Gravity is a universal force that everyone agrees upon, but when it comes to objects in the Universe that are heavyweights in the gravity realm, that title is owned by black holes, followed by neutron stars coming in second place. However, what in the Universe could have given rise to such behemoths boasting such a high gravitational force that, in the case of a black hole, even light cannot escape it! Well, the answer lies in a surprisingly humble beginning, which we will take a look at now.
It all starts with a cloud of gas. Yes, you heard that right… a cloud of gas! This cloud of gas, which is primarily filled with hydrogen, is commonly referred to as a nebula. This cloud of gas increases in density in certain pockets, until it can form a sufficient and stable amount. This gaseous mass, at some point, becomes stable enough and develops a strong gravitational field, which leads to the first stage of the star, also known as a protostar. The gravitational field only increases from here on out, as the protostar sucks in the remaining gas, further increasing the mass of the star and taking in the necessary amount of hydrogen. Hydrogen is the primary fuel that makes the star burn so brightly, driven by the process of nuclear fusion. During the process of nuclear fusion, however, hydrogen atoms are smashed together at the protostar core.
This smashing process continues and results in two very observable phenomena. The first is that nuclear fusion does not entail only the smashing of hydrogen together, but also other higher-order elements. To explain this further, when two hydrogen atoms fuse, they form helium, which then goes on to fuse further, until the element iron is reached. The other phenomenon is that, as the higher elements above hydrogen begin fusing, the mass of the star keeps increasing. The fusion process can only occur until the core contains a limited amount of hydrogen. Once the hydrogen is exhausted, the core becomes unstable because it is unable to continue the fusion process for the heavier elements, such as iron. The instability of the core leads to the end of a star’s life, which will go out in spectacular fashion, only to return like a phoenix from the ashes.
The formation of a neutron star occurs when a giant star’s core collapses, resulting in its death. The explosion at the end of this life cycle is incredibly intense and puts out a phenomenal amount of energy in what is known as a Supernova. Neutron stars are city-sized stellar objects that have a mass 1.4 times greater than the mass of our sun. To put that in simpler terms, their small size accounts for the fact that they are exceptionally densely packed, such that a single teaspoon of a neutron star could weigh billions of tons.
The birth of a neutron star occurs after the death of a red giant. The mass of a red giant is a minimum of 20 times the mass of our Sun. When a star more massive than our Sun explodes, its outer layers explode spectacularly in the form of a supernova. However, what is left behind is a small dense core that continues to collapse; in fact, the gravity is so strong that it makes the protons and electrons on its surface melt into a neutron! This is actually what gave it the name neutron star. The power of the supernova that births a neutron star makes the sun rapidly rotate, causing it to spin several times per second. Neutron stars can rotate as fast as 43,000 times per minute.
Interesting things occur if a neutron star is not alone, but instead part of a binary system. A binary star system is a system in which two stars revolve around a joint center of mass. If the second star has a mass that is less than our sun, it pulls mass from its companion into what is known as Roche Lobe, a balloon-like cloud of material that orbits a neutron star. This makes the secondary star, which is still intact, transfer its mass to the neutron star and ultimately meet its demise.
These strange objects are like another phoenix story, as they spring from the ashes after the demise of a star. According to Einstein’s theory of general relativity, for a black hole to be born of a dying star, it must be at least three times the size of our Sun. The star must be significant enough so that the gravitational force at the time of its demise will exert a force that can overwhelm any other factor. The material in the core of the star must be completely crushed to a small point of infinite density. The funny thing, however, is that our physics don’t work beyond this point, as our mathematical understanding struggles with the concept of infinity.
If this stellar vestige is left alone, the black hole would do very little and pretty much be idle. However, if there are gas and dust particles around the black hole, they will get sucked into the black hole, creating a bright glow as the dust and gases heat up, swirling around the black hold like they are going down a drain. The black hole will incorporate this material into its mass and continue to grow in size. When it comes to black holes, truly fascinating phenomena occur when two of them meet. The powerful gravity of each will attract the other, making them come closer and closer to one another. When they’re in contact, they will be able to shake the very fabric of space-time (the area around and close to it) itself, sending out gravitational waves.
In conclusion, we can say that almost every time a star goes down, it will come roaring back in the form of a neutron star or a gravitational heavy-hitter like a black hole!