by Carl Sagan
These are all familiar chemical elements. We recognize their names. Such stellar nuclear reactions do not readily generate erbium, hafnium, dyprosium, praseodymium or yttrium, but rather the elements we know in everyday life, elements returned to the interstellar gas, where they are swept up in a subsequent generation of cloud collapse and star and planet formation. All the elements of the Earth except hydrogen and some helium have been cooked by a kind of stellar alchemy billions of years ago in stars, some of which are today inconspicuous white dwarfs on the other side of the Milky Way Galaxy. The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.
Some of the rarer elements are generated in the supernova explosion itself. We have relatively abundant gold and uranium on Earth only because many supernova explosions had occurred just before the solar system formed. Other planetary systems may have somewhat different amounts of our rare elements. Are there planets where the inhabitants proudly display pendants of niobium and bracelets of protactinium, while gold is a laboratory curiosity? Would our lives be improved if gold and uranium were as obscure and unimportant on Earth as praseodymium?
The origin and evolution of life are connected in the most intimate way with the origin and evolution of the stars. First: The very matter of which we are composed, the atoms that make life possible, were generated long ago and far away in giant red stars. The relative abundance of the chemical elements found in the Cosmos matches the relative abundance of atoms generated in stars so well as to leave little doubt that red giants and supernovae are the ovens and crucibles in which matter has been forged. The Sun is a second- or third-generation star. All the matter in it, all the matter you see around you, has been through one or two previous cycles of stellar alchemy. Second: The existence of certain varieties of heavy atoms on the Earth suggests that there was a nearby supernova explosion shortly before the solar system was formed. But this is unlikely to be a mere coincidence; more likely, the shock wave produced by the supernova compressed interstellar gas and dust and triggered the condensation of the solar system. Third: When the Sun turned on, its ultraviolet radiation poured into the atmosphere of the Earth; its warmth generated lightning; and these energy sources sparked the complex organic molecules that led to the origin of life. Fourth: Life on Earth runs almost exclusively on sunlight. Plants gather the photons and convert solar to chemical energy. Animals parasitize the plants. Farming is simply the methodical harvesting of sunlight, using plants as grudging intermediaries. We are, almost all of us, solar-powered. Finally, the hereditary changes called mutations provide the raw material for evolution. Mutations, from which nature selects its new inventory of life forms, are produced in part by cosmic rays—high-energy particles ejected almost at the speed of light in supernova explosions. The evolution of life on Earth is driven in part by the spectacular deaths of distant, massive suns.
Imagine carrying a Geiger counter and a piece of uranium ore to some place deep beneath the Earth—a gold mine, say, or a lava tube, a cave carved through the Earth by a river of molten rock. The sensitive counter clicks when exposed to gamma rays or to such high-energy charged particles as protons and helium nuclei. If we bring it close to the uranium ore, which is emitting helium nuclei in a spontaneous nuclear decay, the count rate, the number of clicks per minute, increases dramatically. If we drop the uranium ore into a heavy lead canister, the count rate declines substantially; the lead has absorbed the uranium radiation. But some clicks can still be heard. Of the remaining counts, a fraction come from natural radioactivity in the walls of the cave. But there are more clicks than can be accounted for by radioactivity. Some of them are caused by high-energy charged particles penetrating the roof. We are listening to cosmic rays, produced in another age in the depths of space. Cosmic rays, mainly electrons and protons, have bombarded the Earth for the entire history of life on our planet. A star destroys itself thousands of light-years away and produces cosmic rays that spiral through the Milky Way Galaxy for millions of years until, quite by accident, some of them strike the Earth, and our hereditary material. Perhaps some key steps in the development of the genetic code, or the Cambrian explosion, or bipedal stature among our ancestors were initiated by cosmic rays.
