The formation of super massive black holes


Unfortunately, astrophysicists are unable to view a super massive black hole being formed due to us being very far away from one. Despite this there are theories on how a gigantic black hole can form. Firstly, a small black hole that was formed due to a collapsing star could attract matter and then absorb it to gain mass and increase in size; this is known as a “slow accretion of gas”. On the other hand, a star cluster can collapse which creates a black hole from the largest stars in that cluster, then the powerful gravitational force of the black hole formed can attract the smaller stars in that star cluster. The black hole then swallows the smallest stars which make it have a larger mass and, inevitably, it makes the black hole larger[i].


A collapsing star is the vital component needed for a black hole. The star gets fuelled by nuclear fusion, this is where a hydrogen atom collides with another hydrogen atom to make a helium atom and that collision emits a lot of energy which can be used as thermal energy to maintain or increase the stars temperature, and it can be used as light energy to illuminate the surrounding area. Eventually, the star will run out of hydrogen and will begin to collapse in on itself, the star will then expand and explode violently.

The explosion leaves the collapsed core to attract atoms and will compact them so that the core is very dense, this is called a neutron star. In some cases these neutron stars that are very dense and have a very powerful gravitational pull will not let anything escape, not even light. The probability that the neutron star will become a black hole is mainly dependant on its Schwarzschild radius[ii].

The Schwarzschild radius is the radius required for an object to form into a black hole due to its high density and its high gravitational pull. We can determine the Schwarzschild radius of an object by using this equation[iii].

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The letter R represents the Schwarzschild radius in metres. The letter G represents the gravitational constant or Newton’s constant, the value for this is 6.67×10-11 N m2 kg-2  [iv]. The letter M indicates the mass of the object in kg[v]. C is the speed of light which is three-hundred million ms-1 [vi].Using this equation you can tell that objects like humans or animals have a very small Schwarzschild radius and we are unable to compress these objects into the correct dimensions. However, objects which are many times heavier than our sun have a much larger Schwarzschild radius, which means that these stars are able to make themselves dense enough to form a black hole as they collapse.

Even though there are several speculations about how super massive black holes are formed, we can definitely confirm that when a super massive black hole is formed it emits gravitational radiation7. Gravitational radiation is the release of energy when under the force of gravity also known as gravitational waves; this is the main characteristic of all black holes. Gravitational radiation is caused by oscillating matter when it is in a gravitational field, in this instance the black hole contains matter from stars and planets which are oscillating under the force of gravity[vii] [viii]. The oscillations caused by the super massive black hole are usually referred to as quasi-normal mode oscillations. These quasi-normal mode oscillations have a specific frequency depending on the mass and the spin of the black hole, a black hole with a smaller mass and a larger spin will have a larger quasi-normal frequency7.

[i] Accessed on 25/07/2016 (Cardiff University)

[ii] Accessed on 25/07/2016 (Universe Today)

[iii] Accessed on 02/08/2016 (Hyper Physics)

[iv] Accessed on 02/08/2016 (Universe Today)

[v] Accessed on 02/08/2016 (Yan Ryazantsev, 2013)

[vi] Accessed on 02/08/2016 (Google)

[vii] Accessed on 30/08/2016 (YouTube)

[viii] Accessed on 08/08/2016

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