When you hear the phrase "black hole," it almost certainly evokes a sense of mystery and wonder, perhaps tinged with an element of danger. While the term "black hole" has become synonymous in everyday language with "a place something goes, never to be seen again," most folks are familiar with its use in the astronomy world, if not necessarily with precise features and definitions.
For decades, among the most common refrains summing up black holes has been along the lines of "a place where gravity is so strong, not even light can escape." While this is an accurate enough summary to start with, it's natural to wonder how such a thing could happen to begin with.
Other questions abound. What's inside a black hole? Are there different types of black holes? And what is a typical black hole size, assuming such a thing exists and can be measured? The launch of the Hubble Telescope revolutionized how black holes could be studied.
Basic Black Hole Facts
Before getting deep into the topic of black holes – and bad puns – it's helpful to go over the basic terminology used to define the properties and geometry of black holes.
Most notably, every black hole has at its effective center, a singularity, which consists of matter so compressed that it is almost a point mass. The enormous resulting density produces a gravitational field so powerful that out to a certain distance, not even photons, which are the "particles" of light, can break free. This distance is known as the Schwarzchild radius; in a non-rotating black hole (and you'll learn about the more dynamic type in a subsequent section), the invisible sphere with this radius with the singularity at its center forms the event horizon.
Of course, none of this explains where black holes actually come from. Do they pop up spontaneously and in random places throughout the cosmos? If so, is there any predictability to their appearance? Considering their vaunted power, it would be useful to know if a black hole might be planning to set up shop in the general vicinity of Earth's solar system.
History of Black Holes: Theories and Early Evidence
The existence of black holes was first proposed in the 1700s, but scientists of the day lacked the instruments needed to confirm any of what they had proposed. In the early 1900s, the German astronomer Karl Schwarzchild (yes, that one) used Einstein's theory of general relativity to establish the most physically prominent behavior of black holes – their ability to "trap" light.
In theory, based on Schwarzchild's work, any mass could serve as the basis for a black hole. The only requirement is that its radius after being compressed not exceed its Schwarzchild radius.
The existence of black holes has presented physicists with a conundrum, albeit an alluring one to attempt to resolve. It is believed that thanks to the space-time curvature resulting from the extraordinary force of gravity in the vicinity of the black hole, the laws of physics in effect break down; because the event horizon is inaccessible from human analysis, this conflict is in effect not really a conflict for astrophysicists.
The Size of Black Holes
If one thinks of black hole size as the sphere formed by the event horizon, the density is far different than if the black hole is treated instead only as the ludicrously tiny collapsed star with mass forming the singularity (more on this in a moment).
Scientists believe that black holes can be as tiny as certain atoms, yet possess as much mass as a mountain on Earth. On the other hand, some can be about up to 15 or so times as massive as the sun while still being tiny (but not atomic in size). These stellar black holes are found throughout galaxies, including the Milky Way, in which Earth and the solar system reside.
Still other black holes can be much, much larger. These supermassive black holes can be more than a million times as massive as the sun, and every galaxy is believed to have one at its center. The one at the center of the Milky Way, dubbed Sagittarius A, is large enough to hold a few million Earths, but this volume pales in comparison to the object's mass – estimated to be that of 4 million suns.
Formation of Black Holes
Rather than forming and appearing unpredictably, a threat lightly hinted at previously, black holes are believed to form at the same time as the greater objects in which they "live." Some tiny black holes are believed to have formed at the same time the cosmos itself came into being, at the time of the Big Bang almost 14 billion years ago.
Correspondingly, supermassive black holes within individual galaxies form at the time those galaxies coalesce into existence from interstellar matter. Other black holes form as the consequence of a violent event called a supernova.
A supernova is the implosive, or "traumatic," death of a star, as opposed to a star burning out like a gigantic celestial ember. Such events occur when a star has exhausted so much of its fuel that it begins to collapse under its own mass. This implosion results in a rebound explosion that throws off much of what remains of the star, leaving a singularity in its place.
The Density of Black Holes
One of the aforementioned problems for physicists is that the density of the portion of the black hole regarded as the singularity cannot be computed as anything other than infinite, since it is uncertain how tiny the mass actually is (e.g., how little volume it occupies). To meaningfully compute the density of a black hole, its Schwarzchild radius must be used.
An Earth-mass black hole has a theoretical density of about 2 × 1027 g/cm3 (for reference, the density of water is a mere 1 g/cm3). Such a magnitude is practically impossible to put into the context of everyday life, but the cosmic results are predictably unique. To compute this, you divide the mass by the volume after "correcting" the radius using the relative masses of the black hole and the sun, as shown in the following example.
