Is it possible to harness energy from a black hole? What are some theoretical methods?

 Not only is it possible, it’s theoretically one of the most efficient sources of energy out there.

The black hole itself, by which I mean the event horizon and everything inside, is not a great source of energy. It was a famous discovery by the late Stephen Hawking that despite their gravity, all black holes do emit a little radiation from their event horizon, but it isn’t much. In fact, the bigger the black hole, the less power you get.

But if you throw something in to the black hole, then the fun happens. Almost every black hole is spinning very fast, and this effect pulls material approaching it into what’s called an accretion disk:

While spiraling in, the matter will go faster and faster, and wrap more and more tightly, and before it completes the descent it will start feeling intense friction against itself. Basically, it will give itself a rug burn.

(We’ve only seen this effect happen with gas from stars, but I can’t think of a reason it wouldn’t happen with anything you toss in.)

All that friction turns to heat, then to radiation. This radiation takes the form of X-rays, blasting out perpendicular to the disk:

This is such an intense effect that a lot of the gas falling in will actually disappear. Mass is energy, and energy must come from somewhere. E = mc^2. In this case, a large portion of the mass directly transmutes into X-rays. The black hole is such a gluttonous eater that a lot of what it eats explodes into light before it can get it into its mouth.

This has an absolutely insane matter → energy conversion process. For reference: the only power source we currently have that directly converts matter to energy is nuclear power, which scrapes 0.1%, a mere pittance, of the mass of U235 into energy as it splits apart. The sun’s nuclear fusion is a bit more efficient, converting 0.7% of the mass of its hydrogen into various kinds of light, the other 99.3% settling into helium.

The conversion rate of accretion disk of a black hole can be up 40% .

Forty percent!

One kilogram (~2.2 pounds) of matter converted 40% of the way into energy would yield 36 petajoules, or 10 terawatt-hours. That’s enough to power the entire world for half an hour. A metric ton could power it for three weeks.

So, the potential is there. Now we just need a way to get there :-)

What does a black hole look like? Is it like a funnel or a ball?

 The whole funnel thing is lazy illustration. It’s a convenient way to show “really deep gravity well” by dropping the funnel off the bottom of the illustration.

When a large mass bends spacetime it’s actually more like this - but it’s a lot harder to depict “really deep well” this way.

Now, imagine all those lines being bent even more the closer they would pass by the center. You may *think* “but they’ll come out the other side” - but that’s only true if you’re stuck in a flat Euclidean 3D space like we evolved to live in.

The gravity field around a black hole is so strong that distance doesn’t behave like you expect - even if you bend those lines in the picture 3 or 4 or 10 times further towards the center, they still won’t stick out the other side - they won’t even reach the center. You draw a new line closer than the ones in the grid in the picture, and you bend it more and it still doesn’t reach the center. Draw one close enough, and you need to bend to (nearly) infinity to reach the center…

Fortunately, the event horizon shields us from the *real* weirdness inside…

Some of the other answers have the recent imaging of the supermassive black hole in the galaxy M87. The actual hole isn’t visible, of course. What you see is the radio waves from the area just outside the event horizon. Why did they image a distant black hole and not the one in our own galaxy? Partly because M87 is actively chowing down on stuff that’s falling into it, and the accretion disk is emitting all sorts of light - everything from radio waves to gamma rays. Meanwhile, our own galaxy’s central black hole is relatively quiet, because it’s already chowed down everything that passes too close to it, so it’s relatively quiet compared to M87.

Was the Sagittarius A* black hole ever a star?

 Stars with mass more than 150 Solar Masses are believed to be unstable. The radiation pressure from the core would be so high that it would shed the outer layers of the star eventually leading to loss of mass.

Sagitarius A* weighs around 4 Million Solar Masses.

So in order for it to be a star before becoming a Super Massive Black Hole, it would have to be a star with less than 150 Solar Masses, become a blackhole (in a process which anyways sheds a lot of mass) & then grow from there to acquire a mass of 4 Mn Solar Masses.

Frankly this process would need a LOT more time than the age of the universe. So it could not be a star before becoming a black hole.

Blackholes this big are formed by either of the following three processes:

1. Massive cloud of gas compressed to not just form a star but directly a blackhole. This is believed to be the most common or likely way of formation of Super Massive Blackholes.
2. Blackhole Collisions can also make bigger blackholes. But in order to make blackholes with Millions of Solar Masses, you need loads and loads of Stellar Mass Blackholes. And loads & loads of time.
3. Accretion Disk (the glowy ring around the Blackhole) is another way of adding mass to Blackhole. But its unlikely, because there usually has not been enough time for a blackhole to eat so much for it to grow this much.

How massive is a 100 kilometres diameter black hole? What will happen if a 100 kilometres diameter black hole appear near Sun?

 Okay for context, if our sun became a black hole at this moment, it would have a diameter of 3km. determined by its Schwarzschild radius, which is the radius of the event horizon, or the "point of no return" for light.

Yeah.. that sets the tone for a 100km diameter blackhole. It would have a mass of about 34 times the mass of the Sun. It would be classified as a stellar-mass black hole. Currently the most massive Stellar-mass BH we know in our galaxy is Gaia BH3 with about 32x solar mass, but of course, in our scenario, our BH tops the rank of stellar-mass BHs.

It would definetly disrupt our solar system, possibly causing every object here to fall into it or get ejected into outer space.. and might even disrupt the path of other star systems.

Though honestly, it is still nothing compared to the blackholes found in the centure of galaxies, called Supermassive blackholes such as our milky way’s known as Sagittarius A*. It has a mass of 4.1 MILLION times the mass of our sun and has a radius of 12 MILLION KILOMETER. They are in a league of their own, both technically and figuratively.

Mind boggling numbers huh…

Can you explain the differences between a quasar, black hole, and neutron star?

 Quasar is massive luminous object in space that resides supermassive black hole in it's centre. It emits electromagnetic radiation and accreting materials into it. It is found at the core of galaxy. Its masses million to billion of solar masses.

Other hand, black hole is defined by its event horizon, singularity and an Accretion disc. Black hole can be of stellar massive, super massive and tiny size.

Neutron star is remnant death of star having solar masses 1–2 units. It composes neutrons only.Neutron stars rotate rapidly.

Where do black holes lead?

 A black hole is not really a "hole".

A black hole is a celestial body left over from a giant star that has died where its gravity becomes so great that the curvature of space is very steep, resulting in no matter whatsoever being able to escape its gravity when it gets too close, even light.

Black holes are not like this:

Or this.

A black hole is more like this:

Or like this:

So, instead of a hole, a black hole is a black, three-dimensional object with extraordinary density.

And, if asked "where does a black hole lead" then in this context it is the center of the black hole. What is at the center of a black hole? Experts agree to call this unknown region the singularity .

Previously, black holes had an area called the event horizon, which is the boundary where something is no longer possible to escape from the black hole. In the middle of the boundary in the event horizon is called a singularity, which is a place where mass or density becomes infinite and the curvature of space-time becomes infinite too, because gravity is so strong there.

What happens if someone goes in there?

Gone… and never to return.

At least that's what we expect, because we don't have any data yet on what and how the singularity is filled.