STARS Where Life Begins

When they had heard the king, they went their way; and lo, the star which they had seen in the East went before them, till it came to rest over the place where the child was. When they saw the star, they rejoiced exceedingly with great joy…

−Matthew 2:9-10

For nearly 2,000 years, astronomers, theologians, skeptics and believers have pondered the story of the star of Bethlehem. Did some celestial display actually mark Jesus birth, or is the biblical reference merely figurative? Did a new star really appear in the heavens? Could it have been what contemporary astronomers call a supernova, or exploding star? A comet? Or might the “star” really have been a juxtaposition of two of the brighter planets?

The true answers to these questions may never be learned, though the guesses are improving (see box). In a sense it scarcely matters, for what is most significant about the star of Bethlehem is not whether it existed or what it was, but what it symbolizes. Spangling the night sky, the unattainable stars have always invoked reverence and wonder. It was natural for those recording the birth of Christ to associate the event with a star. Even today a star, gleaming over a creche or twinkling from the top of a Christmas tree, remains the emblem of hope. “It is not difficult to understand why a star was chosen as a symbol to mark the birth of Christ,” muses Astrophysicist Jesse Greenstein of the California Institute of Technology. “Stars are more mysterious and remote than moon or sun gods. At the time of Christ, people all over the world considered them important.”

In one way, the choice of the star was more appropriate than the ancients knew. Like the infant whose birth they symbolize, stars, by living and dying, enable whole new worlds to be born. Conceived in the frigid darkness of space, stars during their lives produce the elements that make life possible and sustain it. When they die, they sow these substances like seeds across the heavens. The elements eventually become part of new stars and planets. Thus in death there is rebirth.

In fact, the earth and its star−the sun−are built in part from the ashes of dead stars, and human beings are litterally star children. People−and all other forms of life on earth−are collections of atoms forged in stellar furnaces. “All of chemistry and therefore all of life has been formed by stars,” says Astrophysicist Patrick Thaddeus of NASA’S Goddard Institute for Space Studies in New York City. “With the exception of hydrogen, everything in our bodies has been produced in the thermonuclear reactions within stars.”

The Mesopotamians believed the stars were gods who controlled their destiny. The Sumerians apparently perceived a regularity in the grouping of stars, and used their knowledge of stellar movements to help mark the passage of the seasons and fix the times for planting and harvesting. The Assyrians assumed that the stars determined man’s fate, and regarded the movement of planets into various constellations as omens of good or evil.

The Greeks philosophized about the physical nature of stars. Xenophanes, who lived in the 6th century B.C., argued that heavenly bodies were luminous clouds, rather than gods. Anaximander of Miletus described the sky as a sphere surrounded on the outside by wheels of fire; the stars, he thought, were the lights of these fires shining through tubelike breathing holes in the sky. Another citizen of Miletus, Anaximenes, believed the stars were fixed like nails to the vault of the heavens. Aristotle maintained that celestial objects were permanent, immutable and perfect. His notion so influenced Greek thought that when the astronomer Hipparchus spotted what seemed to be a new star in 134 B.C., he attributed his discovery to an omission by his predecessors. He also compiled the first accurate star map so that future sky watchers would be spared his dilemma.

In the Middle Ages, Copernicus displaced earth from its position at the center of the solar system. But Aristotle’s thinking continued to dominate astronomy until 1572, when Tycho Brahe observed a bright new star (which scientists now know was a supernova, or exploding star) near the constellation Cassiopeia. Beyond any doubt, it had not previously been visible. Other blows to Aristotelian cosmology followed swiftly. By early in the 17th century, Galileo had used his telescope to discover spots on the sun−demonstrating that the solar complexion was somewhat less than perfect−and to prove that the sky was filled with stars that could not be seen with the naked eye.

In 1718 Astronomer Edmund Halley of comet fame showed that Sirius, Procyon and Arcturus had changed positions−relative to other stars−since Greek times, establishing for the first time that the stars were not fixed in the heavens. By the early 1900s, astronomers had learned that the sun was merely one of billions of stars in a disc-shaped galaxy, or island of stars, then believed by many to constitute the entire universe. In 1920 Harlow Shapley calculated that the galaxy, called the Milky Way, was some 300,000 light years* in diameter, a distance too stupendous for most people to comprehend, and about three times larger than today’s estimates of its size. But the boundaries of the universe were not yet in sight. Using ever larger telescopes, astronomers discovered that some of the “stars” thought to be part of the Milky Way were actually other galaxies−each containing billions of stars and lying far beyond the Milky Way’s outermost limits.

Now, using such instruments as the huge 200-in. optical telescope on Mount Palomar and newer radio, X-ray and gamma-ray telescopes, modern-day stargazers have pushed the frontiers of understanding even closer to the edges of the universe and into the very cores of the stars. With increasing confidence, astrophysicists are answering some of the questions that man has asked from the time he became a rational being: How far away are the stars? What makes them shine? How long have they been there, and will they exist forever?

