What is a Black Hole?

We have all, at some time or another heard of black holes, but what exactly is a black hole and why is it ‘black’?

As I described in my post about star types, most stars grow slowly into massive red giants as they run out of hydrogen. Most then collapse heating up in their final years. This collapse is halted quickly however as most stars don’t have enough mass to create a gravitational pull strong enough to overcome electron degeneracy pressure thus preventing further collapse. In simple terms this is the force that prevents electrons sticking to the protons in the nucleus of the atom. The star can’t release any more energy and it slowly loses its outer layers to space leaving a cooling white dwarf the size of Earth.

More massive stars (those with more than 10 solar masses) continue gravitational collapse past electron degeneracy. This means that in the final moments of a massive star’s life it actually fuses electrons and protons together to form neutrons. This process releases massive amounts of energy which overcomes the gravity and the star rips itself apart in a type II supernovae. The core of the star remains as a small, dense ball of neutrons – a neutron star.

However some stars are so massive that they are even capable of overcoming the force that prevents neutrons from fusing – known as neutron degeneracy. This is far stronger than electron degeneracy and marks the point of no return; once this occurs nothing can stop the gravitational collapse. The mass required for a star to overcome neutron degeneracy is a stellar remnant (that is the remains of the supernova) of about 3.5 solar masses. This is known as the Tolman-Oppenheimer-Volkoff limit, anything more massive will collapse under its own gravity indefinitely. This means that all of the remnant’s mass is concentrated in a tiny area (in compassion to it size). This in turn creates a very small region of space with a massive gravity, and answers the second question – ‘why are black holes black?.

The reason being, a black hole has such a large gravity not even light can escape from its pull. The hole itself is known as the event horizon and it is truly the point of no return. Once past the event horizon nothing, not even light can return. As a weird side note: because light can travel into a black hole you could still see the universe outside if you passed into an event horizon but you would never be seen again.

As there are many kinds of stars and galaxies (as shown in previous posts by Hannah and myself) there are also several kinds of black holes. They are divided into several broad groups based on mass. As the mass of a black hole increases so to does its size.

First is the smallest variety: – Micro black holes are thought to have been caused when the big bang caused the super compression of tiny amounts of matter. Micro black holes have never been observed as they are believed to have event horizons of around a few micrometers – one micrometer is one millionth of a meter.

The next class up is the ‘typical’ black hole – these are stellar black holes, which form when massive stars undergo gravitation collapse as described above. A stellar black can have an event horizon of around 15 miles (24 kilometres) – tiny compared to the size of the original star – larger stars will produce larger black holes due to the increase in mass.

Intermediate-mass black holes – are found in some globular clusters (groups of stars with galaxies). They are several times the mass of stellar black holes but are tiny compared to the final group.

Supermassive black holes – These are the ‘monsters’ found at the centre of galaxies. Sometimes called Active Galactic Nuclei or AGN. The black hole’s accretion disk (the debris orbiting and ‘falling’ into the black hole) creates a massive amount of energy which creates enough light in some cases to allow the AGN to outshine their host galaxy! A supermassive black hole also has a relationship with its host galaxy – it is now believed that the black hole at the centre of a galaxy actually affects its development. Also in those supermassive black holes currently detected another pattern has emerged. A supermassive black hole contains around 1/1000 of its galaxy’s mass. This may sound like a tiny fraction, but bearing in mind that a galaxy weighs several billions of solar masses and is around one hundred thousand light years across compared to a supermassive black hole which has several million solar masses however this is concentrated into around a few million kilometres; the correlation staggering. You can learn more about AGNs here – http://ya.astroleague.org/?p=288 a wonderful post by my colleague HannahH

Black holes can also be split into those that spin and those that don’t. Both types occur at all mass levels.

Perhaps another darker (if you will forgive the pun) aspect of black holes is their ability to grow. Any material that approaches the black hole will be drawn into its event horizon. Once past the event horizon it becomes part of the black hole, which in turn means that the hole has gained mass, its gravity well has increased and it has expanded (slightly).

An artist's impression of a feeding black hole Credit: NASA