Black holes, the celestial monsters that lurk in the depths of the universe, have fascinated astronomers and stargazers for centuries. These cosmic behemoths, formed from the remnants of massive stars, possess a gravitational force so powerful that nothing, not even light, can escape their grasp. In this never-ending journey of exploration, black holes beckon us to venture further into the uncharted territories of the universe, allowing us to glimpse the magnificent forces that shape the cosmos and challenge the limits of human comprehension.
Black Holes have many aspects of their anatomy that make them so distinct and popular:
Event Horizon
The event horizon of a black hole is what truly defines a black hole and gives it its name. It is the surface that we see (and also don’t see) when we look at a black hole. It is the boundary beyond which nothing, not even light, can escape due to the immense gravitational pull of the black hole. It marks the point of no return for anything that crosses it, leading to the black hole's ominous reputation as cosmic "vacuum cleaners." But don’t be fooled, the gravity of the black hole is the same as the gravity of any other massive object. If a black hole the same size as our Sun was to replace our Sun, we would experience the same orbit that we are in now. What we would also experience is an imminent icy death due to the lack of solar radiation and heat. Once an object or even light passes this boundary, it is forever lost as it travels to the black hole's singularity at its center. Since light cannot escape beyond this point to reach our eyes, we view a black ominous abyss. The event horizon is a key characteristic of black holes, shaping our understanding of their mysterious and captivating nature in the cosmos. Fascinatingly, it is this very point that raises profound questions and inspires extensive research into the fundamental laws of physics, gravity, and the nature of the universe.
The Singularity
The singularity at the center of a black hole is a point of infinite density where the laws of physics, as we currently understand them, break down. Within the event horizon, all matter and energy become compacted into this infinitesimal point, resulting in a gravitational pull so strong that not even light can escape. According to the theory of general relativity, gravity can be viewed as the curvature of space time, which can cause planets and other objects to fall into the curvature of more massive objects like the Sun, resulting in an orbit. The singularity in a Black Hole is surrounded by a region of space-time where the curvature becomes infinite, known as the gravitational singularity. Our current understanding of physics is unable to fully describe the behavior of matter and energy at this point, making the singularity a mysterious and fascinating aspect of black hole science.
Accretion Disk
Black holes can gain mass in two ways: by merging with other black holes or through the process of accretion, when the black hole consumes matter. For example, if a star wanders too close to a supermassive black hole, the intense gravity of the black hole will pull the gas from the star, settling into a bright, rapidly-spinning disk. The accretion disk of a black hole is a mesmerizing and intense region of activity. The immense gravitational forces in play cause the matter in the disk to move at incredibly high speeds, generating intense friction and heat. As a result, the accretion disk emits strong X-rays and other high-energy radiation, making it detectable by telescopes and other astronomical instruments. The inner regions of the accretion disk, closest to the event horizon, experience the most extreme conditions, with temperatures soaring to millions of degrees. Accretion disks act as the main light source from a black hole, making them easier to find in comparison to isolated black holes that have already consumed the matter around them and remain shrouded in darkness.
This is a simulation of a star wandering too close to a black hole and is destroyed by its gravity. The gas from the star will orbit the black hole, creating a bright accretion disk.
Types of Black Holes
All black holes are defined by their extremely high densities and ability to capture light past their event horizons, but the sizes of these peculiar objects may range from super tiny structures that we cannot even detect to giants that are billions of times more massive than our Sun.
Stellar-Mass Black Holes
Stellar-mass black holes are the remnants of massive stars that have reached the end of their life cycle and undergone a catastrophic collapse. In order for this to happen, a massive star around 20 times the mass of the Sun will collapse, resulting in a supernova, or a powerful explosion of a star that blows much of the stellar material away. The collapse of the core of the star will leave behind a black hole that is about 26 miles (42 kilometers) in diameter, or about the size of a small city.
What sets black holes apart from their original star is their extreme density, or the high amount of mass packed into a very small volume. The force of gravity near these black holes is so strong because the core of an incredibly massive star that is many times the mass of our own Sun is being squished down to the size of New York City in just seconds. This is a huge amount of mass in a very tiny area. This means that objects can get very close to all of this concentrated mass in the black hole, which means a black hole can exert a higher maximum force than its original star could.
Supermassive Black Holes
Supermassive black holes are celestial objects located at the center of large galaxies. Our Milky Way has its own supermassive black hole named Sagittarius A* (it was found in the constellation Sagittarius). These behemoths possess masses ranging from millions to billions of times that of the Sun, yet they are confined within a region smaller than our solar system (our solar system is still very big). In order to gain this insane amount of mass, these black holes had to have gone through a very different formation process than stellar-mass black holes but we are not sure how.
Ultramassive Black Holes
While supermassive black holes can be millions or billions of solar masses (1 solar mass is the mass of the Sun), ultramassive black holes are between tens of billions of solar masses.
The Small and the Big
Astronomers discovered the first black hole, named Cygnus X-1, in 1971 after detecting X-rays that seemed to come from a blue bright star orbiting a dark object. Since then, we have discovered many stellar-black holes within our home galaxy, leading us to estimate that there may be 10 million to even a billion stellar-mass black holes in the Milky Way alone. As mentioned earlier, virtually ever large galaxy harbors a black hole, so there is no lack of supermassive black holes in the universe with the estimation that there may be 2 trillion galaxies in just the observable universe (how far we can with the light that is reaching us). So we were observing stellar-mass black holes that were the size of cities and had masses between 5 and 10 times the mass of the Sun, and we were also seeing supermassive black holes controlling galaxies with more than 100,000 times the mass of the Sun, but where were the black holes in between? Shouldn’t there be black holes with masses between the relatively tiny mass of a stellar black hole and the unimaginably huge mass of a supermassive or even ultramassive black hole?
Stellar-mass black holes are the most common of all black holes.
Stellar-mass black holes can continue to gain mass by colliding with stars and other black holes.
Almost all of the 50 or so stellar-mass black holes we have found in the Milky Way have been paired with stars. A star that is orbiting nothing must be orbiting a black hole!
This simulation shows a massive star ending its life in a supernova explosion, leaving behind a stellar-mass black hole that formed from the collapse of the star’s core.
Binaries star systems in which two stars are gravitationally bound to each other. In some cases the more massive star will collapse into a stellar-mass black hole and continue to be bound to the star. This has allowed us to detect stellar-mass black holes by observing the star in the system that seems to be gravitationally bound to nothing. In more rare cases when the black hole and star are closer, we see an X-ray binary, in which a black hole is pulling gas off the star into an accretion disk that heats up enough to produce X-rays, a type of light or electromagnetic radiation, that we can detect with special telescopes here on Earth.
An X-ray Binary System