A black hole is an object that does not emit any light. Therefore, it is simply impossible to see it from a great distance. So, how do scientists know that there is such an object somewhere?
Invisible black hole
If you ask the average person what celestial object is the most mysterious, you will likely get the answer “black hole”. These objects are known even to those who have never been interested in astronomy, and the reason for this is their mystery.
The concept of an object whose mass is so large that even light cannot leave it was born in the 18th century. At the beginning of the 20th century, it became clear that mass alone was insufficient; it also had to be contained in a certain volume. The concept of a black hole emerged, decades passed, articles were written about black holes, but no one saw them.
To understand why, we need to have a clear idea of what they are. The blackness that we know from our everyday lives is nothing compared to what this object looks like, because if we pick up a black leather wallet, the screen of a turned-off smartphone, or dark glasses, their surface will still reflect some light. And this never happens with black holes, which absorb all light.
To imagine them, one should rather think of a night somewhere outside a big city when there is no full moon. Even then, the stars’ light will somehow illuminate our planet’s surface. Often, in animations, a black hole can be seen as a completely dark body moving against the background of stars. And this could be true if we forget that the vast majority of such objects should be only 20-30 kilometers in diameter. So, to see such a beautiful picture, you need to be very close to them. Even from a distance of tens of millions of kilometers, not to mention thousands of light years, it is extremely difficult to see the stars disappear behind a patch of darkness. So, how do scientists manage to detect them?
Types of black holes
Before answering this question, it should be mentioned that, in general, black holes can be of almost any mass, and depending on this, the situation with their detection may vary.
It is usually said that these objects cannot have a mass less than a few solar masses. However, this is true only for those that are formed as a result of the evolution of stars in the present. At the beginning of the Universe’s existence, as a result of fluctuations in space-time itself, they could well have been formed even with the mass of a proton.
These black holes are called primary black holes, and the very possibility of their existence is not recognized by many scientists. In addition, due to a phenomenon called Hawking radiation, the smallest of them should have evaporated long ago. So, no one is seriously looking for them.
At the other end of the spectrum are supermassive black holes, and these are the only representatives of this class of objects whose reality is now beyond doubt. Scientists know for sure that they should be found in the centers of galaxies, and their presence there is visible because they actively interact with matter, make stars rotate at high speed, and part of the mass they attract is sometimes thrown out into space in the form of giant jets. We call this phenomenon a quasar, and it is visible throughout the Universe.
There are also black holes of intermediate mass. At least, there should be, because somehow objects formed as a result of supernovae outbursts must evolve into their supermassive cousins. However, they are so elusive that scientists still have only a few candidates for their role.
Finally, there are stellar mass black holes. These are the ones that researchers mostly refer to when they report new sensational discoveries in our Galaxy. So, how do you find a completely black object with a diameter of no more than thirty kilometers, which is located at a distance of hundreds and thousands of light years from us?
Black holes in binary systems
Most stellar-mass black holes that scientists are more or less certain exist are found in multiple star systems. Usually, they are binary, but there are also several known to consist of three objects, one of which is a black hole.
In this case, the easiest way to detect a black hole is through its gravitational interaction with a companion. The fact is that these objects are usually quite massive, even by cosmic standards: from 5 to 30 solar masses, or even more. The vast majority of stars are lighter, and the objects orbit around a common center of mass. Modern astronomical equipment, such as the Gaia space telescope, allows us to obtain astrometric data of unprecedented accuracy, revealing even tiny fluctuations of a star around its equilibrium point. Thus, by studying the barely noticeable movement of a star, scientists can detect its invisible companion, a black hole.
Even more interesting is the situation when a black hole and a companion star orbit each other at close range. In this case, the latter fills its Roche lobe, while the tiny and almost invisible, but much more massive former pulls matter from its surface. The latter begins to rotate around it and forms an accretion disk.
And this disk becomes a source of intense radiation. Some of it falls in the visible range, but the maximum is usually in the X-ray range.