In the beginning God created the heavens and the earth. The earth was without form and void, and darkness was upon the face of the deep; and the Spirit of God was moving over the face of the waters. And God said, “Let there be light, ” and there was light.

−Genesis 1

Most cosmologists−scientists who study the structure and evolution of the universe−agree that the biblical account of creation, in imagining an initial void, may be uncannily close to the truth. The universe, they believe, is the expanding remnant of a huge fireball that was created 20 billion years ago by the explosion of a giant primordial atom. The debris of the fireball, like the fragments of a titanic bomb, is still speeding outward from this cataclysmic blast, which started the process that produces not only stars and planets but also the complex structures of life. This startling concept, called the big bang theory, picked up its first substantial scientific support in 1929, when Astronomer Edwin Hubble used shifts in the spectral lines of light emanating from distant galaxies to calculate that the islands of stars are moving at tremendous speeds away from the earth−and from each other−like dots painted on the surface of an expanding balloon. To some scientists, this outward rush of the galaxies suggested an original cosmic explosion.

In 1965, Princeton Physicist Robert Dicke determined that if the universe indeed began as a fireball filled with intense radiation, a trace of that radiation should still exist and be detectable with a sensitive radio antenna. By a serendipitous coincidence, in the same year Arno Penzias and Robert Wilson of Bell Laboratories were using just such an antenna to listen to radio waves from the Milky Way. They had been puzzled by a faint background noise that seemed to be coming evenly from all parts of the sky. When they heard about Dicke’s work, however, and compared the frequency and intensity of their radiation with his predictions, the mystery faded. Like radio listeners pulling out of the night the signal of a faraway station, they had picked up the hissing echoes of creation.

Building on these discoveries, scientists can now envision a still expanding universe that began almost 20 billion years ago, extends for 20 billion light years and contains 10 billion galaxies −each one an island of hundreds of billions of stars. Looking into the star-filled firmament, astronomers actually perceive a four-dimensional universe, one that has the added measure of time. Traveling at 186,000 miles per second, the light that long ago left distant stars and galaxies is only now reaching the earth. Thus man sees the nearby sun as it was little more than eight minutes ago; the nearest star to the sun. Proxima Centauri, as it was about four years ago; and some of the farther galaxies as they looked billions of years ago. Peering into the heavens then is like looking back into time, and some of the stars that astronomers see may no longer exist. Truly, as André Schwarz-Bart wrote in The Last of the Just: “Our eyes register the light of dead stars.”

As stars die, however, others are born. In our galaxy and in galaxies yet to be discovered, stars are going through a continuous cycle of birth, life and death. Indeed, there are places where the observer who knows what to look for can practically see stars forming before his eyes. These star wombs are great clouds of gas and dust floating in interstellar space. Like the clouds that formed in the expanding primordial fireball shortly after the big bang, they consist mostly of nature’s simplest molecule, hydrogen. A star is born when some force, perhaps a shock wave, drives enough of the hydrogen molecules in a cloud sufficiently close to one another that they are held together by their mutual gravity. As a result, a huge pocket of condensed gas, trillions of miles across, is formed at the edge of the larger cloud. In a model proposed by Astronomers Bruce Elmegreen and Charles Lada of the Harvard-Smithsonian Center for Astrophysics, shock waves from the ignition of earlier massive stars help create the conditions for the birth of other stars from the same cloud.

Under the force of their own gravity, the great clouds of gas slowly begin to contract, raising the pressures and temperatures at their centers. They have become embryonic stars.

The process continues for some 10 million years, during which the clouds shrink to globes more than a million miles in diameter. At this point, temperatures

near the centers of the great

gas balls have reached the critical level of 20 million degrees F., hot enough to cause fusion−the awesome process that occurs in a detonating hydrogen bomb.

Long since stripped of their electrons by the high temperatures, the nuclei of the hydrogen atoms slam together at tremendous speeds, fusing to form helium and releasing huge amounts of energy. Though the nuclear fires have been lit, the actual ignition is hidden deep within the interstellar clouds. “Nature very discreetly pulls the curtain over the act of birth,” says Thaddeus. But the infant star soon makes its presence known, shining through and illuminating the obscuring cloud. This process is occurring in the Orion Nebula (see color page), the illuminated portion of a gigantic cloud of gas and dust that is giving birth to new stars. Some of the stars spawned by the nebula have been formed as recently as the time when the human species first stood upright; the newest offspring are only about 100,000 years old−mere infants by stellar standards.

