So far, we have looked at the extremes of the astrophysical black hole scale, stellar-mass black holes and supermassive black holes. There is a good reason for this. These extremes are the most well-known cases. If we consider all the measured masses of black holes to date, so as of late 2017, we see a cluster of objects in the range of about 5-20 solar masses with some reaching as high as possibly 70-80 solar masses. These are the stellar-mass black holes. There are also a large number of objects towards the right side of this plot, at the highest mass end. These are the supermassive black holes that are thought to reside in the centers of most galaxies. Masses have also been obtained for many other low-mass compact objects. These low-mass stellar remnants populate the far left of this plot and are classified as either neutron stars or white dwarfs. In the middle of this plot, there is a great deal of empty space. Should there not be something lying in the middle? This is one of the many questions perplexing astronomers today. If we were to find black holes lying in the center of this plot, they would be known as intermediate-mass black holes. Although the search for these sources is ongoing, intermediate-mass black holes have proven themselves to be very elusive. Intermediate-mass black holes are just that. They have masses that lie between the heaviest stellar-mass black holes and the lightest supermassive black holes, making them intermediate on the mass scale of the astrophysical black holes, hence the name. But what are these objects, how are they made and why should we care about them? Intermediate-mass black holes weigh more than 100 times the mass of our sun, reaching up to 100,000 times the mass of our sun. They are thought to be too big to form from the death of stars that exist in our universe today. If this is the case, how are they made? One theory suggests that these behemoths were formed early in the universe when it was a much simpler place, chemically speaking that is. The first stars formed when the universe was only about 100 million years old. At this time, the universe contained only the simplest elements, so that was just hydrogen and helium. This early in the universe, stars could become much larger than they are today, sometimes containing upwards of hundreds of solar masses. We have already learned that massive stars burn hotter, brighter, and quicker than their low-mass counterparts. This was true for those first stars too. Such huge stars would have very short lives indeed. We have also seen that massive stars can lose their mass through winds. What we have not yet mentioned though, is that the power of this wind is a function of the star's chemistry. Astronomers have found that the metallicity of a star or the amount of metals it contains, affect the strength of the star's wind. Here is where I should point out a quirk of astronomy. Forget the high school chemistry class for the moment, according to astronomers, the universe is made up of hydrogen, helium, and metals. Anything that contains more than two protons is a metal, strange but true in the astronomical circles. Anyway, back to stellar winds. Therefore, the metallicity of a star or a region give you an indication of how much of these metals are present. When the metallicity is essentially zero, we find that a star loses little to no mass via it's wind, irrespective of its size. This means that those first stars would have lost very little mass by the ends of their lives. At the end of the short life, well, here's another place where it changes from a life of stars today. Given the huge mass contained in these first stars, astronomers think that they may not have ended in a huge explosion as massive stars do today. Instead, it is thought that once the star ran out of fuel, the force of gravity would be so strong that all of the star would collapse directly down to a black hole. The outer envelope of the star, would not be blown away as it is today, it will be dragged down into the black hole to join the core of the star. This stellar death is called direct collapse. This means that the first stars in the universe may have collapsed to form black holes weighing hundreds of solar masses, they would have created intermediate-mass black holes. Now that we know a possible direct way to make intermediate-mass black holes, are there more indirect ways? Well, yes. We can make intermediate-mass black holes by combining two smaller stellar-mass black holes. Stellar-mass black holes are the easiest to see when they are actively feeding from their companion star in a binary system. If the companion is a massive star, then it may also create a black hole at the end of its life. If this happens, we would end up with a black hole binary containing two black holes. Over time, these black holes can spiral in, getting closer and closer together until they merge. When that happens, the merging black holes combine to form single more massive black hole. This is an idea that has been around for a while but recently, has gained traction due to this discovery of black hole mergers with LIGO, the Laser Interferometer Gravitational Wave Observatory. We will be going into more detail about the facilities such as LIGO and the physics behind them in a later module. By combining stellar-mass black holes in this way, it's possible to step up the mass scale to intermediate-mass black hole range. So, for example, if two black holes came together that each weighed in at about 60 solar masses, the result would be a black hole lying in the intermediate-mass black hole range. Some theoretical astronomers have suggested that intermediate-mass black holes could also form by a process known as runaway formation. Runaway formation can only occur in dense regions. Dense regions are areas in space where many stars are clumped closely together, as they are in some stellar clusters. Within the central region of the cluster, you can think of the stars as dancers in a club. They are moving around each other as they travel under the influence of gravity. If two of these stars get too close together, they can start orbiting as binaries do, or they can spiral in towards each other and merge. This new star will have more gravity and attract other nearby stars. As they spiral in and merge, the object at the center will have even more gravity, and the cycle will continue, allowing this object to grow and grow until the gravity of this object is so strong that the supermassive star is forced to collapse to make an intermediate-mass black hole.
