What is a black hole, really?

How does a black hole form?

The most familiar kind starts with a giant star. Stars shine by burning nuclear fuel, and that heat pushes outward against their own gravity. When a star with more than about eight times the Sun’s mass runs out of fuel, NASA explains, its core collapses and the star explodes as a supernova. If the star had around 20 times the Sun’s mass or more, the core keeps collapsing into a black hole. These “stellar-mass” black holes weigh from a few to hundreds of Suns. Nearly all of the ones found so far are paired with a partner star. Such pairs have revealed about 50 suspected or confirmed ones in our galaxy, but NASA says there may be as many as 100 million in the Milky Way alone.

Then there are the giants. Almost every large galaxy, including ours, has a supermassive black hole at its center, with hundreds of thousands to billions of times the Sun’s mass. Ours is called Sagittarius A* (say “A-star”), and it holds about four million Suns’ worth of mass. How the giants got so big so early is still a puzzle. NASA notes that some already existed in the first billion years after the universe began. They grow by swallowing gas, stars and smaller black holes, and by merging when galaxies collide.

The math came first. In 1916, when Einstein’s theory of gravity was new, Karl Schwarzschild found a solution to its equations that describes the space around a round, non-spinning mass. Squeeze that mass small enough and the solution contains a point of no return. Einstein himself did not believe black holes really exist, the Nobel committee notes. In January 1965, Roger Penrose showed that they really can form as a direct result of Einstein’s theory. That work earned him half of the 2020 Nobel Prize in Physics.

How do we know black holes are real?

First, stars. Since the early 1990s, two teams led by Reinhard Genzel and Andrea Ghez have tracked stars near the center of the Milky Way. The stars race around something heavy and invisible. According to the 2020 Nobel Prize announcement, about four million solar masses are packed into a region no larger than our solar system. Genzel and Ghez shared the other half of that prize.

Second, ripples in space. On September 14, 2015, the two LIGO detectors in Louisiana and Washington caught a tiny stretch and squeeze of space itself, called a gravitational wave. LIGO scientists estimated that it came from two black holes of about 29 and 36 times the Sun’s mass, which merged 1.3 billion years ago. About three Suns’ worth of mass was turned into gravitational waves in a fraction of a second. By 2026, the LIGO–Virgo–KAGRA catalog held 390 signals, many of them from black hole pairs.

Third, pictures. The Event Horizon Telescope links radio dishes around the world into one Earth-sized virtual telescope. On April 10, 2019, it released the first image of a black hole’s shadow, at the center of the galaxy M87. That black hole is 55 million light-years away and has 6.5 billion times the Sun’s mass. On May 12, 2022, the team showed Sagittarius A*, about 27,000 light-years away. From here it looks about as big as a doughnut on the Moon would. Both images show a dark center inside a bright ring of glowing gas. For M87, the team said the observations matched their predictions well. For Sagittarius A*, they said the size of the ring agreed with Einstein’s theory.

The strongest objection is that no one has ever seen a black hole directly, since by definition it gives off no light. That is true. Every clue is indirect: moving stars, glowing gas, bent light and ripples in space. But the clues come from different tools and agree with each other. The Nobel committee put it plainly: for the object at our galaxy’s center, a supermassive black hole is the only currently known explanation.

What happens to something that falls in?

It depends on the size of the black hole. Near a small one, gravity pulls much harder on your feet than on your head. NASA’s word for the result is “spaghettification”: you get stretched like a noodle. Strangely, the bigger the black hole, the gentler this stretching is at the horizon. NASA’s Imagine the Universe says that for a black hole of a million Suns, this tidal pull is 10 billion times weaker than for one of 10 Suns. So a traveler falling into a giant black hole would feel far gentler stretching at the horizon than near a small one.

Watchers far away would see something very different. To them, the traveler’s clock seems to tick slower and slower. Her light turns redder and dimmer, and she fades from view just above the horizon. For the traveler, time runs normally, and she keeps falling. Einstein’s theory predicts that the center holds a singularity, a point where matter is crushed to infinite density. NASA is careful here: the singularity may be a real physical thing or only a feature of the math, and astronomers don’t know which. It may mark the place where a better theory, one that joins gravity with quantum physics, is needed.

Do black holes last forever?

Maybe not. In 1974, Stephen Hawking published a short paper in Nature called “Black hole explosions?” He combined quantum physics with Einstein’s gravity and found that black holes should give off a faint glow, now called Hawking radiation. A black hole that glows loses energy, so over enormous spans of time it would shrink and could evaporate completely.

This glow has never been observed. For big black holes it is far too faint. The Stanford Encyclopedia of Philosophy gives the Hawking temperature of Sagittarius A* as about a hundred-trillionth of a degree above absolute zero. The same article calls black hole heat and temperature about as widely accepted as an idea with no direct evidence can be. That is why this page labels Hawking radiation “Supported, still debated.”

Evaporation leads to a deeper puzzle. Quantum physics says the details of the past are never truly erased. The encyclopedia uses a chair as its example: burn it, and in principle you could rebuild it from the smoke, ashes and heat. Throw it into a black hole that later evaporates, and the details seem to vanish. This is the information loss paradox. Hawking first argued that the details really are lost, and later gave up that view. Physicists have proposed many answers, and the encyclopedia sorts them into six broad groups. It is an open question.

What do people often get wrong?

• “Black holes suck everything in.” They don’t. NASA says that from far enough away, a black hole’s gravity acts like that of any object with the same mass. Things fall in only if they get close.
• “A black hole could swallow Earth.” The nearest known one, Gaia BH1, is about 1,500 light-years away. Stars become black holes only if they start with around 20 times the Sun’s mass or more, NASA says, so the Sun will never become one.
• “A collider could make one that eats the planet.” In 2008, a CERN safety review pointed out that cosmic rays have been striking Earth at higher energies for billions of years. Any tiny black holes made by the LHC are expected to decay before they even reach the walls of the detector. Even if some were stable, the review found no danger, because Earth, the Sun and other stars have survived that natural bombardment.
• “Black holes are doorways to other universes.” NASA says they are not shortcuts through space or portals to other dimensions.

How can you check this yourself?

Look at the two Event Horizon Telescope images side by side on the European Southern Observatory’s pages. Read the short Nobel Prize press release from 2020, which explains the star-tracking work in plain words. You can even listen to the “chirp” of the 2015 black hole merger, which LIGO turned into sound. For a friendly walk-through of the whole subject, see our magazine story What Is a Black Hole, Really? And if you wonder how a black hole compares with the cosmic speed limit, read Can Anything Travel Faster Than Light?