The fusion of hydrogen to form helium marks the beginning of a long and stable period in the evolution of the star−a combination of adolescence and middle age that constitutes 99% of the lifespan of a sun-size star. During this period, the tremendous energy radiating from the star’s center neutralizes its gravitational force, and the great glowing orb shrinks no further. But as it must to all stars, death eventually comes. How long a star lives depends on its mass. Generally, the more massive a star is, the shorter its life is. Stars with a mass significantly greater than that of the sun burn their fuel in a profligate manner and die young; a star ten times as massive as the sun, for example, burns 1,000 times faster and survives only 100 million years. The sun, which is some 5 billion years old, is only at the mid-point in life. Smaller stars, on the other hand, are the Methuselahs of the celestial community. A star with one-tenth the mass of the sun can burn for a trillion years.

In Clarke’s haunting story, the stars switch off−like lights in an office building at closing time−when mankind has fulfilled its purpose and determined all the names of God. In fact, stars do go out, but for reasons that are much more complex, and in a variety of ways: some end with a whimper, others with a bang.

The beginning of the end comes when the star has exhausted much of the hydrogen near its core and starts to burn the hydrogen in its outer layers. This process causes the star gradually to turn red and swell to 100 times its previous size, pouring out prodigious amounts of energy. Betelgeuse, in the constellation Orion, is such a “red giant,” visible to the naked eye. When the sun undergoes a similar metamorphosis, it will envelop Mercury and Venus and vaporize the earth. By that time, 5 billion years from now, man’s descendants may have found a new home in an outer planet or beyond.

A star’s red-giant phase lasts until the hydrogen in the layer around the core is exhausted, perhaps as long as a billion years. The stage that follows is short-lived. Its fires banked, the star is deprived of the outward radiation pressure. It contracts violently, driving the core temperature up again, until it reaches 200 million degrees. That is hot enough to ignite the helium, which fuses into a still heavier element: carbon. Its radiation energy restored, the star zooms back toward red-giant status 100 times faster than it took to get there the first time.

What happens after the helium is consumed depends on the size of the star. If a star’s mass is no more than about four times that of the sun, its second red-giant stage may be its death rattle. As the star contracts again, its gravitational energy cannot produce enough heat to fuse carbon into heavier elements. But as its internal temperature rises, the outer envelope expands and cools. Held loosely by gravity, the outer layers then slough off into space in a billow of gas. All that is left behind is the core, which continues contracting into a ball a few thousand miles in diameter with a density of tons per cubic inch. The result is a “white dwarf,” hotter than the surface of the sun but only about the size of the earth and ready to enter a long period of stellar senility. As the millenniums pass, the white dwarf gradually loses its heat, turning first yellow, then red; eventually, its fires burn out entirely, leaving behind a “black dwarf,” a cold cinder in the graveyard of space.

Many large stars manage to lose much of their mass as they evolve, shedding their matter as gas and dust. If they manage to shed sufficient mass, in fact, they can die quietly as white dwarfs. But for stars with a mass greater than four times that of the sun, the end may be far more dramatic. In these giant stars, fusion does not end when all the helium has been converted into carbon. In some of the massive stars, in a catastrophic event known as a supernova, the carbon core explodes, dispersing most of the elements it has produced into space. Stars of more than eight solar masses may go through several more contracting and expanding cycles, forming elements such as magnesium, silicon, sulfur, cobalt, nickel and ultimately iron. When the star has formed an iron core, its fate is sealed. It begins to contract again, but does not have enough gravity to cause fusion of the densely packed nuclei of iron. Instead of being suspended again by the energy of a rekindled nuclear fire, the great mass of the star continues to fall toward the core, unable to resist the pull of its own gravity.

This event is also catastrophic. In a matter of seconds, a star that has lived several million years caves in with a devastating crash, most of its material crushing into an incredibly dense and small sphere at the center. Then, like a giant spring, the star rebounds from this collapse in a massive explosion. The result is another kind of supernova, a fantastic explosion that blows the star to smithereens, dispersing into space most of the remaining elements that it has manufactured during its lifetime. So brilliant is the light from the exploding star that it briefly outshines all of the galaxy’s other billions of stars combined. The last supernova observed in the Milky Way Galaxy was seen by Johannes Kepler in 1604.

What remains after this explosion again depends on the size of the star. Its death throes may leave behind a rapidly spinning, incredibly dense sphere (about ten miles in diameter), consisting only of tightly packed neutrons. Such an object, called a neutron star, or pulsar, has been located in the center of the Crab Nebula, a glowing cloud that is still expanding from a supernova reported by the Chinese in A.D. 1054.

A very massive star may have an even stranger fate. Driven by its own immense gravitation, it collapses through its neutron star stage, crushing its matter into a volume so small that it virtually ceases to exist. The gravity of its tiny remnant is so great that nothing, not even light, can escape from it. All external evidence of its presence disappears, and the star, like the Cheshire cat, vanishes, leaving behind only the grin of its disembodied gravity. Anything that fell into such a “black hole” would quite literally be crushed out of existence.