This is exactly how the first black hole known to scientists appeared. Initially, it was discovered as a source of X-rays – Cygnus X-1. And it was about this very source that Kip Thorne and Stephen Hawking once made a bet on subscriptions to erotic magazines about whether they would be able to prove that it was a black hole or not. Eventually, Thorne won, and the nature of this object was confirmed.
Black holes that are part of binary systems can behave erratically. From time to time, powerful thermonuclear explosions occur in the accretion disks of some of them, which are especially bright in the high-frequency range. They are called X-ray novae.
In general, pairs of space objects that are active in the X-ray range are called X-ray binaries. And everything is very complicated with them. On the one hand, there are several types of them: low-mass, medium-mass, and high-mass. However, this is a characteristic of a larger companion, not of the invisible compact object itself.
On the other hand, a compact object in each of these categories is not necessarily a black hole. It can also be a neutron star or a white dwarf. The actual radiation spectrum of the accretion disk depends on the object around which it was formed, and it changes rather weakly. So, the presence of X-rays is a rather unreliable sign of black holes.
Of course, in the case of double X-ray stars, it is still possible to find out what their invisible component is. However, it all boils down to the same analysis of the star’s oscillations, which are used to infer the mass of the object, and hence its nature.
Tidal destruction events
A special case of detecting a stellar mass black hole is a tidal collapse event. It is an extreme case in a binary system. As mentioned above, if we compare black holes with their companion stars, they are both much smaller in size but much more massive.
As a result, they can not only pull away some of the outer layers of their companion, but also tear it apart and absorb it. This process does not happen instantly, but usually in several stages. And all of them are associated with the release of energy comparable to a supernova explosion.
In this case, there is no possibility of hiding. The tidal destruction event is visible even from quite distant galaxies. So, this is a very reliable way to find a black hole. However, such events are extremely rare and very short-lived.
Gravitational lensing
If a black hole is not a component of a stellar system, its detection is a much more difficult task. After all, in this case, the only way it reveals itself is by blocking and distorting the light of stars. At the same time, the main contribution here is made by its gravitational field, since the radius of the object is too small for us to see the effect of it.
The distortion of light rays by the gravitational force of an object is called the gravitational lens effect. And, theoretically, we should also see the distortion of the star’s shape when a black hole passes in front of it. But in practice, this effect is too weak, so in reality, even the best astronomical instruments can only see the star dim.
This is exactly what happened, for example, in 2011, when the Hubble Space Telescope saw one of the very distant stars begin to fade, and this process lasted for several years. The event was designated OGLE-2011-BLG-0462. It was noticed only in 2022, and only in 2025 was it proved that it was indeed a black hole.
The situation with OGLE-2011-BLG-0462 demonstrates the problem with finding single black holes using microlensing. This is an extremely rare and very subtle phenomenon. You have to constantly examine millions of stars in the hope of seeing one of them fade. And then you have to carefully study a bunch of archival images to estimate the mass of the object. Because gravitational lensing can also be performed by a traveling neutron star. These objects are also practically invisible when they are not part of a star system.
Gravitational waves
Over the past couple of decades, scientists have developed a new way to detect black holes. We are talking about gravitational wave detectors. The latter are vibrations of space itself that propagate in all directions and pass through our planet almost unnoticed. However, sophisticated installations that measure laser beam oscillations with extreme precision can register them.
In turn, gravitational waves are generated by the merger of black holes and neutron stars. In any case, the result of such an event is a black hole, the mass of which can be determined quite accurately regardless of whether there is a star nearby or not. In addition, gravitational waves travel through the entire Universe, so this event can be detected at distances where it is impossible to detect a stellar-mass black hole by other methods.
Scientists look at gravitational wave tracking with great hope. However, this method has its significant limitations. After all, it allows you to record only the events of merging objects, and they are only a brief moment in their history, which can last for billions of years. A lone black hole flying away from the stars remains invisible. In addition, when tracking gravitational waves, it is sometimes quite difficult to tell where they came from. That is, scientists can describe in some detail what masses the black holes had before the merger and how they moved, but where exactly it happened may remain a mystery.
Yet scientists are optimistic about the future of black hole research. No matter how invisible these objects may be, they cannot interact with the world around them at all. This means that by studying the strange behavior of stars, we will eventually find them.
