The 5 Different Types of Black Holes

The Main Features of Black Holes

Black holes form when matter is crushed into a volume so small that its gravity creates an event horizon from which nothing can escape, not even light. Everything within the event horizon appears "black" and falls into a singularity at its center with zero volume and infinite density, according to general relativity. As all astrophysical black holes spin, the singularity is stretched into a flat circle, and the event horizon into an oblate spheroid (a flattened ball).

A region called the ergosphere forms just outside the event horizon where everything—including the fabric of space—is dragged along with the rotation. Also outside the event horizon is a photon sphere, at about 1.5× the horizon radius, where gravity bends light into a circular orbit—a knife-edge balance, where the slightest perturbation sends the light either escaping to infinity or spiralling back in.

An accretion disk of doomed matter, beginning at 1–3× the horizon radius, feeds roughly 1% of black holes, and about 10% of these convert some of the matter into relativistic jets that emanate from the poles at close to the speed of light (see image below).

How Are Black Holes Classified?

The main feature of a black hole is its gravity, reflected in the size of its event horizon—the Schwarzschild radius—which is determined by its mass. These cosmic behemoths are therefore classified by mass (i.e., more mass = more gravity = bigger event horizon).

Black holes can also spin and carry charge, although these features are not used to distinguish between types of astrophysical black hole, which all spin and are expected to be neutral. This is because they are formed from spinning objects and must conserve that angular momentum, while any charge should be neutralized by opposite charges.

Below, the masses of five types of black holes are given in solar masses. The mass of the Sun is 2 × 1030 kg (a two followed by thirty zeros). A black hole with "5 solar masses" is therefore five times the mass of the Sun. 

1. Primordial Black Holes

• Prevalence: unknown (hypothesized)

• Formation: early universe

• Mass range: 10-8 kg – 100,000 solar masses
  
  

Black holes are usually massive because they form from stars with enough gravity to crush matter into the densities required for a singularity. However, density can be achieved in other ways.

In the first second after the Big Bang, the universe was dense enough that random fluctuations in density might have created localized overdense pockets that collapsed under their own gravity, creating primordial black holes.

Some would have been lighter than a feather (a few milligrams) and these will have already "evaporated" by emitting Hawking radiation. Others that were heavier than a small asteroid should still be around today, with some perhaps accumulating matter and growing to "supermassive" sizes over billions of years. 

The existence of primordial black holes was proposed in the 1960s and developed further by Stephen Hawking in the 1970s, although none have been observed. This isn't surprising, given their size and invisibility. Some physicists have even suggested that primordial black holes can explain the mystery of "dark matter" (see video).

2. Stellar Black Holes

• Prevalence: ~100 million per galaxy (~200 billion galaxies)

• Formation: stellar death

• Mass range: 3 – 100 solar masses
  
  

A stellar black hole forms when a massive star exhausts its fuel and collapses. Prior to collapse, the star's fusion of its (mostly hydrogen) fuel exerted an outward pressure that countered the force of gravity and kept it stable ("hydrostatic equilibrium"). Without that pressure, the star throws off its outer layers in a supernova and leaves behind a remnant that collapses into a singularity.

If a star isn't massive, its collapse won't create enough density to form a black hole. For example, our Sun will become a white dwarf, while some bigger stars become neutron stars, both of which are just very dense forms of matter.

Only stars over ~25 solar masses leave behind a remnant that produces enough gravity to create a black hole. This remnant must have a mass over the Tolman–Oppenheimer–Volkoff limit, which is thought to be around 2 to 3 solar masses—recent observations suggest it's close to 2.2.

3. Supermassive Black Holes

• Prevalence: ~1 per galaxy (~200 billion galaxies)

• Formation: direct gas cloud collapse, multiple stellar death, or primordial

• Mass range: 100,000 – 10 billion solar masses
  
  

A supermassive black hole (SMBH) is at the center of nearly every galaxy. Their formation is likely to have required the direct collapse of massive gas clouds in the early universe, or perhaps the collapse of multiple massive stars—which can burn through their fuel in under 10 million years (by comparison, the Sun's lifetime is 10 billion years)—and the merger of their black holes. Another possibility is that SMBHs formed and grew in the early universe from primordial black holes, with galaxies coalescing around them later.

