Black holes are perhaps the most interesting and terrifying objects in the Universe, despite us not really knowing much about them. They most commonly occur from the collapse of a massive star when the core runs out of fuel to ignite the star, collapses, and the outerlayer explodes in a phenomena known as a "supernova" blast. Many articles on a black hole's formation have been written by our team here, but to summarize: As the core of a massive star collapses, its gravity increases. The gravity gets so strong that, at the surface of the collapsing core, the escape velocity (how fast something would need to be traveling to escape the gravity of an object) increases to the speed of light, so nothing (not even light) can escape. Thus, why they call it a "black" hole. You may have already known that, but here are two more facts about black holes that you might not know.

A diagram that depicts the spin of Kerr black holes and stationary ones. (Source)

1: Believe it or not, black holes can spin! That shouldn't come as much of a surprise due to the fact that stars rotate, and black holes are typically formed from the remnants of the collapsed core of a star. They can be formed in a few other ways. One method of which black holes can be formed is through a massive concentration of energy. (Black holes of this type are called Kugelblitz black holes). They might possibly even be formed during particle accelerator collisions, tentatively dubbed as "micro-black holes."

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What happens is this: when the core collapses, the rotation speed increases significantly. As the core of the star condenses further, the rotation speed continues to increase and, if it doesn't have enough mass to become a black hole (typically the mass of 3 of our suns is necessary), the mass of the star condenses to an object only a few kilometers across. Though it is small, it is EXTREMELY heavy. These objects are called neutron stars. (as seen in the diagram on the left) If the magnetic poles of the neutron star are pointed towards our line of sight, we are able to see the light and radiation it emits, classifying the neutron star as a pulsar. Anyways, the same is true for a black hole. Even as the matter shrinks smaller than the event horizon (and is lost to the Universe forever, or at least we think so), the matter is still spinning continuously. 2: Though classified as black by name, 'black' holes aren't always dark. As a matter of fact, they can become spectacularly bright given the right circumstances. This brightness generally occurs when stellar material begins to inch its way towards the event horizon of a black hole, which is the point at which the matter is destined to spiral inwards into a singularity. Sometimes, more material will accumulate just outside of the event horizon than the black hole can devour. Thus, the material builds up and forms an accretion disk. The material in the disk will sometimes twirl around the event horizon in a sideways motion, before actually going around the black hole before being sucked in.

An artist' rendering of an accretion disk. (Source)

As a general rule, the more matter that falls into the black hole, the more matter that will pile up, sort of like an insanely huge intergalactic traffic jam. Some of the matter that's orbiting in towards the event horizon will be orbiting much faster than the matter that's orbiting further out. As a result, the matter will literally will rub together, which generates an astonishing amount of heat (sometimes hundreds of thousands of degrees) through friction. Consequently, the matter in the surrounding disk can become so hot that it appears luminous. Furthermore, the magnetic poles of said black holes can sometimes generate a powerful beam of gamma radiation that an be seen from tremendous distances, wreaking havoc on anything located within a certain distance from the action. So tell us, what obscure black hole fact do you find the most interesting or mind blowing. Besides, you know, the fact that they exist at all!

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