittarius A* is within our Milky Way and is 2500 light years away and is the closest supermassive black hole.

With technological advancement, more black holes will ultimately be found with some of them within our own galaxy.

What happens if an astronaut falls into a black hole?

The simple answer, they die. As we know by now, a black hole’s gravity is incredibly intense therefore, its tidal force is also extreme. If an astronaut fell into a black hole feet first, the force of gravity on their feet would be much stronger than the gravity on their head. Once you fall in, your feet would be pulled with such extreme force that you would stretch. This process is popularly referred to as spaghettification. This happens if they happened to fall just a few kilometers from the black hole. On the other hand, if an astronaut fell in from a further distance, their speed would be almost equal to that of the speed of light and they would die within milliseconds.

Some physicists have speculated that it might be possible to escape a black hole. These physicists speculate that there is a tunnel through space-time that connects black holes to one another. They are referred to as ‘wormholes’. This is a very wild theory that is yet to be substantiated.

Do black holes die?

Previously, it was assumed that black holes existed forever. Stephen Hawkins, however, debunked this theory. In the 1970s, Hawkins and fellow physicist Jacob Beckenstein proved that black holes emit radiation and during this emission, it carries away energy. Therefore, after a period of time, a black hole simply disappears. After calculations, physicists believe that it could take billions of years for a ‘black hole evaporation’ to occur.

A black hole evaporating is often troublesome to astronomers. This is because when an object falls into a black hole, the object’s information disappears permanently in the black hole. Then the black hole evaporation means it also permanently disappears. This poses a big problem for physicists whose equations are always expected to retain information.

Amazing facts about black holes

Black holes are such a fascination to physicists, astronomers, and laypeople alike that many fictional books and movies have been created about them. Here are a few amazing facts about them; 

  • Earlier, it was mentioned that black holes have the ability to alter entire galaxies but other scientific discoveries surmise that they could create entirely new universes. This controversial idea was birthed by Lee Smolin, a physicist based in Canada. He theorized that when a star falls into a black hole and is squeezed down to extreme density, it bounces back and expands again. Once that star expands, it creates a whole new universe. He says this is possible largely because the laws of space-time within a black hole are not as we know them and these conditions could eventually create a new universe.
  • Black holes are also capable of generating more energy than the sun. When material falls into the event horizon, it begins orbiting at extremely high speeds due to the intense gravitational pull of the black hole. Due to the extreme pace of the material, it generates heat of up to billions of degrees Celsius. If you compare, a nuclear fusion turns 0.7% of mass into energy, while a black hole converts 10% of mass into energy.
  • One of the most recent discoveries on black holes is the discovery that occasionally matter is expelled from the accretion disk and thrown into our galaxy at 32 million km/h. In the same respect, supermassive black holes have been known to release enough material that new stars are born. Further studies have also shown that supermassive black holes are able to control how many stars are in a specific galaxy.
  • If an astronaut were to fall into a black hole wearing a watch, an observer would note that his clock would progressively run slower and ultimately stop moving. Space-time within a black hole is not what we know it. If we were to observe, it would take light much longer to reach us and that is why time would seem to stop.
  • Black holes are constantly growing in size and density. Everything that gets in stays in including gas, liquid, or solid material. Physicists, however, believe that black holes do not grow beyond 10 billion solar masses. This is because, above this mass, it would damage their accretion disk.

There is so much to learn and to be discovered about black holes. Astronomers and astrophysicists are constantly learning and discovering and that in itself makes black holes an amazing phenomenon of space.






Leave a Reply

Your email address will not be published. Required fields are marked *

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.