Black holes are astronomical objects with such intense gravitational pull that nothing, not even light, can escape them. The boundary beyond which nothing can escape is known as the event horizon. Despite their darkness, black holes are not empty; they contain matter compressed into an extremely small space. Their presence is inferred through their interaction with nearby matter and electromagnetic radiation, such as visible light. Two primary categories of black holes are observed throughout the universe: stellar-mass black holes, which are three to tens of times the mass of the Sun and are scattered across our Milky Way galaxy, and supermassive black holes, which range from 100,000 to billions of solar masses and reside at the centers of most large galaxies, including our own. Despite current discoveries in the Milky Way, it is believed that hundreds of millions of such black holes exist.

The existence of black holes was first proposed in 1916 by German physicist Karl Schwarzschild, who was attempting to solve Albert Einstein’s general theory of relativity equations. He noticed that the solutions contained a peculiar phenomenon: the theory behaved unusually at a certain radius, now known as the Schwarzschild radius. He concluded that if a mass were compressed into a space smaller than its Schwarzschild radius, its gravity would overwhelm all known forces. Early physicists initially thought such phenomena were unlikely to occur in nature. However, by the late 1930s, it became clear that black holes could indeed form under extreme conditions, as Indian physicist Subrahmanyan Chandrasekhar demonstrated that no force could counteract gravity at a certain density, leading to the formation of black holes only under the most severe conditions.

How Do Black Holes Form?

Stellar black holes form from massive stars that produce light and heat through nuclear fusion. In this process, two lighter atoms fuse to create a heavier atom, releasing energy. These heavier atoms continue to fuse into even heavier atoms in a cycle that keeps the star shining. When a star has a mass about 20 times that of the Sun and nears the end of its life, elements like silicon and magnesium fuse into iron. Iron formation requires more energy than the star can provide, and no force can counterbalance the star’s internal gravity. Consequently, the star collapses, and its core is compressed beyond the Schwarzschild radius, forming a black hole.

Since no known force can halt this collapse, once a black hole forms, the matter continues to be compressed into a singularity, a point of infinite density with no spatial coordinates. The event horizon, a spherical boundary surrounding the black hole, marks the point of no return; anything crossing it cannot escape. To escape a black hole’s pull, one would need to travel faster than the speed of light, which is currently impossible. Supermassive black holes, or monsters with masses millions of times that of the Sun, form by accumulating surrounding matter and merging with other black holes over hundreds of millions of years.

Astronomers have long suspected the existence of an intermediate class of black holes, termed intermediate-mass black holes, weighing between 100 and over 10,000 solar masses. While only a few candidates have been identified indirectly, the most compelling evidence came in May 2019 when gravitational waves from the merger of two stellar-mass black holes revealed a black hole weighing 142 times the mass of the Sun.

What Happens Inside Black Holes?

Inside a black hole, matter is compressed into an extremely small volume due to gravity, reaching a state of singularity within a limited time frame. At this point, the object loses its coordinates, and current physics cannot describe what happens in this environment, as it defies our understanding of physics.

Fortunately, the nearest black holes are thousands of light-years away. Black holes behave like other massive objects in the universe, swallowing nearby matter. However, if the Sun were replaced by a black hole of the same size, Earth’s orbit would remain unchanged, but all plant life would die due to the absence of sunlight.

How Do Scientists Confirm Their Existence?

Scientists cannot observe black holes directly using telescopes that detect X-rays, light, or other forms of electromagnetic radiation. Instead, they infer the presence of black holes by studying their effects on nearby matter. If a cloud of material passes near a black hole, it gets drawn in, a process known as accretion. Similarly, if a normal star passes close to a black hole, it can be torn apart and pulled in, causing the material to accelerate and heat up, emitting X-rays detectable in space. Recent discoveries have provided intriguing evidence of black holes significantly impacting their surroundings, emitting powerful gamma-ray bursts, consuming nearby stars, and influencing the formation of new stars while halting it in other regions.

What Are the Sizes of Black Holes?

The black hole in Cygnus X-1, the closest known black hole to our solar system, has a mass approximately 20 times that of the Sun, which is typical for black holes throughout the universe. NASA estimates there may be between 10 million and a billion black holes within our galaxy.

Cygnus X-1 is located over 6,000 light-years away from our solar system. Although it is the closest known black hole, it is possible that closer black holes exist within 1,000 light-years, though this remains unconfirmed.

At the center of nearly every galaxy, including the Milky Way, lies a supermassive black hole, millions or even billions of times the mass of the Sun. These giant black holes grow to immense sizes by consuming surrounding matter and merging with other black holes over hundreds of millions of years.

What Do Black Holes Look Like?

Black holes, true to their name, do not emit any light, making them appear dark. However, astronomers can detect them through their gravitational effects on nearby objects and their chaotic impacts. For some black holes, particularly supermassive ones, their presence is visible through quasars they produce. When matter falls into a black hole, it compresses and heats up, causing a glowing accretion disk, sometimes brighter than the entire host galaxy. These black holes can also emit jets of highly energized particles at nearly the speed of light, extending tens of thousands of light-years.

Another way to “see” black holes is during their mergers. When two black holes collide, they send ripples through spacetime known as gravitational waves. Although these waves are incredibly faint, sensitive instruments on Earth can detect them. To date, astronomers have recorded 50 such black hole merger events.

In 2019, the first real image of a black hole was captured when astronomers used the Event Horizon Telescope, a network of dishes spanning the globe, to photograph the glowing disk of material surrounding a black hole with a mass over 6 billion times that of the Sun, located in the galaxy M87, approximately 55 million light-years away. The image appears as a distorted orange ring due to the impossibility of capturing the black hole itself, as light cannot escape from it. Instead, astronomers observed the shadow of the black hole against the glowing material surrounding it.