Explosions, Mergers, And Wanderings: What Happens To Black Holes

Black Holes

Astrophysicist Sergey Popov on how black holes are born, live and travel through galaxies, and people try to study them

Astronomy is interesting because we cannot directly experiment with the objects of study. But thanks to the invention of new telescopes, and Astronomical satellites, we have a variety of data on many objects, including the most mysterious of them – black holes. This allows you to better understand how they are arranged, how they appeared and what happens to them in the Universe.

A black hole is a region inside space with gravity so strong that it sucks in everything around it, including light. RAS professor Sergei Popov explains that black holes do not have one clear definition, and even this is one of the options. If you ask different scientists – astrophysicists and physicists – they will approach the answer from different angles. There are encyclopedic dictionaries that fix definitions and give specific answers, but there is no single correct formulation.

Four Types Of Black Holes

All bodies bend space and time around them. The more compact the body, the more noticeable this effect. The compact is not just about size. A small body with a large mass will also be compact. Black holes are an extreme case. We can take anything: a cloud of gas or a neutron star, begin to compress them, and, in the end, any of these objects will turn into a black hole. Everything changes there. Information from inside cannot get outside. Therefore, black holes sometimes seem not so much interesting objects as mysterious. We don’t really know what’s going on inside.

Therefore, there are two approaches to the study of black holes. The first one is more physical: we are talking about the properties of black holes, including internal ones. Here, for the time being, we restrict ourselves to theoretical studies. The second is astrophysical. There are four main types of black holes in astrophysics.

The most famous are supermassive black holes. Every large galaxy has a central black hole. There are black holes with masses ranging from several thousand solar masses to tens of billions. There is such a black hole in the center of our galaxy.

The second popular type is stellar-mass black holes. Stars evolve: a thermonuclear reaction takes place in the depths, and light elements turn into heavy ones. But this synthesis ends sooner or later. An iron core is formed. And this iron core begins to collapse. If this collapse is not stopped, a black hole will form. Here, of course, we will not get any billions of solar masses, because we start from a stellar mass – this is 20, maybe 200 solar masses. The black hole will obviously be a little lighter because not all of the star’s matter will get inside.

Sometimes a separate type of black hole of intermediate masses is distinguished – something in between stars and supermassive black holes. So far, there is no understanding of how these objects arose. Maybe several massive stars merged with each other in a dense cluster, for a short time formed a star with a mass of, say, a thousand solar masses. And it didn’t collapse.

Finally, there are the so-called primordial black holes, which should form in the young universe. The beauty of such primordial black holes is that they appear before anything else – before stars and galaxies. And they can have very different masses: from quite large to very microscopic. These objects have a number of unique properties. I really want to find them, but so far a variety of attempts have not led to anything.

Therefore, in reality, we are dealing with supermassive black holes and stellar-mass black holes, and we also have good candidates for intermediate-mass black holes.

Observations of matter around a black hole

By definition, no signal can get out of the depths of the hole. The black hole itself is not visible, it has no surface, only the horizon. Therefore, when studying real black holes, supermassive or stellar-mass black holes, we, first of all, see the processes that occur around them. The main way to study is to observe the matter that heats up in the vicinity of a black hole.

Let’s consider the simplest example. There is a black hole at the center of the galaxy. There is always some gas in there, it is attracted to the black hole. If the gas falls evenly into the black hole, it will emit almost nothing, but if it orbited around a bit, then this movement will become more and more noticeable as the gas approaches the black hole.

And finally, a gravitational disk can form near the black hole – due to friction, the gas is heated to a high temperature. In supermassive black holes, we typically see optical and ultraviolet radiation from the disk, while in lighter stellar-mass black holes, we see X-rays. In this case, the source of matter is the second star.

Stars, especially massive ones, love to be born in pairs. Often a situation arises when matter from one star flows to another. This second object could be a compact object: a white dwarf, a neutron star, or a black hole. Again, a disk is formed, and we see its radiation. Thus, the first good candidates for black holes were discovered, for example, the famous source Cygnus X-1.

But in the vast majority of binary systems, this overflow of matter is absent. It turns out that astronomers are looking under the lantern – where it is easier to find. We see those objects that themselves betray their presence with powerful radiation.

Star-Black Hole: Observations Of Binary Systems

When people generally understood that black holes could exist, a simple idea arose. Imagine once upon a time there were two stars. One turned into a black hole. But we keep seeing the second star. What do they do if they form a binary system when combined? They revolve around a common center of mass. Then it becomes important to fix the movement of the visible star – to get its spectrum and by the shift of the spectral lines, that is, by the Doppler effect, notice: the star either moves towards you, or moves away from you.

This is the rotation around the center of mass in a binary system. If you can prove that the second component is massive enough, has a mass greater than three solar masses, and is invisible, then it can only be a black hole. This idea was put forward in the early 1960s. But then it was not possible to open a single object, and only in the last few years articles began to appear,

Observations here are quite complex. Because at the same time it is necessary to prove that the second component is actually invisible, and not just very weak. This is quite difficult to do because the first star is usually the brightest one. Its light makes it difficult to see the second dim component. Then it is important to prove that the second object is light and that it is a black hole, and not a neutron star, white or red dwarf.

Until recently, all good candidates ended up being discarded. But this year, a paper finally appeared where, apparently, people actually saw such an inactive black hole in a binary system. It is very important to discover this because at the moment we do not understand very well which stars give rise to black holes and which give rise to neutron stars.

