Towards thee I roll, thou all-destroying but unconquering whale; to the last I grapple with thee; from hell’s heart I stab at thee; for hate’s sake I spit my last breath at thee.
So utters Captain Ahab of the Pequod at his mammoth, yet elusive, quarry, the object of his obsessive search—the great White Whale, and the title beast of Herman Melville's renowned 1851 novel: Moby-Dick; or, The Whale. It’s hard to find a better example of a determined and single-minded quest to find something so massive, yet so mysterious.
But poor old Captain Ahab isn’t alone, even today: some astronomers might be considered his unlikely compatriots. Outer space is home to its own massive, yet mysterious and elusive, leviathans—not White Whales, but Black Holes. The scientists who hunt them down and study them need not destroy everything in the process, as Captain Ahab does, but sometimes they do have to be just as single-minded, determined, and unceasing in their quest. Tracking down and learning about black holes turns out, in its own way, to be every bit as arduous and demanding as finding Moby Dick. But unlike the hunt for Moby Dick, the search for these celestial leviathans serves the beneficial purpose of expanding human knowledge, rather than trying to exact revenge on a force of nature.
You may wonder why scientists bother trying to find black holes. The answer is, black holes are extreme. Studying extremes is one way we can learn about the limits of what is possible in our Universe. For example, the biggest animal that has ever lived on this planet is indeed a whale. And Moby Dick, as a sperm whale, was the largest of the toothed whale species, and with the largest brain of any animal ever. Marine scientists study whales to help us learn more about life on the Earth, and many of us hope to be lucky enough to see a whale up close.
Likewise, black holes are among the most massive discrete objects in the Universe, with enormously powerful gravitational pull, and learning more about them is interesting for its own sake. But beyond that, if we can figure out something as extreme as black holes, then in the process, we also gain fundamental knowledge about our Universe, to help us answer many other questions about why and how things turned out to be the way they are.
In this series of four articles, we'll explore more about black holes, the white whales of the cosmos—where they come from, how we can find them, how old they might be, how massive they are, and why we think they do some of the things they do. To date, we have only some of the answers, so we’ll also be pointing to where our present understanding is incomplete.
Where Do Black Holes Come From, and What Are They?
It's easiest to start talking about black holes by describing the best-understood way certain ones can form. These are called “stellar black holes,” and they are the final destiny of some individual, big stars. How big? No one can answer that question for certain yet—but definitely bigger than our Sun. As long as one of these big stars still has nuclear fuel to burn, outward pressure and energy make the star shine and keep it from collapsing. But when the fuel runs out, the star begins to fall in on itself. As this happens, the star heats up again, but not in a stable or sustainable way. Instead, the reheated star eventually blows off its outer layers in a spectacular explosion, driven by the collapse of the core.
Even after the star has exploded, its leftover mass is still enormous—because the star was so huge to begin with. That still-enormous mass collapses again. But this time, nothing can stop its inward plummet. The repulsive forces between neutrons in the atoms of the star's core are too weak to withstand the crush of collapsing stellar material. When the unstoppable collapse of the star's core is complete, the result is a stellar black hole.
Black holes actually come in different varieties, and each might have different origins—stellar black hole formation being only one of them. For example, so-called primordial black holes, if they do indeed exist, would have formed when the Universe was extremely young, and we think many such primordial black holes would have a mass much smaller than that of a star. But other black holes, by contrast, have mass far greater than can be explained by the collapse of an individual star, with some being so colossal that we call them “supermassive” black holes—and these behemoths will be the focus of the rest of this article.
The origins of supermassive black holes are a bit of a mystery. We can't just assume they form the same way stellar black holes do, because we don't know of any stars big enough for to create them. Some theories suggest supermassive black holes collapsed directly from the huge clouds of gas that accompany the formation and growth of a galaxy. We know that the rate of star formation and the growth of galaxies peaked when the Universe was between 1 and 4 billion years old, so perhaps the rate of supermassive black hole formation peaked over that timescale as well.
Others theories of formation of supermassive black holes postulate they are the result of lots of smaller black holes coming together through gravity and merging. But it's also possible these are just stellar black holes after all—older, heftier ones that have been around long enough to vacuum up stars and dust around them, and grow and grow and grow.
Finding out how the different kinds of black holes form is one of the main goals of scientists who study them. The more we can learn about how these extreme objects come into being, the more we know about our Universe and everything in it.
In the next part of this series, we'll look at how we can find black holes, and how old they are.
Herman Melville, Moby-Dick. A Norton Critical Edition. Parker, Hershel, and Harrison Hayford (eds). (2001). Second Edition, New York and London: W.W. Norton & Company.
Skidmore, W., TMT International Science Development Teams, & Science Advisory Committee, "TMT Detailed Science Case 2015", Research in Astronomy and Astrophysics, 15, 1945. DOI 10.1088/1674-4527/15/12/001. See also https://arxiv.org/abs/1505.01195
Many thanks to my colleagues Matthias Schöck (TMT System Scientist) and Warren Skidmore (TMT Instrument System Scientist), for helping with this blog entry.