Video on black holes and what is inside a black hole
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In 2016, two scientists at the California Institute of Technology, Mike Brown and Konstantin Baytgin, hypothesized the existence of a ninth planet in our solar system. With mass ten times greater than that of Earth, this planet could explain the weird orbits of some objects far beyond the planet Neptune. Since then they — and other scientists — have been looking for Planet 9 to no avail. Then in August 2020, different scientists put forth another hypothesis: that the weird orbits of these objects could be explained by a collection of bodies — one of which is Planet 9 — and that all these are black holes.
Well, that’s all very interesting but what is a black hole?
It is not empty space. It is in fact a region of spacetime where gravity is so strong that nothing — not even light can escape it — and if light cannot escape it, we cannot see it — hence black hole. Scientists think that the smallest black holes are as small as an atom but with the mass of a mountain.
When a big heavy star — over five times the mass of the Sun — runs out of fuel it collapses onto itself, causing an explosion called a supernova. That explosion squeezes the remaining matter into a tiny space, forming a black hole and giving it its heavy-duty gravity. These are stellar-mass black holes, which can be up to ten to hundred times more massive than the Sun.
This by the way will not happen to the Sun because it is not massive enough. Our star will have a much less explosive, though no less spectacular death, when, in about five billion years it will turn into a red giant and will engulf Mercury, Venus and possibly even Earth. Eventually, it will become a white dwarf with just about fifty percent of its current mass. However, that’s a long time away.
There are also supermassive black holes, which are inordinately humungous, the largest type of black holes, with mass of millions or even billions suns (generally supermassive black holes are those above one million solar masses). Some scientists have started labelling black holes above ten billion solar masses as ultra-massive black holes. Over the years, observations have revealed that at the center of most galaxies is a supermassive black hole, formed at the same time as the host galaxy.
(Check out this map of 25,000 black holes)
Even our galaxy, the Milky Way has a supermassive black hole at its center, corresponding to the location of a bright and compact radio source known as Sagittarius A*. This black hole is equal to about four million suns, but this mass is compacted inside a small space and that’s why the gravity is so high. Research is still ongoing on the formation of supermassive black holes but scientists think that they can grow by accreting matter due to their extremely high gravity or even by merging with other black holes.
(Read Stupendously Large Black holes)
Primordial black holes may have formed directly from external pressure and density fluctuations in the first moments of the Big Bang. These could have accreted matter, eventually becoming supermassive black holes. So, primordial black holes could have been the seeds of the existing supermassive black-holes.
Fun fact: black holes were predicted by Einstein’ theory of relativity but he did not believe they could exist because the idea was so bizarre. Many scientists have since then contributed to the theory that black holes do — in fact — exist. Initially, the idea of a massive body that even light could not escape its gravity was proposed by English astronomer and clergyman Jon Michell in 1784.
How do we know black holes exist?
Remember that immense gravity we talked about? That’s how. This gravity pulls material into the center of the black hole and we can see this material falling into it by the electromagnetic radiation it emits. Scientists study stars orbiting or flying by a black hole. When they are close enough to a black hole, they emit high energy light, which they can then observe with satellites and telescopes.
Black holes have some well-known characteristics:
Event Horizon
An event horizon is the defining feature of a black hole. This is the boundary in spacetime through which matter and light can pass only inward into the black hole. Once matter or light have passed the event horizon, that’s it, there is no escape. Any object approaching the event horizon appears to slow down to an observer and seems to take an infinite time to crossover, till it fades away and can no longer be seen. Typically, this happens very quickly though, taking less than a second. However, if you were to fall into a black hole, time would appear normal to you.
Singularity
At the center of a black hole may lie the singularity — a region where the curvature of spacetime becomes infinite. The singularity has zero volume but contains all the mass of the black hole. If you fall into a black hole and reach the singularity, you will go through something called spaghettification. You will be stretched so that you look like spaghetti, and then torn apart. After that, you will be crushed into infinite density and your mass will be added to that of the black hole. Which I think is very cool! Black holes can be static and rotating and both types can have a singularity.
Active Galactic Nuclei
As gas and dust from stars and other objects, orbiting a super massive black hole at the center of a galaxy, falls into it, it forms an accretion disk. This disk emits electromagnetic radiation that can be captured by our instruments. The extreme gravity of the black hole compresses the accretion disk till it reaches millions of degrees kelvin and forms the bright active galactic nucleus at the center of a galaxy.
Quasars are extremely luminous active galactic nuclei, in which a supermassive black hole with mass ranging from millions to billions of times the mass of the Sun is surrounded by a gaseous accretion disk. The power radiated by quasars is enormous and we are able to detect the electromagnetic radiation they emit with our instruments. A blazar is an active galactic nucleus emitting a jet composed of ionized matter, traveling nearly at the speed of light.
For most of the time we have known about black holes we were never able to directly observe them. Then, in 2016, scientists announced the detection of the merger of two black holes. The merger produced gravitational waves (also predicted by Einstein), which were captured by special instruments here on Earth. Since then gravitational waves produced by many such mergers have been detected, some even indicating the presence of intermediate mass black holes i.e those between stellar mass and supermassive ones.
Our first ever view of what a black hole looked like was in April 2019, when a direct image of a supermassive black hole and its vicinity was revealed through observations made by the Event Horizon Telescope Collaboration. This super-massive black hole lives at the center of a supergiant galaxy called Messier 87 (fifty five million light years away from Earth). That image confirms scientists’ predictions of black holes and their event horizon.
And just in September 2020, a new comparison of that image with earlier Event Horizon Telescope data shows that the brightest spot of the ring around the black hole changes — seeming to “wobble” as scientists put it.
[Update] And on May 12, 2022, The Event Horizon Telescope Collaboration did it again! This time they captured an image of the black hole inside our own Milky Way galaxy.
Black holes are cool and definitely worth studying. Check out this cool animation comparing the sizes of different black holes.
All images Creative Commons and Public Domain / NASA Goddard/ NASA/ European Space Agency (ESA) / European Southern Observatory (ESO) / Event Horizon Telescope
Music: Title music: Hovering Thoughts by Spence (YouTube Music Archive)
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Saima Baig
I am a Chartered Environmentalist from the Royal Society for the Environment, UK and co-owner of DoLocal Digital Marketing Agency Ltd, with a Master of Environmental Management from Yale University, an MBA in Finance, and a Bachelor of Science in Physics and Mathematics. I am passionate about science, history and environment and love to create content on these topics.