Because black holes emit no light or other radiation, their existence, predicted by the laws of relativity, cannot be confirmed by direct observation, but it can be inferred. Astronomers have identified a powerful X-ray source in the constellation Cygnus. Some suspect the source, which has been labeled Cygnus Xl, may be just such a black hole. It appears to be rotating with a visible star around a common center of gravity−a dead partner of a dualstar system. Scientists believe material from the glowing star is being drawn into the black hole with such force that the material becomes hot enough to emit X rays.

While neutron stars and black holes can result from the death of massive stars, the explosions that precede them create elements essential to the birth of new stars and spread through the universe the materials essential to life. “Stars have two purposes,” says Stanford University Astrophysicist Robert Wagoner. “They give energy in the form of light, and they produce the heavy elements that we are made of.”

Indeed, scientists believe hydrogen and helium were the only two elements in the primordial universe. But when stars formed in the clouds of these two gases, they began the manufacture of the other elements now found in nature. That this sequence occurred seems to be supported by spectral-line evidence in starlight. Older stars, formed when the universe was young, have only traces of the heavier elements. Stars born more recently have more of the heavy elements produced by their predecessors. Those currently forming in interstellar dust clouds can be expected to have significant proportions of the atoms produced in celestial forges. Says Thaddeus of the clouds: “We can see the lovely fertilizer, this compost heap just sitting there waiting to be consumed in star formation.”

The great interstellar clouds also contain another kind of fertilizer. In 1963 a team of researchers from the Massachusetts Institute of Technology and the Lincoln Laboratory used a radio telescope to discover the hydroxyl radical (two-thirds of the water molecule) in space. Since then, more than three dozen molecules have been found floating in the galactic clouds, including those of methane, formaldehyde, ammonia, hydrogen cyanide, ethyl alcohol and carbon monoxide.

These findings were particularly exciting in light of a classic experiment carried out in 1953 by Stanley Miller and Harold Urey at the University of Chicago. They discovered that when electric sparks were sent through water vapor, ammonia and methane in a sealed container, they combined to form amino acids, the building blocks of protein found in living organisms. Says Astrophysicist Herbert Friedman of the Naval Research Laboratory in Washington: “We believe the gas in space can form complex molecules that can eventually lead to life.”

The implications are staggering.

Though the space between the stars seems hostile to the formation of life, the evidence that organic chemistry is not unique to earth makes it probable that life exists on planets elsewhere. The universe contains billions of sunlike stars built from the remains of earlier stellar explosions. Many of them may well have planets, which some scientists believe condense from a disc of gas and dust that forms around a developing star.

The search for life elsewhere in the universe is already under way. NASA scientists are still analyzing the data gathered by the Viking landers that touched down on Mars last summer, and will conduct more experiments to discover if life exists on the solar-system planet that most closely resembles our own. Radioastronomers, meanwhile, have beamed coded signals toward the stars to let any other civilization know that intelligent life exists on earth; they have also been listening with their huge antennas, hoping to pick up the message of an extraterrestrial society eager to communicate with other beings.

How long will the stars keep burning? Most astrophysicists believe the universe lacks sufficient matter to stop its expansion. Thus, they say, the universe will continue to expand indefinitely, and the stars in time will consume the vast supply of hydrogen. Star formation will slow and then stop, and the last stars will blink out, bringing an end to all activity in the universe.

Still, says Astronomer Geoffrey Burbidge of the University of California at San Diego, “cosmology has much in common with religion; both rely on a very small measure of information and a very large measure of belief.” However, for all scientists have learned in recent years about the universe, the abyss of the incomprehensible, the limits of the unknowable, remain to challenge human logic and intelligence. To the ultimate question−what existed before the big bang most of modern science is mute. Says Northwestern University Astronomer J. Allen Hynek: “In science, it’s against the rules to ask questions when we have no way of approaching the answers.”

But within the gigantic framework of the universe that can be perceived and studied, some astronomers cling to a more optimistic belief: there is enough matter in the universe to halt the expansion, and the onrushing star-filled galaxies will eventually slow to a stop, then begin rushing back through space until they crash together to reform the primordial atom. Then, say the optimists, the giant atom will explode again, sending its fragments flying outward to recreate the cosmos and life itself in an oscillating, never-ending cycle.

Whichever scenario is correct, says Astrophysicist Greenstein, “I find a certain pleasure and honor in belonging to the universe of stars, of these events that have created the materials of which the earth and I are made.” It is a sentiment many can echo. The final consolation has always been, as humanity looking upward measured its own finiteness against the infinity of the stars, that it is better to have been for a season, even a moment, than not to have been at all. The stars thus are no less symbols in their newly understood mortality than they were, seemingly eternal in their courses, in remote times.

* A light year, the distance traveled by light in one year, is some 6 trillion miles.