You’ve probably heard of black holes existing in the universe and may even have studied the phenomena in a high school science class. According to NASA, scientists speculate that they can range in size from a mere atom to supermassive ones the size of a million suns combined.
While countless artists have used their imaginations to depict these mysterious objects, you might be surprised to find out that, in spite of the size of some of them, they cannot be seen with either the naked eye or with the most powerful telescopes. How can something potentially so massive hide so well? To understand this, we must learn about what black holes are and how they are formed.
Britannica states that a black hole is a cosmic body of gravity that is so strong nothing can escape from it — not even rays of light. For a black hole to form, a massive star needs to collapse on itself. When stars age, they will grow in mass until the center can no longer support the body. Upon collapsing, the star will explode into a supernova, sending the outermost layers hurling into space. The remaining bulk of the star caves in on itself, crushing and compressing the star into an object with “zero volume and infinite density.” This object is what is known as a black hole.
A black hole consists of two parts: the event horizon and the singularity. The event horizon is the perimeter outside of the black hole itself, which is known as the point of no return. Crossing the event horizon means that you would need an escape velocity stronger than that of the speed of light in order to move away from the black hole. As nothing we know of has a velocity greater than light, nothing we know could ever escape from a black hole when it gets too close.
The event horizon forms a radius around the black hole that is called the Schwarzschild radius. Named after famed German astronomer Karl Schwarzschild, this radius is calculated by determining the mass of the object that collapsed to make the black hole. Schwarzschild surmised large collapsed stars that should be emitting radiation would not, as this radiation would have been pulled into the black hole.
The singularity is the center of the black hole itself. As light cannot escape, it means that a black hole cannot be technically seen in the traditional sense. So how do we know that they exist?
Scientists will detect, rather than see, black holes. This is done in several ways. By observing how stars are reacting, scientists can determine if they are orbiting a black hole — assuming there is high-energy light being emitted from the star (via NASA). The extraordinarily high amount of gravitational pull on nearby objects creates observable behavior, as well, giving observers a good notion that these objects are being affected by a black hole.
Cool Cosmos further explains that this force of gravity from the black hole will cause gases to be sucked closer to it, creating a gaseous disc that will rapidly rotate around it. When the disc rotates at a certain speed, the gas molecules will become very hot and emit X-rays. These X-rays can be seen here on Earth by the scientists who observe the night sky.
Only the most massive stars will ever become black holes. Small and medium-sized stars do not have the potential mass to trigger the powerful collapse necessary to begin the chain reaction to form a black hole. Stars that lack this mass will eventually begin to die and become white dwarfs or neutron stars.
Worried that our sun will collapse upon itself and suck all life on Earth into a black hole? Don’t be. Our sun is a medium-sized star and wouldn’t have the necessary mass to become a black hole. It’s also worth mentioning that even if our sun could become a black hole, we wouldn’t be alive to witness it. Long before a star dies, it swells with mass and heat, the result of which would scorch our planet of all life long before it collapsed.