Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)
SECTION 5. WHEN THE UNIVERSE TURNS BAD
Chapter 31. GALACTIC ENGINES
Galaxies are phenomenal objects in every way. They are the fundamental organization of visible matter in the universe. The universe contains as many as a hundred billion of them. They each commonly pack hundreds of billions of stars. They can be spiral, elliptical, or irregular in shape. Most are photogenic. Most fly solo in space, while others orbit in gravitationally linked pairs, familial groups, clusters, and superclusters.
The morphological diversity of galaxies has prompted all manner of classification schemes that supply a conversational vocabulary for astrophysicists. One variety, the “active” galaxy, emits an unusual amount of energy in one or more bands of light from the galaxy’s center. The center is where you will find a galactic engine. The center is where you will find a supermassive black hole.
The zoo of active galaxies reads like the manifest for a carnival grab-bag: Starburst galaxies, BL Lacertae galaxies, Seyfert galaxies (types I and II), blazars, N-galaxies, LINERS, infrared galaxies, radio galaxies, and of course, the royalty of active galaxies—quasars. The extraordinary luminosities of these elite galaxies derive from mysterious activity within a small region buried deep within their nucleus.
Quasars, discovered in the early 1960s, are the most exotic of them all. Some are a thousand times as luminous as our own Milky Way galaxy, yet their energy hails from a region that would fit comfortably within the planetary orbits of our solar system. Curiously, none are nearby. The closest one sits about 1.5 billion light-years away—its light has been traveling for 1.5 billion years to reach us. And most quasars hail from beyond 10 billion light-years. Possessed of small size and extreme distance, on photographs one can hardly distinguish them from the pointlike images left by local stars in our own Milky Way, leaving visible-light telescopes quite useless as tools of discovery. The earliest quasars were, in fact, discovered using radio telescopes. Since stars do not emit copious amounts of radio waves, these radio-bright objects were a new class of something or other, masquerading as a star. In the we-call-them-as-we-see-them tradition among astrophysicists, these objects were dubbed Quasi-Stellar Radio Sources, or more affectionately, “quasars.”
What manner of beast are they?
One’s ability to describe and understand a new phenomenon is always limited by the contents of the prevailing scientific and technological toolbox. An eighteenth-century person who was briefly, but unwittingly, thrust into the twentieth century would return and describe a car as a horse-drawn carriage without the horse and a lightbulb as a candle without the flame. With no knowledge of internal combustion engines or electricity, a true understanding would be remote indeed. With that as a disclaimer, allow me to declare that we think we understand the basic principles of what drives a quasar. In what has come to be known as the “standard model,” black holes have been implicated as the engine of quasars and of all active galaxies. Within a black hole’s boundary of space and time—its event horizon—the concentration of matter is so great that the velocity needed to escape exceeds the speed of light. Since the speed of light is a universal limit, when you fall into a black hole, you fall in for good, even if you’re made of light.
HOW, MIGHT YOU ASK, can something that emits no light power something that emits more light than anything else in the universe? In the late 1960s and 1970s, it didn’t take long for astrophysicists to discover that the exotic properties of black holes made remarkable additions to the theorists’ toolbox. According to some well-known laws of gravitational physics, as gaseous matter funnels toward a black hole, the matter must heat up and radiate profusely before it descends through the event horizon. The energy comes from the efficient conversion of gravity’s potential energy into heat.
While not a household notion, we have all seen gravitational potential energy get converted at some time in our terrestrial lives. If you have ever dropped a dish to the floor and broken it, or if you have ever nudged something out the window that splattered on the ground below, then you understand the power of gravitational potential energy. It’s simply untapped energy endowed by an object’s distance from wherever it might hit if it fell. When objects fall, they normally gain speed. But if something stops the fall, all the energy the object had gained converts to the kind of energy that breaks or splatters things. Therein is the real reason why you are more likely to die if you jump off a tall building instead of a short building.
If something prevents the object from gaining speed yet the object continues to fall, then the converted potential energy reveals itself some other way—usually in the form of heat. Good examples include space vehicles and meteors when they heat up while descending through Earth’s atmosphere: they want to speed up, but air resistance prevents it. In a now-famous experiment, the nineteenth-century English physicist James Joule created a device that stirred a jar of water with rotating paddles by the action of falling weights. The potential energy of the weights was transferred into the water and successfully raised its temperature. Joule describes his effort:
The paddle moved with great resistance in the can of water, so that the weights (each of four pounds) descended at the slow rate of about one foot per second. The height of the pulleys from the ground was twelve yards, and consequently, when the weights had descended through that distance, they had to be wound up again in order to renew the motion of the paddle. After this operation had been repeated sixteen times, the increase of the temperature of the water was ascertained by means of a very sensible and accurate thermometer…. I may therefore conclude that the existence of an equivalent relation between heat and the ordinary forms of mechanical power is proved…. If my views are correct, the temperature of the river Niagara will be raised about one fifth of a degree by its fall of 160 feet. (Shamos 1959, p. 170)
Joule’s thought-experiment refers, of course, to the great Niagara Falls. But had he known of black holes, he might have said instead, “If my views are correct, the temperature of gas funneled toward a black hole will be raised a million degrees by its fall of a billion miles.”
