Abstract
Everyone has heard about those most exotic of objects - Black Holes. But how many really know about them; how they are formed, their locations and what happened if you happened to fall into one? Read this chapter to find out all (well almost) of the answers to these questions.
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Notes
- 1.
The term black hole, was coined by John Wheeler in the latter 1960s. However, that such objects might exist is not a new idea, as Cambridge professor and amateur astronomer John Michell wrote in 1783. He suggested that stars with escape velocities exceeding that of light might exist. Furthermore, in1796, the mathematician Pierre-Simon, Marquis de Laplace, made similar calculations using Newton’s theory of gravity and called the objects “dark bodies.”
- 2.
There are some theoretical astrophysicists who have suggested that for the most massive stars, a black hole may form at the centre of such stars before a neutron star can form.
- 3.
In 1783, the British astronomer Rev. John Mitchell realized that using Newton’s laws of gravity, a situation could occur whereby an object 500 times the radius of the Sun but with the same density would have an escape velocity greater than the speed of light. Although he didn’t know it, he was talking about a black hole.
- 4.
If something that has no size can correctly be called an object.
- 5.
It has been suggested that by its very nature, we will never be able to fully describe or even understand the singularity at the centre of a black hole.
- 6.
The reasons why naked singularities cannot exist has been given the name “The Cosmic Censorship Hypothesis” and deals with the geometry of spacetime. Basically, what is implies is that if they do exist, Einstein’s General Relativity needs a revision.
- 7.
Rotating black holes are not spherical but have an oblate shape. Their description however is far more complex and will be discussed in the rotating black hole section.
- 8.
In some cases, a supernova remnant that does not have a central pulsar or neutron star may have a black hole at its center.
- 9.
Don’t worry if you are unfamiliar with angular momentum, it is just a physics attribute that all spinning objects possess with the proviso that the total angular momentum of a closed system remains constant.
- 10.
These are Schwarzschild and Reissner–Nordström black holes, with the former having no electric charge, and latter having a charge.
- 11.
Sir Roger Penrose was awarded one half of the 2020 Nobel Prize in Physics “for the discovery that black hole formation is a robust prediction of the general theory of relativity”.
- 12.
Stephen Hawking died in March 2014, aged 76, and was undoubtedly one of the world’s greatest scientists.
- 13.
Virtual particles exist in pairs, a particle and antiparticle which can be of any kind. These pairs exist for an extremely short time, and then annihilate. But in some cases, the pair may be torn apart using external energy so that they avoid annihilation and become actual particles, such as in the scenario we are describing.
- 14.
There are some relativists who propose that in an unimaginably distant future, black holes will indeed “evaporate.” We will long be gone for this issue to worry us.
- 15.
That is, if we ignore the immense amount of radiation being formed around a black hole and the debris from stars that have been literally torn apart.
- 16.
I imagine that you are surprised at the simplicity of these two equations, considering we are dealing with General Relativity, which has extremely difficult mathematics. I know I was when I first saw and used them.
- 17.
To be completely accurate, for stellar mass black holes, her light would have been red shifted completely into the radio regime, and so would she be invisible to the naked eye. In fact, the light is so far redshifted that no known telescope could detect it.
- 18.
We discuss these in the chapter on active galaxies.
- 19.
Sagittarius A* is now believed to be made of two components: SgrA East and SgrA West. The former is a supernova remnant, and the latter is an ultra-compact, non-thermal source, i.e., a black hole.
- 20.
Recent analysis suggests that the density around the center of the galaxy is about a million times greater than any known star cluster. It is probably made up of living stars, dead stars, gas, and dust, and of course a black hole.
- 21.
To get a sense of scale, consider that Mercury is 46 million km from the Sun at perihelion.
- 22.
Now for a surprise. Astronomers have suggested that instead of a supermassive black hole, it is instead a mass of dark matter. Further measurements await for confirmation of this eye-opening statement. “E A Becerra-Vergara et al, Hinting a dark matter nature of Sgr A* via the S-stars, Monthly Notices of the Royal Astronomical Society: Letters (2021)”.
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Inglis, M. (2023). Black Holes. In: Astrophysics Is Easy!. The Patrick Moore Practical Astronomy Series. Springer, Cham. https://doi.org/10.1007/978-3-031-16805-5_14
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DOI: https://doi.org/10.1007/978-3-031-16805-5_14
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