Black hole, the mysterious abyss in our cosmos, still stays obscure to us. Despite their elusive nature, our awe and interest in this enigmatic cosmic phenomenon intensifies as we uncover the layers of its intricate configuration, wondering with anticipation to one day disclose the laws that govern our universe.
There are over 100 million black holes in the Milky Way galaxies although detection is difficult, as the majority of the observation is based upon indirect observational methods using telescopes and theoretical models. However, astrophysicists and astronomers are still interested in studying black holes because they can be used to test the fundamental theories proposed by physicists. For example, black holes are used to test Einstein’s general relativity theory that was first published in 1915, in which it correctly indicates that when a gigantic star dies, it will leave a highly dense core behind with its mass more than 3 times of the mass of the sun; this is later verified through the discovery of the existence of black holes in 1964, Cygnus X-1. Through this, studying black holes help us to understand more about the universe, especially when black holes are also concerned with many other cosmic bodies and their behaviours.
In this article, we will be mainly discussing the formation of the black holes and their configuration and structure.
Formation
Black holes can be formed from two pathways, one is from the gravitational collapse of massive stars in which they have exhausted their nuclear fuel, whereas the other way is from the direct collapse of gas clouds, which also collapse under gravity but bypass the step of star formation.
The first way: collapse of massive stars forms the majority of the black holes we have currently discovered. Some well-known examples include Cygnus X-1, V404 Cygni and GRO J1655-40; interestingly, they are all binary (or double) star systems consisting of a black hole and a companion star. This is because the binary star system allows observable effects and interactions to be identified which makes it easier and more reliable about indication of the presence of black holes. The collapse of massive stars is due to the exhaustion of the nuclear fuels; within the core of the stars, nuclear fusion occurs, turning hydrogen into helium. This process releases energy in the form of photons thus providing an outwards pressure that balances the force of gravity, preventing the stars from collapsing. However, when the stars have “exhausted their nuclear fuel”, meaning they do not have sufficient hydrogen to carry out the nuclear fusion reaction, gravity becomes significant and the stars then go through gravitational collapse, forming black holes.
The other way: collapse of massive gas clouds is a theoretical conjecture as there has been insufficient evidence to support the hypothesis. This process is very similar to the collapse of massive stars as it is also the collapse due to the exhaustion of nuclear fuel and the significant gravitational force. However, this channel is expected to result in black holes that are more massive and are thought to happen in the early state of the universe. Gas clouds are regions that consist of gases, mainly hydrogen gas but also may contain other elements, it has an important role in the space in which it leads to the formation of stars and planetary systems.
Black holes consist of different parts including event horizon and singularity.
Event Horizon
Event horizon is an invisible boundary around the black hole in which beyond this region, nothing can escape, not even light. The shape of the event horizon can varies depends on different types of black holes; for a static black hole, known as the Schawarschild black hole, the event horizon is spherical whereas for a rotating black hole, known as the Kerr black hole, is more complicated but it can be an oblate shape.
The Kerr black hole rotates around its own axis and it has a region called the ergosphere which is located outside the event horizon; this characteristic is not present in the Schwarzschild black hole. In this region, an object cannot remain at rest due to the frame-dragging of the black hole which is proposed by Einstein’s general relativity theory; as black hole rotate, it also cause the spacetime around it to rotate along with it within the ergosphere and hence the object within this spacetime are also forced to move. In order for the object to remain at rest relative to the observer outside the region, it needs to move at the same speed as the rotation of black hole, which is nearly or even faster than the speed of light. As this is almost impossible, it implies that objects cannot stay static within the ergosphere. This object can either move in the same direction as the black hole’s rotation in the co-rotating region or move in the opposite direction in the counter-rotating region.
Singularity
When a star or gas cloud collapses, all the mass will be concentrated into a point which is known as the singularity hence it is an extremely dense core, located at the centre of a black hole. At this point, the gravitational force becomes infinite which indicates that normal laws of physics will no longer apply. As distance to singularity decreases, the gravitational force intensifies, together with the tidal effect, which leads to a phenomenon called spaghettification; this causes objects to be stretched into long and thin shapes. As the singularity is hidden by the event horizon and has infinite density, the current knowledge is insufficient to describe what happens within and beyond the singularity, therefore there are many theories concerning wormholes and time travel being proposed regarding black holes but all are not supported by evidence.
Categories of Black Hole
There are three main different types of black holes, mainly divided by the size of black holes: stellar-mass black hole, intermediate black hole and supermassive black hole.
| Types of black hole | Mass | Origin |
| Stellar-mass black hole | 2-150 M☉* | Star |
| Intermediate black hole | 102–105 M☉ | Unclear |
| Supermassive black hole | 106–109 M☉ | Unclear but it is believed that this type of black hole grows by feeding on smaller objects such as stellar-mass black holes and neutron stars. |
*M☉ = mass of the sun
At the end of this article, it is important to realise that even though our knowledge on black holes is almost empty, as technology advances, we will be able to solve the problems on this mysterious body and carry on with our magnificent study of the universe in the future.
Bibliography/Reference
https://en.wikipedia.org/wiki/Black_hole#Innermost_stable_circular_orbit_(ISCO)
https://science.nasa.gov/astrophysics/focus-areas/black-holes/
https://www.space.com/15421-black-holes-facts-formation-discovery-sdcmp.html