On July 4, in the year 1054, Chinese astonomers recorded what they called a “guest star” in the constellation of Taurus, the Bull. A star never before seen became brighter than any star in the sky. Halfway around the world, in the American Southwest, there was then a high culture, rich in astronomical tradition, that also witnessed this brilliant new star.* From carbon 14 dating of the remains of a charcoal fire, we know that in the middle eleventh century some Anasazi, the antecedents of the Hopi of today, were living under an overhanging ledge in what is today New Mexico. One of them seems to have drawn on the cliff overhang, protected from the weather, a picture of the new star. Its position relative to the crescent moon would have been just as was depicted. There is also a handprint, perhaps the artist’s signature.
This remarkable star, 5,000 light-years distant, is now called the Crab Supernova, because an astronomer centuries later was unaccountably reminded of a crab when looking at the explosion remnant through his telescope. The Crab Nebula is the remains of a massive star that blew itself up. The explosion was seen on Earth with the naked eye for three months. Easily visible in broad daylight, you could read by it at night. On the average, a supernova occurs in a given galaxy about once every century. During the lifetime of a typical galaxy, about ten billion years, a hundred million stars will have exploded—a great many, but still only about one star in a thousand. In the Milky Way, after the event of 1054, there was a supernova observed in 1572, and described by Tycho Brahe, and another, just after, in 1604, described by Johannes Kepler,† Unhappily, no supernova explosions have been observed in our Galaxy since the invention of the telescope, and astronomers have been chafing at the bit for some centuries.
Supernovae are now routinely observed in other galaxies. Among my candidates for the sentence that would most thoroughly astonish an astronomer of the early 1900’s is the following, from a paper by David Helfand and Knox Long in the December 6, 1979, issue of the British journal Nature: “On 5 March, 1979, an extremely intense burst of hard x-rays and gamma rays was recorded by the nine interplanetary spacecraft of the burst sensor network, and localized by time-of-flight determinations to a position coincident with the supernova remnant N49 in the Large Magellanic Cloud.” (The Large Magellanic Cloud, so-called because the first inhabitant of the Northern Hemisphere to notice it was Magellan, is a small satellite galaxy of the Milky Way, 180,000 light-years distant. There is also, as you might expect, a Small Magellanic Cloud.) However, in the same issue of Nature, E. P. Mazets and colleagues of the Ioffe Institute, Leningrad—who observed this source with the gammaray burst detector aboard the Venera 11 and 12 spacecraft on their way to land on Venus—argue that what is being seen is a flaring pulsar only a few hundred light-years away. But despite the close agreement in position Helfand and Long do not insist that the gamma-ray outburst is associated with the supernova remnant. They charitably consider many alternatives, including the surprising possibility that the source lies within the solar system. Perhaps it is the exhaust of an alien starship on its long voyage home. But a rousing of the stellar fires in N49 is a simpler hypothesis: we are sure there are such things as supernovae.
The fate of the inner solar system as the Sun becomes a red giant is grim enough. But at least the planets will never be melted and frizzled by an erupting supernova. That is a fate reserved for planets near stars more massive than the Sun. Since such stars with higher temperatures and pressures run rapidly through their store of nuclear fuel, their lifetimes are much shorter than the Sun’s. A star tens of times more massive than the Sun can stably convert hydrogen to helium for only a few million years before moving briefly on to more exotic nuclear reactions. Thus there is almost certainl
y not enough time for the evolution of advanced forms of life on any accompanying planets; and it will be rare that beings elsewhere can ever know that their star will become a supernova: if they live long enough to understand supernovae, their star is unlikely to become one.
The essential preliminary to a supernova explosion is the generation by silicon fusion of a massive iron core. Under enormous pressure, the free electrons in the stellar interior are forceably melded with the protons of the iron nuclei, the equal and opposite electrical charges canceling each other out; the inside of the star is turned into a single giant atomic nucleus, occupying a much smaller volume than the precursor electrons and iron nuclei. The core implodes violently, the exterior rebounds and a supernova explosion results. A supernova can be brighter than the combined radiance of all the other stars in the galaxy within which it is embedded. All those recently hatched massive blue-white supergiant stars in Orion are destined in the next few million years to become supernovae, a continuing cosmic fireworks in the constellation of the hunter.
