The basics of black holes explained

What are black holes and what have we learned in the century since they were first theorised?


Black holes are so dense that they suck up any material that passes too close; even light can’t escape their clutches. Image Credit: NASA/JPL-Caltech


A black hole is a region of space where matter has become so densely packed that its gravity will let nothing escape – including light.

Time comes to a stop at its edge, and its centre may hide a point of infinitely small volume and infinite density, where all laws of physics break down.

Possessing the terrifying power to devour anything unlucky enough to pass within their grasp, it is little surprise that these cosmic chasms have become a staple of sci-fi since the term ‘black hole’ was coined in 1964.

Almost 50 years before this discovery, Karl Schwarzchild’s solutions to Einstein’s theory of general relativity had predicted how a black hole could form.

In 1916, Schwarzchild predicted that if matter could be drawn tightly enough together, it could suffer a cataclysmic collapse to an infinitesimal point called a singularity, a bottomless pit in the fabric of space-time.

He also provided a useful distance from the centre of the black hole you would not want to go beyond, the Schwarzchild radius.

This marks the boundary where the speed required to escape the gravitational pull of a black hole is equal to the speed of light.

Beyond this point, matter and light are forever trapped.

This is why we are not able to actually see inside a black hole.

Given that they emit nothing, not even light, how do we know that black holes even exist?

The answer is that we have observed their effects on other celestial bodies and material that we can see.

The light fantastic

One of the brightest clues is the accretion disc; a flattened band of gas, dust and other debris from a star that has come close to the black hole but not quite fallen in.

The particles within the accretion disc are accelerated to tremendous speeds by the black hole’s gravity, in the process releasing heat, X-rays and gamma rays, which can be seen by dedicated observatories.

By also measuring the orbit of the star around the hidden object, scientists can infer the latter’s mass and size, and thereby confirm it is indeed a black hole.

Using this technique, tens of stellar-mass black holes have been found.

Similarly, many hundreds of supermassive black holes, which sit at the heart of most galaxies, have been discovered by observing the fast orbits of stars and gas, and seeing stars being literally torn apart by the strong gravitational field.

Our Galaxy has a black hole at its heart too, designated Sgr A*

More recently, astronomers at the Laser Interferometer Gravitational Wave Observatory (LIGO) and the Virgo Interferometer have been using a technique called interferometry to detect gravitational waves – ‘ripples’ in the fabric of space-time caused by violent and energetic cosmic processes – which can be formed by the merger of two black holes.

With black hole detections only numbering in the thousands, you might assume that these objects are rare – vanishingly so, you might say.

Yet there are actually thought to be between 10 million and one billion black holes in our Milky Way alone.

Radiation emanating from accretion discs is one way we can infer that a black hole exists. Credit: ESO/L. Calçada
Radiation emanating from accretion discs is one way we can infer that a black hole exists.
Credit: ESO/L. Calçada

Despite their abundance, there is no reason to panic: black holes will not devour Earth nor the Universe.

This is because, at a distance, their gravitational pull is no more compelling than a star of the same mass.

If we were to replace the Sun with a black hole of the same mass, the planets would continue orbiting in exactly the same way, it would just be very dark and cold.

So, the only way Earth could be swallowed into the abyss would be if it happened to stray across a wandering black hole’s event horizon, something that is not going to happen any time soon, and likely never will.

As with many other celestial bodies, black holes can be split into classes:

Miniature black holes

Still hypothetical, these black holes have a mass smaller than the Sun.

They were first proposed by Stephen Hawking in 1971, who suggested they may have formed in the early Universe.

Some experts claimed miniature black holes might appear in the collisions created by the Large Hadron Collider, but none has been detected so far.

Stellar-mass black holes

These black holes, between about 4 and 100 solar masses, are thought to be the most abundant of the four classes.

Formed from the core-collapse of massive stars at the end of their lives, the nearest known one is V616 Monocerotis.

It is located about 3,000 lightyears away, and is between 9-13 times the mass of the Sun.

Intermediate-mass black holes

Ranging from 100 to 100,000 solar masses, only a handful of these black holes have been discovered.

They have been proposed as the seeds of supermassive black holes.

A Japanese team recently announced finding one close to the Milky Way’s own supermassive black hole, adding fuel to the idea that these titans are formed by the merger of their smaller cousins.

Supermassive black holes

These can be anywhere from between 100,000 and 50 billion times the mass of the Sun.

They exist at the heart of most large galaxies; even the Milky Way has one, Sgr A*.

Its 1974 discoverers, Bruce Balick and Robert Brown, added an asterisk to signify the discovery was ‘exciting’.

Sgr A* is 4.1 million times more massive than the Sun.


Ben Skuse is a mathematician turned science writer