Banner by Troy Barlow

Going Down the Rabbit Hole: What are Black Holes?

by Emily Forsyth

Swirling just out of reach of astronomers and letting nothing, not even light, escape their pull, black holes have always been objects of great curiosity for astronomers and non-astronomers alike. They continue to gain interest as more information is gathered and black holes are featured in entertainment. But what are they exactly? And why do astronomers care about them so much?

Albert Einstein’s general theory of relativity predicted black holes way back in 1916. Later, in 1967, John Wheeler gave Einstein’s prediction its famous modern name, the “black hole.” Black holes cannot be viewed like stars and planets, as light itself is pulled into its incredibly strong gravitational field, which makes them difficult to find. To notice them, astronomers must wait for matter, such as interstellar dust or a star, to pass close enough to a black hole for its gravity to pull the matter in, a process known as accretion. This matter accelerates into the black hole and heats up, sending x-rays out into space that may or may not be detected by astronomers on Earth. Out of all of the data sent out by these massive giants, astrophysicists can pick up only a miniscule amount of it because of their location in space, leaving most black holes undetected. 

 But sometimes, by chance, we can pick up the x-rays sent out, like in 1971, when Einstein’s predictions were confirmed, and the first black hole was detected. Unfortunately, Einstein never lived to see his predictions unfold, but thousands of excited astronomers continued to build off of his predictions to learn more about these strange new entities. In 2017, researchers captured the first image of a black hole, yet they faced a dilemma. The massive size of the data prevented the researchers from putting the image together immediately. It wasn’t until nearly two years later that researchers finally witnessed the fruits of their labors and the picture came together, finally showing them the bright swirling mystery. They had captured a picture of a supermassive black hole. This is one of the three types of black holes discovered to date, which are classified by size: stellar black holes, intermediate black holes and supermassive black holes. 

Stellar black holes are the smallest and most plentiful kind of black hole. They are created by the death and collapse of stars greater than three times our Sun’s mass. Stars less massive than this will collapse into a core and then become a neutron star or a white dwarf. Stellar black holes are massive, yet their diameter is roughly only as large as the diameter of New York City. These black holes are abundant in galaxies, with an estimated 100 million in the Milky Way alone. 

Until 2004, scientists were unaware of intermediate black holes. These are less massive than supermassive black holes, but more massive than stellar black holes, weighing in with masses anywhere from 36,000 to 316,000 times that of our Sun. The first hypothesized detection of one was in 2014, where one was thought to be spotted in the arm of a spiral galaxy. There is little research on these midsized black holes, but their origin is believed to be a cluster of stars colliding and collapsing inward. More recent research predicts that these intermediate black holes could reside at the center of dwarf galaxies. 

The largest class of black holes is the supermassive black hole. These can be a whopping several million or billion times as massive as our own solar system’s Sun, yet around the same size. There are many hypotheses for how these giants form. They could be the result of many smaller black holes merging, the collapse of massive gas clouds, the collapse of a cluster of stars or from large collections of the elusive dark matter. The only thing known for sure about dark matter is that it makes up 27 percent of the universe, making the creation of supermassive black holes hard to track. Though little is known of their birth, scientists are sure of their home in the universe once they exist. They live at the center of galaxies and gain mass by pulling in the plenitude of matter that exists at the center of galaxies. One even inhabits the center of our own Milky Way galaxy. 

The intriguing properties of black holes are what captures the attention of both astronomers and the public. Their massive gravitational force is strong enough that not even light itself can escape its tug if the light travels too close. Black holes consist of an event horizon and a singularity. The event horizon is an invisible boundary around the black hole that marks the point of no return; if any matter, or light itself, passes this frontier, it falls into the black hole with no possibility of leaving. It will fall until it reaches the singularity, or center, of the black hole. Nobody is certain what happens at the singularity. Another interesting characteristic of black holes is that, nearing the event horizon, time slows down relative to the time observed by people far away. This baffling effect means that if you were able to somehow teleport to a planet near the event horizon, live a day and teleport back home to earth, everyone on earth would have aged many years while you looked exactly the same. 

So, what would happen if a human fell into a black hole? (Theoretically, of course.) Famous astrophysicist Neil DeGrasse Tyson answers this with a touch of dark humor. Provided they could survive long enough, if a person began falling toward a black hole feet first, the gravity that their feet would feel relative to their head would be so much greater that they would split in half. They would then split into smaller and smaller pieces, undergoing a process humorously coined as “spaghettification.” 

These ideas are entertaining to think about, but why do black holes actually matter so much to astronomers? Studies of black holes lead to an understanding of the effect they have on their surroundings, especially those of supermassive black holes. Knowledge about the relationship that supermassive black holes have with their home galaxy can provide astronomers with insight as to why the centers of galaxies act and move the way they do. For example, astronomers have noticed a depletion in the number of stars created over massive periods of time, which they now believe is due to the ultrafast winds given off by black holes. Understanding black holes links back to helping us understand the universe we live in and the laws which govern it, a goal which we all hold as a part of natural human curiosity.