It is also very interesting to know what speed black holes acquire at birth. These data will help to understand how the collapse occurred. This is a very complex process that we are trying to simulate on computers, but so far it has not been possible to do this in small details. More precisely, it is obtained in many different ways. To choose the right one, we need to compare the results with real data.

The System Disintegrated: Observations of a Lonely Black Hole

Not all moving stars indicate the presence of a black hole nearby – the offset must be large enough. Until recently, it was not possible to get close to such stars. Fortunately, the European satellite Gaia is in orbit, the task of which is to measure the exact position of the stars. This gives us the ability to get statistics on black holes and should eventually lead to a much better understanding of how they are created.

If the binary system broke up, and the star exploded, then the situation is worse than with a black cat in a black room: a black hole flies by itself in black space. But it turns out that the general theory of relativity gives us the opportunity to learn about the presence of a black hole. This is possible due to the effect of gravitational lensing. Imagine that we are watching a star, and its brightness begins to increase symmetrically in all spectra, which means that some massive body flew between us and the star and warped space-time.

The problem is that the likelihood of this happening is small. If you want to take a beautiful photo, for example, an airplane against the background of the moon, then simply pointing the camera at the sky, you are unlikely to take such a beautiful picture. But the probability will increase if there is an airport near you and planes fly frequently. Accordingly, in order to

There are about 400 billion stars in the galaxy and several hundred million black holes. After many years of observations, candidates for events finally appeared, where we identify the presence of a single black hole, which did nothing, but simply flew, but accidentally amplified the light of a distant star and thereby gave itself away. This makes it possible to determine the mass of a black hole, which is important for studying how stars end their lives.

Neutron stars have a typical mass of one and a half to two solar masses. Black holes usually have seven to ten solar masses. There are very few objects in the middle of these masses. With the help of the Gaia satellite, which detected the effect of gravitational lensing, it was possible to detect a black hole that falls into this mass distribution gap. This is critical to understanding stellar evolution and ultimately brings us back to the question of which stars turn into black holes.

Observations of a system of two black holes

Now the most exotic astronomical observations are the search for gravitational wave bursts. This is the best way to study black holes in large numbers with accurate masses. Massive stars are mostly born in binary systems. There may be such a situation that both stars turned into black holes, and the system survived. Two black holes revolve around a common center of mass. They lose energy due to the emission of gravitational waves – this can be considered the absolute friction of the curvature of space. That is, energy is taken away from the binary system, and black holes approach each other.

The closer they come to each other, the more intense the process of emission of gravitational waves. As a result, two black holes merge, and a large amount of energy is released, but in a very exotic form – in the form of gravitational waves. Only in the 21st century were installations created that really made it possible to fix this phenomenon. In September 2015, the LIGO probes detected for the first time a gravitational wave burst from the merger of two black holes. Since then, 100 gravitational wave bursts have been observed.

Such observations allow us to determine another remarkable parameter – the black holes approach asymmetrically. One black hole may be more massive than another, and the direction of the axes of rotation and speed may not coincide. Therefore, gravitational waves will also be emitted asymmetrically – they carry away energy and momentum. But momentum must be maintained.

Therefore, when black holes merge, and gravitational waves carry away more momentum somewhere, and somewhere less, the black hole should start moving: fly to where fewer impulses have flown to compensate for everything. The question arises: how fast can a black hole move? It turns out that very fast – thousands of kilometers per second. Moreover, everything depends on the ratio of the masses of black holes and on how the axes of rotation were directed.

This effect of the gravitational wave rocket is very important. Physicists came up with it back in the 1960s, and then astrophysicists remembered it in the 1990s and realized that it can actually be observed. A speed of hundreds of thousands of kilometers per second, for example, allows a black hole to fly out of a galaxy and start wandering in intergalactic space. That is, it is not empty at all, in fact, a large number of black holes plow its expanses. For the first time, this was proved thanks to the processing of data from LIGO installations.

The black hole in our galaxy

Of course, the black hole would not fly away if it formed right next to the central black hole of its galaxy. In this region, right near any central black hole, stars rotate. There is also a black hole in the center of our galaxy – it is very small and light. But she has a strong effect on her inner circle. The sun does not revolve around the central black hole, it is controlled by the galaxy, and the stars that are near the black hole are controlled by it. This is interesting because we cannot see a black hole.

We have access to either the substance around, or objects. Stars revolve around the central black hole with some kind of orbital period. For a long time, the most famous star was the one that orbited a black hole in 15 years. That is, to get good information, you need to observe for a long time. Where are the stars which are even closer to the black hole? In the center of the galaxy, they are difficult to observe, since this region is heavily covered by dust.

But surveillance tools are becoming more sensitive, and so records are constantly being set. Stars are being discovered that are getting closer and closer to the black hole. A star was discovered that makes a revolution in 9-plus years. And finally, another record was set this year – scientists have found a star that makes an orbit around a black hole in just four years.

This is also very interesting because the closer we get to a black hole, the stronger all the effects of general relativity manifest themselves. But there is a limit: if the star gets too close to the black hole, then the tidal forces will simply tear it apart. An accretion disk appears around the black hole for a short time, and before that, the calm black hole becomes active – it begins to devour this matter.

Studying black holes is very difficult, but interesting. The last year has been rich in discoveries in this area. Summarizing what has been said, we are getting a lot of new information about black holes from various directions, and even about those that were very difficult to observe before.

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