Very rarely, a SMBH is thrown out of its galaxy by a merger with another galaxy. In this case, both SMBHs interact and produce gravitational waves that can be stronger in one direction (e.g., if their spins are counter-aligned). After merging, the combined SMBH recoils away from this direction to conserve momentum. This likely occurred in the 3C 186 galaxy.

At the center of the Milky Way is Sagittarius A*, which is 4 million solar masses and 27,000 light-years away (image below). This is small for SMBHs. The black hole in the nearby Andromeda galaxy (2.5m light-years) is ~100m solar masses. The black hole in the Messier 87 galaxy (55m light-years) is ~6.5b solar masses (image below). The largest SMBH may be in the Phoenix A galaxy (8.6b light-years) at ~100b solar masses. 

4. Quasars

• Prevalence: 5%–10% of supermassive black holes (SMBHs)

• Formation: accretion of matter around SMBHs

• Mass range: 100,000 – 10 billion solar masses
  
  

Some SMBHs gather a large accretion disk in which matter is smashed together, emitting all kinds of EM radiation (radio waves, infrared, visible light, ultraviolet, X-rays, and gamma-rays). These SMBHs, which are hundreds or thousands of times brighter than their host galaxies, are a type of "active galactic nuclei" called quasars.

The brightest quasar in the night sky (3C 273) would appear as bright as the Sun from 33 light-years away. This quasar swallows matter equivalent to a thousand Suns per year, or ten Earths per second.

Quasars become an ordinary SMBH when their accretion disk is depleted, which is why they were more common in the early universe, although a galactic merger can create one. The nearest quasar is therefore 600 million light-years away.

About 10% of quasars are "radio loud" and emit relativistic jets that are strong sources of radio and gamma-ray waves (see images below). The jets are produced when a quasar is spinning rapidly and has a "corona" of diffuse matter that produces a strong magnetic field (the field appears to have a helical, DNA-like structure).

The word quasar comes from the obsolete term "quasi-stellar radio source." Some quasars whose jets point toward Earth—making them appear even brighter—are classified as "blazars."

5. Intermediate-Mass Black Holes

• Prevalence: unknown (hypothesized)

• Formation: repeated black hole mergers or primordial

• Mass range: 100 – 100,000 solar masses
  
  

Intermediate-mass black holes (IMBHs) cover the mass range between stellar and supermassive black holes. Like SMBHs, they might have grown out of primordial black holes in the early universe, perhaps before many grew into SMBHs.

Consequently, IMBHs might still exist at the center of smaller "dwarf" galaxies because the mass of galactic nuclei is related to the mass of their host galaxy. However, these are typically in the range of a few thousand to 100,000 solar masses, with many overlapping the lower end of the SMBH range.

At the lower mass range, IMBHs can be formed by the merger of stellar black holes. However, such mergers are often closer to 100 solar masses and could be classed as "stellar."

Nevertheless, black hole mergers might be a mechanism for producing IMBHs of ~1000 solar masses in globular star clusters, where thousands of stars are packed so densely that black holes frequently collide and merge. Some evidence points to an 800 solar mass IMBH in the Messier 4 cluster (image below), suggesting that a star cluster can behave like a little galaxy with a (relatively) little black hole in the middle. 

Ongoing Research About Black Holes

In sum, stellar black holes are the remnants of collapsed stars, and supermassive black holes sit in the centers of galaxies, with some of the latter called quasars because of their bright accretion disks and jets. Additionally, there is growing evidence for intermediate-mass black holes between the stellar and supermassive range, as well as smaller primordial black holes, but that research is ongoing.

Indeed, the amount of scientific interest in black holes rivals the enormity of the objects themselves. New telescopes and imaging methods are continually being developed, and new discoveries are continually being made. A decade from now, we will know much more than we do today.

Sources

The following websites can be used to verify the information contained in this article and to obtain new information about black hole discoveries.

• NASA - Types of Black Hole

• Event Horizon Telescope

• James Webb Space Telescope

• European Southern Observatory

• Hubble Space Telescope