Death By Black Hole & Other Cosmic Quandaries

by Neil DeGrasse Tyson

  Equivalently, one can ask the simple question: How quickly does the force of gravity change as you draw nearer to an object? The equations of gravity show that gravity changes more and more swiftly as you near the center of an object. Smaller black holes allow you to get much closer to their centers before you enter their event horizons, so the change of gravity over small distances can be devastating to fallers-in.

  A common variety of black hole contains several times the mass of the Sun, but packs it all within an event horizon only about a dozen miles across. These are what most astronomers discuss in casual conversations on the subject. In a fall toward this beast, your body would begin to break apart within 100 miles of the center. Another common variety of black hole reaches a billion times the mass of the Sun and is contained within an event horizon that is nearly the size of the entire solar system. Black holes such as these are what lurk in the centers of galaxies. While their total gravity is monstrous, the difference in gravity from your head to your toes near their event horizons is relatively small. Indeed, the tidal force can be so weak that you will likely fall through the event horizon in one piece—you just wouldn’t ever be able to come back out and tell anybody about your trip. And when you do finally get ripped apart, deep within the event horizon, nobody outside the hole will be able to watch.

  As far as I know, nobody has ever been eaten by a black hole, but there is compelling evidence to suggest that black holes in the universe routinely dine upon wayward stars and unsuspecting gas clouds. As a cloud approaches a black hole, it hardly ever falls straight in. Unlike your choreographed feet-first fall, a gas cloud is typically drawn into orbit before it spirals to its destruction. The parts of the cloud that are closer to the black hole will orbit faster than the parts that are farther away. Known as differential rotation, this simple shearing can have extraordinary astrophysical consequences. As the cloud layers spiral closer to the event horizon they heat up, from internal friction, to upwards of a million degrees—much hotter than any known star. The gas glows blue-hot as it becomes a copious source of ultraviolet and x-ray energy. What started as an isolated, invisible black hole (minding its own business) has now become an invisible black hole encircled by a gaseous speedway, ablaze with high-energy radiation.

  Since stars are 100 percent certified balls of gas, they are not immune from the fate that greeted our hapless clouds. If one star in a binary system becomes a black hole, then the black hole does not get to eat until late in the companion star’s life, when it swells to become a red giant. If the red giant grows large enough, then it will ultimately get flayed, as the black hole peels and eats the star, layer by layer. But for a star that just happens to wander into the neighborhood, tidal forces will initially stretch it, but eventually, differential rotation will shear the star into a friction-heated disk of highly luminous gas.

  Whenever a theoretical astrophysicist needs an energy source in a tiny space to explain a phenomenon, well-fed black holes become prime ammunition. For example, as we saw earlier, the distant and mysterious quasars wield hundreds or thousands of times the luminosity of the entire Milky Way galaxy. But their energy emanates primarily from a volume that is not much larger than our solar system. Without invoking a supermassive black hole as the quasar’s central engine, we are at a loss to find an alternative explanation.

  We now know that supermassive black holes are common in the cores of galaxies. For some galaxies, a suspiciously high luminosity in a suspiciously small volume provides the needed smoking gun, but the actual luminosity depends heavily on whether stars and gas are available for the black hole to shear them apart. Other galaxies may have one too, in spite of an unremarkable central luminosity. These black holes may have already eaten all the surrounding stars and gas, leaving no evidence behind. But stars near the center, in close orbit to the black hole (not too close to be consumed), will have sharply increased speeds.

  These speeds, when combined with the stars’ distance from the center of the galaxy, are a direct measure of the total mass contained within their orbits. Armed with these data, we can use the back of an envelope to calculate whether the attracting central mass is, indeed, concentrated enough to be a black hole. The largest known black holes are typically a billion solar masses, such as what lurks within the titanic elliptical galaxy M87, the largest in the Virgo Cluster of galaxies. Far down the list, but still large, is the 30-million solar mass black hole in the center of the Andromeda galaxy, our near neighbor in space.

