Are there different types of black holes?

Category: Space
Published: August 2, 2022

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Yes, there are different types of black holes. The most straightforward way to classify black holes is according to their mass. You may think that because a black hole is in essence just a clump of matter that is dense enough to trap light, black holes of all masses should exist. In other words, black holes should exist along a continuous range of masses. However, that is not what we find in practice. The only types of black holes that have been firmly established to exist are stellar-mass black holes and supermassive black holes.

Artistic rendering of a black hole. Public Domain Image, source: Christopher S. Baird

Stellar-mass black holes are formed from the gravitational collapse of a single star or from the merger of two neutron stars. Therefore, stellar-mass black holes have masses similar to the masses of stars. More specifically, stellar-mass black holes have masses ranging from about 3 times the mass of our sun to about 50 times the mass of our sun. In contrast, supermassive black holes have a mass greater than about 50,000 times the mass of our sun and are typically millions to billions times the mass of our sun. Supermassive black holes are far too large to have formed from the gravitational collapse of a single star. However, scientists do not currently know how supermassive black holes form. Supermassive black holes are always found at the center of a galaxy and almost all galaxies have a supermassive black hole at its center. This seems to suggest that each supermassive black hole is formed as part of the formation of its galaxy.

Interestingly, there seems to be zero or very few black holes with a mass between that of stellar black holes and supermassive black holes. The range between about 50 times the mass of our sun to about 50,000 times the mass of our sun seems like a huge range over which black holes typically do not exist. Any black hole with a mass in this range is called an intermediate black hole. A few decades ago, intermediate black holes were thought to not exist at all. However, recent observations seem to suggest that intermediate black holes may exist but are very rare. There may be many reasons why intermediate black holes are very rare, but one reason is likely the most important. This reason is that there are not any common physical mechanisms in the universe that can collapse matter down to a black hole of intermediate size. Most stars are too small to collapse down to intermediate black holes and whatever galactic mechanism produces supermassive black holes seems to involve masses that are too large to produce intermediate black holes. This is an ongoing area of research.

Also interestingly, there seems to be no black holes that have a mass smaller than that of the stars (which spans a huge range from the mass of planets down to masses smaller than that of electrons). Termed mini black holes or micro black holes, the laws of physics as currently understood seem to suggest that it is indeed physically possible for them to exist. However, scientists cannot find any evidence of mini black holes existing. Perhaps mini black holes can exist but there is no natural physical mechanism that can produce them. Or perhaps mini black holes cannot exist for fundamental physical reasons. If mini black holes did exist, it is likely that they would quickly evaporate away to nothing through Hawking radiation. This is also an ongoing area of research.

These concepts are summarized in the table below, where the numbers shown are approximate and the values for mass are that value times the mass of our sun.

Another way to classify black holes is according to physical structure. The point-of-no-return nature of black holes means that most of the information that enters a black hole is destroyed or permanently locked away from the rest of the universe. For instance, there are no such things as rocky black holes or gaseous black holes like there are rocky planets and gaseous planets. All the rocks, gases, and dust particles that fall into a black hole get crushed down to a featureless speck of mass or ring of mass. Similarly, there are no such things as hot black holes or cold black holes. Also, there is no difference between a black hole formed from regular matter and a black hole formed from antimatter (although I should note here that there are not actually clumps of antimatter in our universe that are large enough to form black holes). The very nature of a black hole leads it to collapse all of its mass and energy down to an indistinguishable clump and to smooth out all irregularities and asymmetries. Because everything becomes indistinguishable within a black hole, the word "mass" in this context actually refers to mass and energy.

However, a black hole does indeed retain a few properties that are externally measurable: its overall mass, its overall electric charge, and its overall spinning rate. Note that a few other properties of a black hole such as radius and magnetic moment are externally measurable, at least in principle, but these are not independent parameters. In other words, they are directly dependent, and arise from, the black hole's mass, charge, and spin. These three properties are the only independent, externally-observable black hole properties. If two isolated black holes had the same mass, charge, and spin, they would be indistinguishable.

The reason that the total mass, total charge, and total spin of a black hole are measurable from outside of the black hole despite being internal properties is that they obey universal conservation laws. Another way of saying this is that these properties are connected to fundamental symmetries in spacetime and therefore affect spacetime curvature.

We can therefore classify black holes according to mass, charge, and spin. I have already described classifying black holes by mass. If we just focus on charge and spin, we can make the following classification categories: black holes that are not spinning and have no net electric charge (Schwarzschild black holes), black holes that are spinning and have no net electric charge (Kerr black holes), black holes that are not spinning and do have a net electric charge (Reissner-Nordstrom black holes), and black holes that are spinning and do have a net electric charge (Kerr-Newman black holes). In our universe, black holes are almost always spinning (because they form from spinning bodies of matter) and almost always have zero net electric charge (because of the tendency of electric charge to attract opposite types of electric charge and self-neutralize). Therefore, Kerr black holes are by far the most common. These concepts are summarized in the table below.

Combining all of the concepts in this article, we see that the most common black holes in our universe are spinning, uncharged stellar-mass black holes and spinning, uncharged supermassive black holes.

Topics: black hole, charge, energy, gravity, mass, spin