What is a Black Hole?


Black holes is one of the most popularized exotic space objects. Even if you are far from astronomy, chances are you still heard about black holes. What black holes really are? How do they form and how do we know they exist?

Let’s start with a gravity explanation. Go outside and throw a stone as far as you can (just please don’t hurt anyone :-) Your stone will fall down on Earth, because there is a gravity force between any two objects which have masses. Stone has a mass (and you can feel it like stone’s weight), Earth has a mass (and very big one!), so these two masses create a force which we call gravity.

If you climb somewhere high in the mountains and throw the same stone exactly the same way it will fly further, because gravity force weakens with the distance. In university we even measured different gravity force on different building floors, as you climb up the stairs you can see how gravity force weakens (this is one of the reasons why most of the modern athletic world records were created in high altitude stadiums). So, the point is — the further you are from the object the lower gravity force is, the closer you are to the object — the stronger gravity force is.

Gravity is different in different parts on Earth. Thats right, nothing is very simple in the real world. Image from: sciencefriday.com

Gravity is different in different parts on Earth. That's right, nothing is very simple in the real world. Image from: sciencefriday.com

Now let’s talk about launching satellites. You saw how your stone is falling back to Earth after you threw it. This is not what we want when we launch satellites. What is the real difference between your stone throw and satellite launch? The main difference is in speed. Use your imagination a bit: imagine if you are stronger than you are, then you obviously will throw stone further. Now imagine that you are a superman, if a superman throws a stone it can fly so far, that it will turn around the Earth and hit him in the head from behind. Minimum speed required for the stone to fly around the Earth (very close to the surface) is called first cosmic speed and equals about 7.8 km/s (4.8 mi/s). This speed is different for different altitudes, that’s why high flying satellites have lower speeds than low flying satellites. If you want to launch a spacecraft to some other planet, then you need a higher velocity, about 11.2 km/s (6.9 mi/s) for Earth. This is called second cosmic velocity or escape velocity and it means how fast you should fly to be able to completely escape gravitational field.

We use rockets to give our satellites enough speed for them not to fall back down on Eaarth. Image from: ilslaunch.com

We use rockets to give our satellites enough speed for them not to fall back down on Earth. Click for bigger picture. Image from: ilslaunch.com

I wrote about what is happening in the Sun’s core in my other introductory astronomy article about the Sun. Basically it is a huge hydrogen bomb non stop blast. Why the Sun is still in one piece? Why it didn’t brake down because of all those blasts happening there 24/7? It’s a gravity force which keeps it together. Pieces of Sun are trying to fly away, but Sun is so heavy, that it’s gravity keeps it in one piece. Every star is in a balance between these two forces. Gravity is trying to collapse the star, while wild blasts of energy is trying to expand the star. Star lives while it’s balanced.

What happens when star runs out of fuel? Those thermonuclear processes inside the star consume hydrogen, when star runs our of hydrogen nothing stops it from collapsing anymore. Star begins to shrink and it’s density grows. Imagine that you are standing right there on the star surface. As star shrinks your weight will increase: star mass stays the same, your mass stays the same, but distance between you and the star center decreases, so gravity force grows. As gravity force grows, cosmic velocities also grow, so you need higher and higher speeds to leave the star as it shrinks.

Star lives while there is a balance between two forces. Image from: astro.psu.edu

Star lives while there is a balance between two forces. Image from: astro.psu.edu

If star is heavy enough nothing can possibly stop it from total shrinking (at least nothing we currently know of), so it will be collapsing and in one point the second cosmic velocity for this star will reach the speed of light. Currently we know that nothing can travel faster than light, so at that point nothing can possibly leave this star. This kind of collapsed star is called a black hole.

It took me quite a while to explain what black holes really are, but it’s much better now when you know that there is nothing mysterious there — just the object with gravity field is so intense that even light can not escape from it. If you will stand on the “surface” of black hole and turn on your flashlight then light beam from it will bend, fly around a black hole and hit you in the head from behind :-). You can make black hole out of anything if you shrink it enough, you don’t need collapsing stars for it. If you will collapse our Earth into the ball with this radius, it will become a black hole too.

All things described above is a pure theory, we could come up with it (and we did) just on paper, without any observations. Why are we so sure that black holes exist? They don’t shine (they are black holes, remember?), they don’t reflect the light, can we possibly detect them? Yes we can! When charged particles are captured by black hole they don’t just fall on it right away — they are spinning around it as they fall, imagine some kind of spiral with every spin they come closer. Every accelerated charged particle emits electromagnetic energy and we are able to capture that energy. While we can’t capture black hole itself we can capture the things it is doing to it’s neighborhood. For example many stars are in fact double stars, our single Sun actually belongs to a minority of star population. Let’s say we have a double star and one of them collapses into a black hole. How will it affect the neighbor? Black hole will suck it’s material, spin it very fast around and ultimately consume. But again, while star material spins around the black hole we can detect radiation it emits.

Artist impression of black hole consuming a star. This kind of events we can detect with our telescopes. Image from: nrao.edu

Artist impression of black hole consuming a star. This kind of events we can detect with our telescopes. Image from: nrao.edu

OK, enough fancy talk for today, if you want to learn more about black holes you can search about how do they evaporate, this is an introductory article, so I try not to cover much of advanced stuff.

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