  Beginning to feel “black hole envy”? You are entirely justified: the one in the Milky Way’s center checks in at a mere 4-million solar masses. But no matter the mass, death and destruction are their business.

  SECTION 6

  SCIENCE AND CULTURE

  THE RUFFLED INTERFACE BETWEEN COSMIC DISCOVERY AND THE PUBLIC’S REACTION TO IT

  THIRTY-FOUR

  THINGS PEOPLE SAY

  Aristotle once declared that while the planets moved against the background stars, and while shooting stars, comets, and eclipses represented intermittent variability in the atmosphere and the heavens, the stars themselves were fixed and unchanging on the sky and that Earth was the center of all motion in the universe. From our enlightened perch, 25 centuries later, we chuckle at the folly of these ideas, but the claims were the consequence of legitimate, albeit simple, observations of the natural world.

  Aristotle also made other kinds of claims. He said that heavy things fall faster than light things. Who could argue against that? Rocks obviously fall to the ground faster than tree leaves. But Aristotle went further and declared that heavy things fall faster than light things in direct proportion to their own weight, so that a 10-pound object would fall ten times faster than a 1-pound object.

  Aristotle was badly mistaken.

  To test him, simply release a small rock and a big rock simultaneously from the same height. Unlike fluttering leaves, neither rock will be much influenced by air resistance and both will hit the ground at the same time. This experiment does not require a grant from the National Science Foundation to execute. Aristotle could have performed it but didn’t. Aristotle’s teachings were later adopted into the doctrines of the Catholic Church. And through the Church’s power and influence Aristotelian philosophies became lodged in the common knowledge of the Western world, blindly believed and repeated. Not only did people repeat to others that which was not true, but they also ignored things that clearly happened but were not supposed to be true.

  When scientifically investigating the natural world, the only thing worse than a blind believer is a seeing denier. In A.D. 1054, a star in the constellation Taurus abruptly increased in brightness by a factor of a million. The Chinese astronomers wrote about it. Middle Eastern astronomers wrote about it. Native Americans of what is now the southwestern United States made rock engravings of it. The star became bright enough to be plainly visible in the daytime for weeks, yet we have no record of anybody in all of Europe recording the event. (The bright new star in the sky was actually a supernova explosion that occurred in space some 7,000 years earlier but its light had only just reached Earth.) True, Europe was in the Dark Ages, so we cannot expect that acute data-taking skills were common, but cosmic events that were “allowed” to happen were routinely recorded. For example, 12 years later, in 1066, what ultimately became known as Halley’s comet was seen and duly depicted—complete with agape onlookers—in a section of the famous Bayeux tapestry, circa 1100. An exception indeed. The Bible says the stars don’t change. Aristotle said the stars don’t change. The Church, with its unmatched authority, declares the stars don’t change. The population then falls victim to a collective delusion that was stronger than its members’ own powers of observation.

  We all carry some blindly believed knowledge because we cannot realistically test every statement uttered by others. When I tell you that the proton has an antimatter counterpart (the antiproton), you would need $1 billion worth of laboratory apparatus to verify my statement. So it’s easi
er to just believe me and trust that, at least most of the time, and at least with regard to the astrophysical world, I know what I am talking about. I don’t mind if you remain skeptical. In fact, I encourage it. Feel free to visit your nearest particle accelerator to see antimatter for yourself. But how about all those statements that don’t require fancy apparatus to prove? One would think that in our modern and enlightened culture, popular knowledge would be immune from falsehoods that were easily testable.

  It is not.

  Consider the following declarations. The North Star is the brightest star in the nighttime sky. The Sun is a yellow star. What goes up must come down. On a dark night you can see millions of stars with the unaided eye. In space there is no gravity. A compass points north. Days get shorter in the winter and longer in the summer. Total solar eclipses are rare.

  Every statement in the above paragraph is false.