Sample problem: A black hole has the mass of about 3.9 million (3.9 × 106) suns, with the sun's mass being 1.99 × 1033 grams, and is assumed to be a sphere with a Schwarzchild radius of 3 × 105 cm. What is its density?
First, find the effective radius of the sphere forming the event horizon by multiplying the Schwarzchild radius by the ratio of the mass of the black hole to that of the sun, given as 3.9 million:
(3 × 105 cm) × (3.9 × 106) = 1.2 × 1012 cm
Then compute the volume of the sphere, found from the formula V = (4/3)πr3:
V = (4/3)π(1.2 × 1012 cm)3 = 7 × 1036 cm3
Finally, divide the mass of the sphere by this volume to obtain the density. Because you are given the mass of the sun and the fact that the black hole's mass is 3.9 million times greater, you can compute this mass as (3.9 × 106)(1.99 × 1033 g) = 7.76 × 1039 g. The density is therefore:
(7.76 × 1039 g)/(7 × 1036 cm3) = 1.1 × 103 g/cm3.
Types of Black Holes
Astronomers have produced different classification systems for black holes, one based on mass alone and the other based on charge and rotation. As noted in passing above, most (if not all) black holes rotate about an axis, like Earth itself.
Classifying black holes based on mass yields the following system:
- Primordial black holes: These have masses similar to that of Earth. These are purely hypothetical and may have formed through regional gravitational disturbances in the immediate aftermath of the Big Bang.
- Stellar mass black holes: Mentioned previously, these have masses between about 4 and 15 solar masses and result from the "traditional" collapse of a larger-than-average star at the terminus of its lifespan.
- Intermediate mass black holes: Unconfirmed as of 2019, these black holes – about a few thousand times as massive as the sun – may exist in some star clusters, and also later may blossom into supermassive black holes.
- Supermassive black holes: Also mentioned previously, these boast between a million to a billion solar masses and are found at the centers of large galaxies.
In an alternative scheme, black holes can be categorized according to their rotation and charge instead:
- Schwarzschild black hole: Also known as a static black hole, this type of black hole does not rotate and has no electric charge. It is therefore characterized by its mass alone.
- Kerr black hole: This is a rotating black hole, but like a Schwarzschild black hole, it has no electrical charge.
- Charged black hole: These come in two varieties. A charged, non-rotating black hole is known as a Reissner-Nordstrom black hole, while a charged, rotating black hole is called a Kerr-Newman black hole.
Other Black Hole Features
You would be right to have begun wondering how scientists have drawn so many confident conclusions about objects that by definition cannot be visualized. Much knowledge of black holes has been inferred by the behavior and appearance of relatively nearby objects. When a black hole and a star are close enough together, a special kind of high-energy electromagnetic radiation results and can tip off alert astronomers.
Large gas jets can sometimes be seen projecting from the "ends" of a black hole; sometimes, this gas can coalesce into a vaguely circular form known as an accretion disk. It is further theorized that black holes emit a kind of radiation called, appropriately, black hole radiation (or Hawking radiation). This radiation may escape the black hole owing to the formation of "matter-antimatter" pairs (e.g., electrons and positrons) just outside the event horizon, and the subsequent emission of only the positive members of these pairs as thermal radiation.
Before the launch of the Hubble Space Telescope in 1990, astronomers had long puzzled over very distant objects they named quasars, a compression of "quasi-stellar objects." Like supermassive black holes, the existence of which was discovered later, these rapidly whirling high-energy objects are found at the centers of large galaxies. Black holes are now regarded as the entities that drive the behavior of quasars, which are found only enormous distances because they existed in the relative infancy of the cosmos; their light is just now reaching Earth after some 13 billion years in transit.
Some astrophysicists have proposed that galaxies that appear to be different basic types when viewed from Earth may in fact be the same type, but with different sides of them presented toward Earth. Sometimes, the quasar energy is visible and provides a sort of "lighthouse" effect in terms of how Earth instruments record the quasar's activity, whereas at other times galaxies appear more "quiet" because of their orientation.
- Swinburne University Astronomy Online: Black Hole
- NASA: What Is a Black Hole?
- The Hubble Space Telescope: Black Holes and the Quasar Connection
- Chemistry Libretexts: Density of Black Holes
- University of Alaska-Fairbanks: The Composition of a Black Hole
- Cornell University – Ask and Astronomer: What Is the Density of a Black Hole?