What Is a Black Hole?
Types, Formation & the Science Explained

Not every star becomes a black hole. Only stars that begin their lives with at least 20 times the mass of the Sun have enough gravitational pressure at their cores to collapse into a black hole when they die. Stars like our Sun end their lives gently — they shed their outer layers as a planetary nebula and leave behind a white dwarf, a dense Earth-sized ember of carbon and oxygen that slowly cools over billions of years.

For a star 20+ solar masses, the story is far more violent. Throughout its life, the star fuses lighter elements into heavier ones — hydrogen into helium, helium into carbon, carbon into neon, neon into oxygen, oxygen into silicon — building an onion-like structure of nested fusion shells. Each stage burns faster than the last. Silicon fusion into iron takes about one day. And iron is the endpoint: fusing iron does not release energy, it consumes it.

When the core is converted to iron, the star loses the outward radiation pressure that had been holding it up against gravity for millions of years. In less than a second, the iron core — an object roughly the size of Earth with the mass of 1.5 Suns — collapses inward at nearly a quarter of the speed of light. The core crushes past the density of a white dwarf, past the density of a neutron star, and becomes a singularity: a point of infinite density where the known laws of physics break down. The outer layers of the star, suddenly unsupported, crash down onto this newborn black hole and rebound outward in a titanic supernova explosion that can briefly outshine an entire galaxy.

The Singularity

At the center of a black hole, general relativity predicts that all the mass is crushed into an infinitely dense point — the singularity. Here, the curvature of spacetime becomes infinite, and the laws of physics as we know them cease to apply. Most physicists believe that a complete theory of quantum gravity — one that unifies general relativity and quantum mechanics — will replace the singularity with something physically meaningful. But we do not yet have that theory. The singularity remains the deepest mystery in astrophysics.

Spaghettification

This is a real scientific term — not a joke. As you approach a black hole, the difference in gravitational pull between your head and your feet becomes enormous. For a stellar-mass black hole, this tidal force would stretch you into a long, thin strand of atoms — like spaghetti — long before you reached the event horizon. For a supermassive black hole like Sagittarius A*, the event horizon is so large that the tidal forces at the boundary are mild. You would cross the event horizon without noticing — but you would still be pulled inexorably toward the singularity and destroyed within seconds.

On April 10, 2019, the Event Horizon Telescope (EHT) collaboration revealed the first direct image of a black hole: a glowing orange ring surrounding a dark central shadow. The image showed the supermassive black hole at the center of galaxy M87, 55 million light-years away. The orange ring is not the event horizon itself — it is the photon sphere, a region where light orbits the black hole before either falling in or escaping. The dark center is the black hole's shadow, roughly 2.5 times larger than the actual event horizon due to the extreme bending of light by gravity.

The EHT is not a single telescope. It is a planet-sized array of eight radio observatories spanning from Hawaii to Spain to the South Pole, synchronized by atomic clocks and combined through a technique called very-long-baseline interferometry (VLBI). Together, they create a virtual telescope with the resolving power needed to read a newspaper in New York from a sidewalk café in Paris. The data from each observatory — petabytes of it — was physically shipped on hard drives to central processing centers because the volume exceeded what the internet could transfer.

In May 2022, the EHT released its second image: Sagittarius A*, the black hole at the center of our own galaxy. At 27,000 light-years away and 4 million solar masses, it appears far smaller in the sky than M87* despite being 1,600 times closer — because M87* is 1,600 times more massive. The Sgr A* image looks similar: a glowing ring around a dark shadow, confirming that general relativity's predictions hold true across black holes of vastly different masses.

At the exact center of the Milky Way — 27,000 light-years away in the direction of the constellation Sagittarius — lies a supermassive black hole called Sagittarius A* (pronounced "A-star"). It has a mass of 4.15 million Suns packed into a region smaller than Mercury's orbit. Astronomers proved its existence by tracking the orbits of individual stars near the galactic center over decades. The most famous of these, a star called S2, orbits Sgr A* once every 16 years, reaching speeds of 7,650 km/s — nearly 3% the speed of light — at its closest approach. These precise orbital measurements, conducted primarily at the Keck Observatory and the Very Large Telescope, earned Reinhard Genzel and Andrea Ghez the 2020 Nobel Prize in Physics.

Sgr A* is relatively quiet — it is not actively feeding on large amounts of matter. If it were, the X-ray and radio emission from its accretion disk would make the galactic center one of the brightest objects in the sky. Instead, it flickers faintly in infrared and X-ray, with occasional flares as small clumps of gas fall in. NASA's James Webb Space Telescope recently detected rapid flickering from the innermost region of Sgr A*'s accretion disk — variations on timescales of seconds to minutes — indicating processes occurring just outside the event horizon.