AS YOU MIGHT suspect, black holes enjoy a prodigious appetite for stars that wander too close. A paradox of galactic engines is that their black holes must eat to radiate. The secret to powering the galactic engine lies in a black hole’s ability to ruthlessly and gleefully rip apart stars before they cross the event horizon. The tidal forces of gravity for a black hole elongate the otherwise spherical stars in much the same way that the Moon’s tidal forces elongate Earth’s oceans to create high and low oceanic tides. Gas that was formerly part of stars (and possibly ordinary gas clouds) cannot simply gain speed and fall in; the gas of previously shredded stars impedes wanton free fall down the hole. The result? A star’s gravitational potential energy gets converted to prodigious levels of heat and radiation. And the higher the gravity of your target, the more gravitational potential energy was available to convert.
Faced with the proliferation of words to describe oddball galaxies, the late Gerard de Vaucouleurs (1983), a consummate morphologist, was quick to remind the astronomical community that a car that has been wrecked does not all of a sudden become a different kind of car. This car-wreck philosophy has led to a standard model of active galaxies that largely unifies the zoo. The model is endowed with enough tweakable parts to explain most of the basic, observed features. For example, the funneling gas often forms an opaque rotating disk before it descends through the event horizon. If the outward flow of radiation cannot penetrate the disk of accreted gas, then radiation will fly out from above and below the disk to create titanic jets of matter and energy. The observed properties of the galaxy will be different if the galaxy’s jet happens to be pointing toward you or sideways to you—or if the ejected material moves slowly or at speeds close to the speed of light. The thickness and chemical composition of the disk will also influence its appearance as well as the rate at which stars are consumed.
To feed a healthy quasar requires that its black hole eat up to ten stars per year. Other less-active galaxies from our carnival shred many fewer stars per year. For many quasars, their luminosity varies on time scales of days and even hours. Allow me to impress you with how extraordinary this is. If the active part of a quasar were the size of the Milky Way (100,000 light-years across) and if it all decided to brighten at once, then you would first learn about it from the side of the galaxy that was closest to you, and then 100,000 years later the last part of the galaxy’s light would reach you. In other words, it would take 100,000 years for you to observe the quasar brightening fully. For a quasar to brighten within hours means that the dimensions of the engine cannot be greater than light-hours across. How big is that? About the size of the solar system.
With a careful analysis of the light fluctuations in all bands, a crude, but informative three-dimensional structure can be deduced for the surrounding material. For example, the luminosity in x-rays might vary over a time scale of hours but the red light might vary over weeks. The comparison allows you to conclude that the red light-emitting part of the active galaxy is much larger than the x-ray emitting part. This exercise can be invoked for many bands of light to derive a remarkably complete picture of the system.
If most of this action takes place during the early universe in distant quasars, then why does it no longer happen? Why are there no local quasars? Do dead quasars lurk under our noses?
Good explanations are available. The most obvious is that the core of local galaxies ran out of stars to feed the engine, having vacuumed up all stars whose orbits came too close to the black hole. No more food, no more prodigious regurgitations.
A more interesting shut-off mechanism comes from what happens to the tidal forces as the black hole’s mass (and event horizon) grows and grows. As we will see later in this section, tidal forces have nothing to do with the total gravity felt by an object—what matters is the difference in gravity across it, which increases dramatically as you near an object’s center. So large, high-mass black holes actually exert lower tidal forces than the smaller, low-mass black holes. No mystery here. The Sun’s gravity on Earth dwarfs that of the Moon’s on Earth yet the proximity of the Moon enables it to exert considerably higher tidal forces at our location, a mere 240,000 miles away.
It’s possible, then, for a black hole to eat so much that its event horizon grows so large that its tidal forces are no longer sufficient to shred a star. When this happens, all of the star’s gravitational potential energy converts to the star’s speed and the star gets eaten whole as it plunges past the event horizon. No more conversion to heat and radiation. This shut-off valve kicks in for a black hole of about a billion times the mass of the Sun.
These are powerful ideas that do indeed offer a rich assortment of explanatory tools. The unified picture predicts that quasars and other active galaxies are just early chapters in the life of a galaxy’s nucleus. For this to be true, specially exposed images of quasars should reveal the surrounding fuzz of a host galaxy. The observational challenge is similar to that faced by solar system hunters who try to detect planets hidden in the glare of their host star. The quasar is so much brighter than the surrounding galaxy that special masking techniques must be used to detect anything other than the quasar itself. Sure enough, nearly all high-resolution images of quasars reveal surrounding galaxy fuzz. The several exceptions of uncloaked quasars continue to confound the expectations of the standard model. Or is it that the host galaxies simply fall below the detection limits?
The unified picture also predicts that quasars would eventually shut themselves off. Actually, the unified picture must predict this because the absence of nearby quasars requires it. But it also means that black holes in galactic nuclei should be common, whether or not the galaxy has an active nucleus. Indeed, the list of nearby galaxies that contain dormant supermassive black holes in their nuclei is growing longer by the month and includes the Milky Way. Their existence is betrayed through the astronomical speeds that stars achieve as they orbit close (but not too close) to the black hole itself.
Fertile scientific models are always seductive, but one should occasionally ask whether the model is fertile because it captures some deep truths about the universe or because it was constructed with so many tunable variables that you can explain anything at all. Have we been sufficiently clever today, or are we missing a tool that will be invented or discovered tomorrow? The English physicist Dennis Sciama knew this dilemma well when he noted:
Since we find it difficult to make a suitable model of a certain type, Nature must find it difficult too. This argument neglects the possibility that Nature may be cleverer than we are. It even neglects the possibility that we may be cleverer to-morrow than we are to-day. (1971, p. 80)