Are There Different Types of Black Holes?

Yes, there are different types of black holes. The most straightforward way to classify black holes is according to their mass. While one might intuitively think that black holes should exist along a continuous range of masses, the reality is different. The types of black holes that have been firmly established to exist are stellar-mass black holes and supermassive black holes.

Stellar-Mass Black Holes

Stellar-mass black holes are formed from the gravitational collapse of a single star or from the merger of two neutron stars. Their masses are similar to those of stars and typically range from about $3$ to $50$ times the mass of our sun ($M_{\odot}$). More specifically, we can express this as:

$$\text{Mass}_{\text{stellar}} \approx [3 M_{\odot}, 50 M_{\odot}]$$

Supermassive Black Holes

Supermassive black holes, on the other hand, have a mass greater than about $50,000$ times the mass of our sun, often reaching millions to billions of solar masses:

$$\text{Mass}_{\text{supermassive}} > 50,000 M_{\odot}$$

These black holes are so large that they cannot form from the gravitational collapse of a single star. They are typically found at the center of galaxies, suggesting that their formation is closely linked to the formation of the galaxy itself. Interestingly, there seems to be a significant gap in mass between stellar-mass and supermassive black holes, with very few (if any) black holes existing in the intermediate mass range.

Intermediate Black Holes

Black holes with masses between $50$ and $50,000$ times the mass of our sun are known as intermediate black holes. The existence of these black holes was once doubted, but recent observations indicate they may exist, albeit rarely. This rarity may be due to the absence of common physical mechanisms that can collapse matter into black holes of this intermediate size. The mass range can be expressed as:

$$\text{Mass}_{\text{intermediate}} \approx [50 M_{\odot}, 50,000 M_{\odot}]$$

Mini Black Holes

On the lower end of the mass spectrum, there are mini black holes (or micro black holes), which would have a mass less than that of stellar black holes (i.e., less than $3 M_{\odot}$). Although theoretical physics suggests that such black holes could exist, there is currently no evidence of their existence. If they did exist, they would likely evaporate quickly due to Hawking radiation.

Summary of Black Hole Types

The various types of black holes can be summarized in the following table:

Black Hole Type	Mass Range (in $M_{\odot}$)	Abundance	Formation	Physically Allowed
Mini Black Hole	less than $3$	rare or none	unknown	maybe
Stellar-Mass Black Hole	$3$ to $50$	common	collapse of a star	yes
Intermediate Black Hole	$50$ to $50,000$	rare or none	unknown	yes
Supermassive Black Hole	$50,000$ to billions	common	part of galaxy formation	yes

Classification by Physical Structure

In addition to classification by mass, black holes can also be classified according to their physical structure. Unlike rocky or gaseous planets, black holes do not have such distinct characteristics. All matter, regardless of its form (rocks, gases, etc.), that falls into a black hole is crushed to a singular point of mass.

Observable Properties

Despite the internal nature of black holes, they do retain a few measurable properties from the outside: their overall mass, electric charge, and spin. Notably, if two black holes have the same mass, charge, and spin, they are indistinguishable from one another. The classification based on these three properties can be summarized as follows:

1. Schwarzschild Black Holes: non-spinning and uncharged
2. Kerr Black Holes: spinning and uncharged (most common)
3. Reissner-Nordstrom Black Holes: non-spinning and charged
4. Kerr-Newman Black Holes: spinning and charged

Summary of Common Black Hole Types

The most common types of black holes in our universe can be summarized as follows:

Black Hole Type	How Common
Schwarzschild Black Hole	rare
Kerr Black Hole	common
Reissner-Nordstrom Black Hole	rare
Kerr-Newman Black Hole	rare

In conclusion, black holes are fascinating objects that come in various types based on mass and physical properties. The most prevalent types are the spinning, uncharged stellar-mass and supermassive black holes, while the existence of intermediate and mini black holes remains an intriguing area of ongoing research.