  Many people (perhaps most people) believe one or more of these statements and spread them to others even when a firsthand demonstration of falsehood is trivial to deduce or obtain. Welcome to my things-people-say rant:

  The North Star is not the brightest star in the nighttime sky. It’s not even bright enough to earn a spot in the celestial top 40. Perhaps people equate popularity with brightness. But when gazing upon the northern sky, three of the seven stars of the Big Dipper, including its “pointer” star, are brighter than the North Star, which is parked just three fist-widths away. There is no excuse.

  And I don’t care what else anyone has ever told you, the Sun is white, not yellow. Human color perception is a complicated business, but if the Sun were yellow, like a yellow lightbulb, then white stuff such as snow would reflect this light and appear yellow—a snow condition confirmed to happen only near fire hydrants. What could lead people to say that the Sun is yellow? In the middle of the day, a glance at the Sun can damage your eyes. Near sunset, however, with the Sun low on the horizon and when the atmospheric scattering of blue light is at its greatest, the Sun’s intensity is significantly diminished. The blue light from the Sun’s spectrum, lost to the twilight sky, leaves behind a yellow-orange-red hue for the Sun’s disk. When people glance at this color-corrupted setting Sun, their misconceptions are fueled.

  What goes up need not come down. All manner of golf balls, flags, automobiles, and crashed space probes litter the lunar surface. Unless somebody goes up there to bring them back, they will never return to Earth. Not ever. If you want to go up and not come down, all you need to do is travel at any speed faster than about seven miles per second. Earth’s gravity will gradually slow you down but it will never succeed in reversing your motion and forcing you back to Earth.

  Unless your eyes have pupils the size of binocular lenses, no matter your seeing conditions and no matter your location on Earth, you will not resolve any more than about five or six thousand stars in the entire sky out of the 100 billion (or so) stars of our Milky Way galaxy. Try it one night. Things get much, much worse when the Moon is out. And if the Moon happens to be full, it will wash out the light of all but the brightest few hundred stars.

  During the Apollo space program, while one of the missions was en route to the Moon, a noted television news anchor announced the exact moment when the “astronauts left the gravitational field of Earth.” Since the astronauts were still on their way to the Moon, and since the Moon orbits Earth, then Earth’s gravity must extend into space at least as far as the Moon. Indeed, Earth’s gravity, and the gravity of every other object in the universe, extends without limit—albeit with ever-diminishing strength. Every spot in space is teeming with countless gravitational tugs in the direction of every other object in the universe. What the announcer meant was that the astronauts crossed the point in space where the force of the Moon’s gravity exceeds the force of Earth’s gravity. The whole job of the mighty three-stage Saturn V rocket was to endow the command module with enough initial speed to just reach this point in space because thereafter you can passively accelerate toward the Moon—and they did. Gravity is everywhere.

  Everybody knows that when it comes to magnets, opposite poles attract while similar poles repel. But a compass needle is designed so that the half that has been magnetized “North” points to Earth’s magnetic north pole. The only way a magnetized object can align its north half to Earth’s magnetic north pole is if Earth’s magnetic north pole is actually in the south and the magnetic south pole is actually in the north. Furthermore, there is no particular law of the universe that requires the precise alignment of an object’s magnetic poles with its geographic poles. On Earth the two are separated by about 800 miles, which makes navigation by compass a futile exercise in northern Canada.

  Since the first day of winter is the shortest “day” of the year, then every succeeding day in the winter season must get longer and longer. Similarly, since the first day of summer is the longest “day” of the year, then every succeeding day in the summer must get shorter and shorter. This is, of course, the opposite of what is told and retold.

  On average, every couple years, somewhere on Earth’s surface, the Moon passes completely in front of the Sun to create a total solar eclipse. This event is more common than the Olympics, yet you don’t read newspaper headlines declaring “a rare Olympics will take place this year.” The perceived rarity of eclipses may derive from a simple fact: for any chosen spot on Earth, you can wait up to a half-millennium before you see a total solar eclipse. True, but lame as an argument because there are spots on Earth (like the middle of the Sahara Desert or any region of Antarctica) that have never, and will not likely ever, host the Olympics.

