When Galileo Galilei first pointed his homemade telescope toward the sky in 1610, he couldn’t have possibly imagined that he was about to change astronomy forever. His discovery of Jupiter’s moons in particular had two major effects: it provided compelling evidence for the Copernican theory, and it recognised the use of the telescope as a key element in unravelling the mysteries of the universe.

In the centuries following Galileo’s discovery we have built larger and ever-more powerful telescopes, some of which are conveniently placed in space, and have come to map the cosmos with a fair degree of precision. And although there are plenty of things that we still don’t know about the universe, one thing we can be absolutely certain of is that it is an extremely intriguing place.

A place crammed with beautiful galaxies, colourful nebulas and hot stars that are often orbited by planets, which in turn are revolved by rings and moons. A home to fast-spinning pulsars, ultra-compact neutron stars, wandering comets, icy asteroids, and countless other treasures.

But hidden among these jewels is a swarm of elusive, almost invisible objects: black holes, the most mysterious and fascinating objects that we know of, certainly the most feared. After all science fiction has frequently depicted them as evil monsters wandering in the universe vacuuming everything they encounter. But is that really what black holes are?

Well, not exactly. Technically, a black hole is a region in space where a massive amount of matter is compressed into a very tiny area, and its gravitational pull is so strong that nothing, not even light, can escape! Black holes, however, do not roam through the universe, randomly swallowing things. Just like any other celestial body, they obey the laws of gravity: they follow precise orbits and are orbited by other bodies.

It is true though that anything swallowed by a black hole is gone for good. This is because at the “event horizon,” the so called “point of no return” or the boundary of the black hole, gravity is so strong that in order to escape, matter would have to travel faster than the speed of light. As this is physically impossible, nothing can escape!

The idea of black holes (initially called “frozen stars”) has been around for centuries although the term “black hole” was first used only in 1967 by the American astronomer John Wheeler. It was in these years that scientific attention shifted back to black holes, with some of the greatest minds such as Wheeler, Oppenheimer, Penrose, Hawking and many others making significant contributions.

Today we are fairly confident that most black holes form from the remnant of large stars. The idea is that when a massive star consumes all its fuel and dies, all it leaves behind is a dense core. If this core has a certain mass (over three times the mass of the Sun), it collapses on itself under the effect of gravity to become a black hole.

But while the theory is elegant and widely accepted, the direct observation of a black holes remained fairly complicated, considering that not even light can escape to reveal them. Astronomers, however, are very clever people and eventually figured out a way: to spot a black hole all they needed to do was observing its effects on nearby matter.

Imagine a black hole passing through a region of interstellar dust or crossing paths with a close star: the extreme gravitational pull would draw matter toward the event horizon and in doing so the matter would accelerate and heat up, emitting fancy x-rays that can be easily observed by astronomers.

That is precisely what astronomers did in 2019 when the Event Horizon Telescope (EHT), a combination of eight ground-based radio telescopes, captured the image of the supermassive black hole at the centre of the galaxy M87 interacting with nearby matter. The first image ever recorded of a black hole was served.

Black holes are quite common in the universe and, according to recent studies, there may be billions of them in our own galaxy alone. In fact, a supermassive black hole called Sagittarius A* sits at the core of the Milky Way, and astronomers estimate that there is one at the heart of virtually every large galaxy.

So far, scientists and astronomers have identified three types of black holes whereas a fourth type, primordial black holes, is still only theoretical although scientific evidence of their existence is mounting. These are the main classes of black holes:

• Primordial – While these black holes are still only theoretical, scientists believe they formed in the initial stages of the universe, a fraction of second after the Big Bang. At this stage, the universe was not homogeneous and areas that were denser and hotter than others may have quickly collapsed into black holes.
  According to some theories, primordial black holes may even explain one of the biggest mysteries of the universe: dark matter. Such theories suggest that primordial black holes may in fact be a type of dark matter called MACHOs (MAssive Compact Halo Objects,), the glue that keep galaxies together.

• Stellar mass (5 to 100 solar masses) – These are the “ordinary” black holes, which form when a star more than thirty times the mass of the Sun explodes into a supernova at the end of its cycle, leaving behind a dense core. This new core than collapses on itself under the effect of gravity and a stellar mass black hole is formed. They are relatively small but incredibly dense, with an impressive gravitational force.

• Intermediate mass (100 to 10,000 solar masses) – As the name suggest, these black holes represent a midway between stellar mass and supermassive black holes. Scientists ignored their existence for decades until in March 2020 Hubble spotted what it is believed to be a 50,000 solar mass black hole. Intermediate black holes could form when stars in a cluster collide in a chain reaction or from the merger of two or more stellar mass black holes.

• Supermassive (greater than 1 million solar masses) – These black holes are millions or even billions of times as massive as the Sun and are found at the core of major galaxies: they are the seeds around which the galaxies are constructed. For instance, Sagittarius A*, the supermassive black hole at the core of our galaxy, is roughly four million times the mass of the Sun whereas the black hole photographed in 2019 weighs a billion solar masses. Supermassive black holes are thought to be formed from the merger of hundreds or thousands of tiny black holes.

Luckily for us, the nearest black holes are thousands of light-years away from Earth. At such distance there is no danger for us or our planet; as we saw, black holes obey the law of gravity like any other celestial body. If we were to replace the Sun with an equal-mass black hole, our orbit and that of all planets in the Solar System would remain unchanged.

On the other hand, if someone, say a curious astronaut, was to wander too close to a black hole, this would have catastrophic consequences. As the astronaut approached the black hole, he would be stretched head-to-toe into a long string of particles before even reaching the event horizon, in a process known as “spaghettification.”

So, the bottom line is: unless you want to become spaghetti, better stay away from back holes!