Black hole

A black hole is an astronomical body so compact that its gravity prevents anything, including light, from escaping. According to Albert Einstein's theory of general relativity, when a mass becomes dense enough, it forms a black hole. The point of no return around a black hole is called the event horizon. Once something crosses this boundary, it cannot escape.

Black holes typically form when very massive stars collapse at the end of their life cycle. After forming, a black hole can grow by pulling in nearby matter. Many galaxies, including our own Milky Way, have huge black holes called supermassive black holes at their centers.

Scientists can detect black holes by watching how they affect nearby matter and light. For example, matter falling toward a black hole forms a hot, glowing disk called an accretion disk. When black holes collide and merge, they produce ripples in space called gravitational waves that we can detect. By studying the movements of stars, astronomers have found many black holes in binary systems and confirmed the presence of a giant black hole at the center of the Milky Way.

History

Main article: History of black hole physics

The first simulated image of a black hole, published by Jean-Pierre Luminet in 1979 and featuring the characteristic shadow, photon sphere, and lensed accretion disk. The disk is brighter on one side due to Doppler beaming.

The idea of a body so massive that even light could not escape was first proposed in the late 18th century by English astronomer and clergyman John Michell and independently by French scientist Pierre-Simon Laplace. Both scholars suggested very large stars that could trap light due to their strong gravity.

Later, Albert Einstein developed his theory of general relativity, which helped explain how massive objects like black holes can form. In the mid-1900s, scientists discovered that some stars could collapse into extremely dense objects called black holes. Observations in recent decades have confirmed that black holes exist in space.

Definition

A black hole is a place in space where gravity is so strong that nothing, not even light, can escape. Scientists identify black holes by measuring the mass of objects and using theories about how gravity can cause matter to collapse. When an object has more mass than three times that of the Sun, it is likely to be a black hole. Another way to think about a black hole is as a region where space is pulling inward faster than light can travel.

Properties

Black holes are special objects in space with such strong gravity that not even light can escape from them. According to Einstein's theory of general relativity, a black hole forms when a lot of mass is squeezed into a very small area. The edge of a black hole, where gravity becomes too strong for anything to escape, is called the event horizon.

A black hole is mostly described by just three main features: its mass, its electric charge, and how fast it spins. These three features tell us everything we can know about a black hole. The simplest kind of black hole, called a Schwarzschild black hole, has only mass and no charge or spin. Other types, like rotating black holes, have different properties based on how they spin or if they have any charge.

Classification

Black holes are grouped based on how they form and how much they weigh. One type, called stellar black holes, comes from stars collapsing. These can range from about 10 to 100 times the mass of the Sun. Another type, primordial black holes, may have formed right after the Big Bang and could be very small.

There are also intermediate-mass black holes, which are bigger than stellar black holes but smaller than the giant ones found in the centers of galaxies. The largest black holes, called supermassive black holes, are found at the centers of most big galaxies and can be millions or even billions of times the mass of the Sun.

Black hole classifications
Class	Approx. mass	Approx. radius
Ultramassive black hole	10⁹–10¹¹ M_☉	>1,000 AU
Supermassive black hole	10⁶–10⁹ M_☉	0.001–400 AU
Intermediate-mass black hole	10²–10⁵ M_☉	10³ km ≈ R_Earth
Stellar black hole	2–150 M_☉	30 km
Micro black hole	up to M_Moon	up to 0.1 mm

Structure

Relativistic jets from the supermassive black hole in Centaurus A extend perpendicularly from the galaxy.

Black holes are fascinating objects in space. Their gravity is so strong that nothing, not even light, can escape from them. This makes them act like invisible sinks for matter and light.

Black holes can have powerful streams of energy called relativistic jets, shooting out from their poles at nearly the speed of light. They also have accretion disks—flat, swirling disks of gas and dust that heat up and glow brightly as they fall toward the black hole. The point of no return around a black hole is called the event horizon, and once something crosses this boundary, it cannot escape.

Formation

Black holes are formed when very large stars collapse under their own gravity. This can happen when a star runs out of fuel and can no longer support itself, leading to a powerful explosion called a supernova. Sometimes, two smaller objects called neutron stars can merge to form a black hole.

Other ideas about how black holes form include very special conditions in the early universe, or possibly in high-energy collisions, though these are still just theories. In the early universe, areas of space that were denser than others might have collapsed into black holes. Scientists also think about tiny black holes that could form in particle collisions, but these would disappear very quickly.

Evolution

After a black hole forms, it can change in several ways. It might grow by merging with other black holes or stars. It can also gain mass by pulling in nearby matter, a process called accretion of matter. Over very long periods, tiny particles called Hawking radiation might slowly cause a black hole to shrink and disappear.

When matter falls toward a black hole, it forms a fast-moving disk around the black hole. The friction in this disk heats it up, making it glow brightly. This glowing can be seen from far away using special telescopes. Some of the biggest and brightest objects in space, like active galactic nuclei and quasars, are powered by this process.

Observational evidence

A Chandra X-Ray Observatory image of Cygnus X-1, which was the first strong black hole candidate discovered

Millions of black holes are thought to exist in our Milky Way galaxy, formed when stars collapse. Even small galaxies likely contain many black holes, though only a few have been found. Since black holes do not give off light, scientists must look for clues from objects orbiting them or from special waves in space caused by black holes moving together.

One way scientists study black holes is by watching stars that orbit an invisible object at our galaxy's center. These stars move as if pulled by a supermassive black hole. Another way is by using special telescopes that can "see" the shadow of a black hole, or by detecting special waves in space created when black holes spin around each other and merge. These methods help scientists learn more about these mysterious objects.

Areas of investigation

Information loss paradox

Scientists wonder if information disappears inside black holes. According to some ideas, a black hole only shows us three things: its mass, its charge, and how it spins. This makes it seem like all other details about what fell into the black hole are gone forever. When black holes slowly lose energy by sending out special kinds of radiation, this information still seems missing. This mystery is called the black hole information paradox, and solving it could help us understand how tiny particles and space-time work together.

Two galaxies from the first billion years after the Big Bang. The galaxy on the left hosts a luminous quasar at its center.

Supermassive black holes in the early universe

Observations show that very bright objects, powered by huge black holes, existed when the universe was very young. These black holes are thought to have formed from the collapse of giant stars, but scientists are still unsure how they grew so big so quickly. Some ideas include smaller black holes merging together, very large clouds of gas collapsing directly into black holes, or special conditions that let black holes grow faster than normally thought possible.

Alternatives to black holes

In fiction

The idea of black holes has inspired many artists and scientists. In her book Conjuring the Void: the Art of Black Holes, Lynn Gamwell explores how art and science work together, using black holes as an example. Stories and movies often use black holes to teach science concepts and show exciting adventures.

The black hole and accretion disk used in the movie Interstellar, without lens flare. Interstellar's visual effects team used relativity to visualize gravitational lensing around the black hole.

Black holes appear in many science fiction stories and films. Early examples include the 1928 novel The Skylark of Space and the 1935 short story Starship Invincible. Popular movies like Interstellar and High Life show black holes using real science, though they sometimes take creative freedom. These stories often use black holes to explore time travel and fast journeys through space.