  Want a few more? At high noon, the Sun is directly overhead. The Sun rises in the east and sets in the west. The Moon comes out at night. On the equinox there are 12 hours of day and 12 hours of night. The Southern Cross is a beautiful constellation. All of these statements are wrong too.

  There is no time of day, nor day of the year, nor place in the continental United States where the Sun ascends to directly overhead. At “high noon,” straight vertical objects cast no shadow. The only people on the planet who see this live between 23.5 degrees south latitude and 23.5 degrees north latitude. And even in that zone, the Sun reaches directly overhead on only two days per year. The concept of high noon, like the brightness of the North Star and the color of the Sun, is a collective delusion.

  For every person on Earth, the Sun rises due east and sets due west on only two days of the year: the first day of spring and the first day of fall. For every other day of the year, and for every person on Earth, the Sun rises and sets someplace else on the horizon. On the equator, sunrise varies by 47 degrees across the eastern horizon. From the latitude of New York City (41 degrees north—the same as that of Madrid and Beijing) the sunrise spans more than 60 degrees. From the latitude of London (51 degrees north) the sunrise spans nearly 80 degrees. And when viewed from either the Arctic or Antarctic circles, the Sun can rise due north and due south, spanning a full 180 degrees.

  The Moon also “comes out” with the Sun in the sky. By invoking a small extra investment in your skyward viewing (like looking up in broad daylight) you will notice that the Moon is visible in the daytime nearly as often as it is visible at night.

  The equinox does not contain exactly 12 hours of day and 12 hours of night. Look at the sunrise and sunset times in the newspaper on the first day of either spring or fall. They do not split the day into two equal 12-hour blocks. In all cases, daytime wins. Depending on your latitude, it can win by as few as seven minutes at the equator up to nearly half an hour at the Arctic and Antarctic circles. Who or what do we blame? Refraction of sunlight as it passes from the vacuum of interplanetary space to Earth’s atmosphere enables an image of the Sun to appear above the horizon several minutes before the actual Sun has actually risen. Equivalently, the actual Sun has set several minutes before the Sun that you see. The convention is to measure sunrise by using the upper edge of the Sun’s disk as it peeks above the horizon; similarly, sunse
t is measured by using the upper edge of the Sun’s disk as it sinks below the horizon. The problem is that these two “upper edges” are on opposite halves of the Sun thereby providing an extra solar width of light in the sunrise/sunset calculation.

  The Southern Cross gets the award for the greatest hype among all eighty-eight constellations. By listening to Southern Hemisphere people talk about this constellation, and by listening to songs written about it, and by noticing it on the national flags of Australia, New Zealand, Western Samoa, and Papua New Guinea, you would think we in the North were somehow deprived. Nope. Firstly, one needn’t travel to the Southern Hemisphere to see the Southern Cross. It’s plainly visible (although low in the sky) from as far north as Miami, Florida. This diminutive constellation is the smallest in the sky—your fist at arm’s length would eclipse it completely. Its shape isn’t very interesting either. If you were to draw a rectangle using a connect-the-dots method you would use four stars. And if you were to draw a cross you would presumably include a fifth star in the middle to indicate the cross-point of the two beams. But the Southern Cross is composed of only four stars, which more accurately resemble a kite or a crooked box. The constellation lore of Western cultures owes its origin and richness to centuries of Babylonian, Chaldean, Greek, and Roman imaginations. Remember, these are the same imaginations that gave rise to the endless dysfunctional social lives of the gods and goddesses. Of course, these were all Northern Hemisphere civilizations, which means the constellations of the southern sky (many of which were named only within the last 250 years) are mythologically impoverished. In the North we have the Northern Cross, which is composed of all five stars that a cross deserves. It forms a subset of the larger constellation Cygnus the swan, which is flying across the sky along the Milky Way. Cygnus is nearly twelve times larger than the Southern Cross.