The awesome supernova explosion ejects into space most of the matter of the precursor star—a little residual hydrogen and helium and significant amounts of other atoms, carbon and silicon, iron and uranium. Remaining is a core of hot neutrons, bound together by nuclear forces, a single, massive atomic nucleus with an atomic weight about 1056, a sun thirty kilometers across; a tiny, shrunken, dense, withered stellar fragment, a rapidly rotating neutron star. As the core of a massive red giant collapses to form such a neutron star, it spins faster. The neutron star at the center of the Crab Nebula is an immense atomic nucleus, about the size of Manhattan, spinning thirty times a second. Its powerful magnetic field, amplified during the collapse, traps charged particles rather as the much tinier magnetic field of Jupiter does. Electrons in the rotating magnetic field emit beamed radiation not only at radio frequencies but in visible light as well. If the Earth happens to lie in the beam of this cosmic lighthouse, we see it flash once each rotation. This is the reason it is called a pulsar. Blinking and ticking like a cosmic metronome, pulsars keep far better time than the most accurate ordinary clock. Long-term timing of the radio pulse rate of some pulsars, for instance, one called PSR 0329 + 54, suggests that these objects may have one or more small planetary companions. It is perhaps conceivable that a planet could survive the evolution of a star into a pulsar; or a planet could be captured at a later time. I wonder how the sky would look from the surface of such a planet.
Neutron star matter weighs about the same as an ordinary mountain per teaspoonful—so much that if you had a piece of it and let it go (you could hardly do otherwise), it might pass effortlessly through the Earth like a falling stone through air, carving a hole for itself completely through our planet and emerging out the other side—perhaps in China. People there might be out for a stroll, minding their own business, when a tiny lump of neutron star plummets out of the ground, hovers for a moment, and then returns beneath the Earth, providing at least a diversion from the routine of the day. If a piece of neutron star matter were dropped from nearby space, with the Earth rotating beneath it as it fell, it would plunge repeatedly through the rotating Earth, punching hundreds of thousands of holes before friction with the interior of our planet stopped the motion. Before it comes to rest at the center of the Earth, the inside of our planet might look briefly like a Swiss cheese until the subterranean flow of rock and metal healed the wounds. It is just as well that large lumps of neutron star matter are unknown on Earth. But small lumps are everywhere. The awesome power of the neutron star is lurking in the nucleus of every atom, hidden in every teacup and dormouse, every breath of air, every apple pie. The neutron star teaches us respect for the commonplace.
A star like the Sun will end its days, as we have seen, as a red giant and then a white dwarf. A collapsing star twice as massive as the Sun will become a supernova and then a neutron star. But a more massive star, left, after its supernova phase, with, say, five times the Sun’s mass, has an even more remarkable fate reserved for it—its gravity will turn it into a black hole. Suppose we had a magic gravity machine—a device with which we could control the Earth’s gravity, perhaps by turning a dial. Initially the dial is set at 1 g* and everything behaves as we have grown up to expect. The animals and plants on Earth and the structures of our buildings are all evolved or designed for 1 g. If the gravity were much less, there might be tall, spindly shapes that would not be tumbled or crushed by their own weight. If the gravity were much more, plants and animals and architecture would have to be short and squat and sturdy in order not to collapse. But even in a fairly strong gravity field, light would travel in a straight line, as it does, of course, in everyday life.
Consider a possibly typical group of Earth beings at the tea party from Alice in Wonderland. As we lower the gravity, things weigh less. Near 0 g the slightest motion sends our friends floating and tumbling up in the air. Spilled tea—or any other liquid—forms throbbing spherical globs in the air: the surface tension of the liquid overwhelms gravity. Balls of tea are everywhere. If now we dial 1 g again, we make a rain of tea. When we increase the gravity a little—from 1 g to, say, 3 or 4 g’s—everyone becomes immobilized: even moving a paw requires enormous effort. As a kindness we remove our friends from the domain of the gravity machine before we dial higher gravities still. The beam from a lantern travels in a perfectly straight line (as nearly as we can see) at a few g’s, as it does at 0 g. At 1000 g’s, the beam is still straight, but trees have become squashed and flattened; at 100,000 g’s, rocks are crushed by their own weight. Eventually, nothing at all survives except, through a special dispensation, the Cheshire cat. When the gravity approaches a billion g’s, something still more strange happens. The beam of light, which has until now been heading straight up into the sky, is beginning to bend. Under extremely strong gravitational accelerations, even light is affected. If we increase the gravity still more, the light is pulled back to the ground near us. Now the cosmic Cheshire cat has vanished; only its gravitational grin remains.
When the gravity is sufficiently high, nothing, not even light, can get out. Such a place is called a black hole. Enigmatically indifferent to its surroundings, it is a kind of cosmic Cheshire cat. When the density and gravity become sufficiently high, the black hole winks out and disappears from our universe. That is why it is called black: no light can escape from it. On the inside, because the light is trapped down there, things may be attractively well-lit. Even if a black hole is invisible from the outside, its gravitational presence can be palpable. If, on an interstellar voyage, you are not paying attention, you can find yourself drawn into it irrevocably, your body stretched unpleasantly into a long, thin thread. But the matter accreting into a disk surrounding the black hole would be a sight worth remembering, in the unlikely case that you survived the trip.
Thermonuclear reactions in the solar interior support the outer layers of the Sun and postpone for billions of years a catastrophic gravitational collapse. For white dwarfs, the pressure of the electrons, stripped from their nuclei, holds the star up. For neutron stars, the pressure of the neutrons staves off gravity. But for an elderly star left after supernova explosions and other impetuosities with more than several times the Sun’s mass, there are no forces known that can prevent collapse. The star shrinks incredibly, spins, reddens and disappears. A star twenty times the mass of the Sun will shrink until it is the size of greater Los Angeles; the crushing gravity becomes 1010 g’s, and the star slips through a self-generated crack in the space-time continuum and vanishes from our universe.
Black holes were first thought of by the English astonomer John Mitchell in 1783. But the idea seemed so bizarre that it was generally ignored until quite recently. Then, to the astonishment of many, including many astronomers, evidence was actually found for the existence of black holes in space. The Earth’s atmosphere is opaque to X-rays. To determine whether astronomical objects emit such short wavelengths of light, an X-ray telescope must be carried aloft
. The first X-ray observatory was an admirably international effort, orbited by the United States from an Italian launch platform in the Indian Ocean off the coast of Kenya and named Uhuru, the Swahili word for “freedom.” In 1971, Uhuru discovered a remarkably bright X-ray source in the constellation of Cygnus, the Swan, flickering on and off a thousand times a second. The source, called Cygnus X-1, must therefore be very small. Whatever the reason for the flicker, information on when to turn on and off can cross Cyg X-1 no faster than the speed of light, 300,000 km/sec. Thus Cyg X-1 can be no larger than [300,000 km/sec] × [(1/1000) sec] = 300 kilometers across. Something the size of an asteroid is a brilliant, blinking source of X-rays, visible over interstellar distances. What could it possibly be? Cyg X-1 is in precisely the same place in the sky as a hot blue supergiant star, which reveals itself in visible light to have a massive close but unseen companion that gravitationally tugs it first in one direction and then in another. The companion’s mass is about ten times that of the Sun. The supergiant is an unlikely source of X-rays, and it is tempting to identify the companion inferred in visible light with the source detected in X-ray light. But an invisible object weighing ten times more than the Sun and collapsed into a volume the size of an asteroid can only be a black hole. The X-rays are plausibly generated by friction in the disk of gas and dust accreted around Cyg X-1 from its supergiant companion. Other stars called V861 Scorpii, GX339-4, SS433, and Circinus X-2 are also candidate black holes. Cassiopeia A is the remnant of a supernova whose light should have reached the Earth in the seventeenth century, when there were a fair number of astronomers. Yet no one reported the explosion. Perhaps, as I. S. Shklovskii has suggested, there is a black hole hiding there, which ate the exploding stellar core and damped the fires of the supernova. Telescopes in space are the means for checking these shards and fragments of data that may be the spoor, the trail, of the legendary black hole.