Stars experience a cycle of birth and death.
The life of any star is a constant battle against the force of gravity, which tries to pull the star in on itself. Against this unremitting force, stars deploy a variety of countervailing strategies. Some of these strategies allow stars to stave off collapse temporarily and some strategies allow them to stave it off forever. But nothing can protect the largest stars from eventual collapse into a black hole, the ultimate victory of gravity over matter.
The Birth of Stars
All stars begin as diffuse clouds of dust in deep space. Somewhere in the cloud is a place where matter has gathered by chance more thickly than elsewhere, and the force of gravity exerted by the clump pulls in neighboring materials. This makes the clump more massive and increases its gravitational attraction, so even more material is pulled in. It’s not hard to guess the outcome of this process—the cloud starts to collapse around the original concentration of matter.
As the contraction progresses, the pressure and temperature at the center increases. First, electrons are torn off their parent atoms, creating a plasma. Then as the contraction continues, the nuclei in the plasma start moving faster and faster until, at last, nuclei approaching each other are moving so fast that they can overcome the electrical repulsion that exists between their protons. The nuclei come together and nuclear fusion begins—the nuclear fires ignite. Energy from fusion pours out from the core, setting up a pressure in the surrounding gas that balances the inward pull of gravity. When the energy reaches the outer layers, it moves off into space in the form of electromagnetic radiation and the stabilized cloud begins to shine. A star has been born.
The primary fuel for the fusion reaction is hydrogen. Two protons (the nuclei of hydrogen atoms) come together to form deuterium (an isotope of hydrogen consisting of one proton and one neutron) and some other particles. Subsequent collisions of the deuterium with other protons eventually produce helium-4, a nucleus consisting of two protons and two neutrons. In symbolic form, the nuclear reaction can be written as:
4 protons → helium + energy + leftover particles
As in nuclear reactions, the conversion of mass (in this case some of the mass of the four initial protons) supplies the energy.
While the star is contracting and stabilizing itself, some interesting events are taking place out in its periphery. The original cloud will, in general, have some small rotation. As the contraction starts, the rotation speed increases. The cloud is like an ice skater who, when she pulls her arms in while spinning, spins faster. If nothing counters it, contraction will increase the spin until the star is torn apart. There are two ways for the nascent star to avoid this fate: it can split into two, forming a double star system, or it can form planets. In both cases, the spin is transferred from the body of the star to the revolution of the stars or planets around each other. Most stars seem to take the double star route—at least two thirds of those you see in the sky are multiple star systems. The search for planetary systems around other stars has revealed several stars with multiple planets, and astronomers are searching for a system like our own.
Stellar Lifetimes
The appetite of stars for hydrogen is truly prodigious. The sun, for example, consumes some 700 million tons each second, with about 5 million tons being converted into energy (primarily in the form of gamma rays). Yet so large is our star that it has burned its hydrogen at this rate for 4.6 billion years and will continue to do so for more than five billion years before running out of fuel.
How long will a star live? That of course depends on how much hydrogen it has and how fast it is consumed. Oddly enough, the larger a star is, the shorter its lifetime. The reason for this seeming paradox is simple: the bigger a star is, the greater is the gravitational force trying to make it collapse and the more hydrogen has to be burned to keep the star stable. The sun, a quite ordinary star, has enough fuel to keep gravity at bay for ten billion years, but a star thirty times as massive as the sun must burn its fuel in such a profligate way that it will shine for only a few million years. A star much smaller than the sun, on the other hand, will live for tens of billions of years—longer than the age of the universe. The star just pokes along, doling out miserly bits of energy into space as it husbands its hydrogen throughout a long and frugal life.
The Death of Stars
Profligate or miserly, every star must eventually burn up all of its hydrogen, filling the core with helium ash. When the hydrogen is gone, the outward force generated by the nuclear reactions disappears and gravity resumes its inevitable inward march. The inner parts of the star start to contract and warm up. For a star like the sun, the interior heating temporarily produces more energy as hydrogen burns just outside the core and the outer regions of the star are pushed farther outward, creating what astronomers call a red giant. Five billion years from now the body of the sun will extend out past the present orbits of Mercury and Venus and will scorch Earth’s lifeless surface.
The core continues to contract even as the outer layers puff up, and soon the core becomes so hot that helium, the ash of the hydrogen fire, itself starts to fuse. In a series of reactions, three helium nuclei come together to form nuclei of carbon. Once the helium is consumed (a process that may take only a few minutes in a star like the sun), the collapse starts again in earnest. The bloated outer layers are blown off, while the inner region continues to contract. There is no more fuel to burn, so something else has to stop the collapse. That “something else,” for the sun and stars like it, is related to the behavior of electrons. Electrons in the star cannot overlap—they need elbow room. Only so many can be crammed together in a given volume. When the core has collapsed down to the size of the Earth, its electrons will have reached the point where they cannot be further compressed and the star will be stabilized forever, with gravity pushing in and the electrons pushing out. A star held up by pressure from its electrons is called a white dwarf. It generates no internal energy, having used up all its fuel, but continues to glow for a long time as it cools off.
Currently, theorists believe that stars with masses up to eight times that of the sun will end up as white dwarves. Made almost entirely of carbon nuclei, such a star is truly, as our childhood rhyme told us, “like a diamond in the sky.”
If the star is very massive, however, its death is much more spectacular. It burns through its hydrogen quickly. Then, after a short collapse, it starts burning helium to make carbon. When the helium is exhausted and the inevitable collapse starts again, the temperatures at the center of the star get so high that even the carbon starts to fuse. This pattern continues, the ashes of each fire serving as fuel for the next as the star desperately tries to stave off the inevitable. In the final stages of nuclear burning, iron starts to be produced. Iron is the ultimate nuclear ash. It is impossible to get energy from iron by allowing it to fuse with another nucleus, and it is impossible to get energy from it by fission. As the star’s core clogs up with iron, there is no way for the star to generate more energy. Again the collapse starts, but this time the force exerted by the electrons is not enough to overcome gravity Electrons are forced inside the protons in the core, neutrons are produced, and the core shrinks quickly to a sphere of neutrons—a neutron star—about ten miles across. The force of gravity and the pressure of neutrons against each other balance and, providing the force of gravity is not too strong, the core stabilizes.
Supernovae and Their Consequences
When the core collapse occurs, the outer parts of the star find that the rug has been pulled out from under them. They start to fall inward, meet the rebounding neutron core and a flood of neutrinos created in nuclear reactions, and the star literally tears itself apart. For the space of about half an hour, shock waves crisscross the stellar carcass, creating temperatures in which all chemical elements up to uranium and plutonium are synthesized in a wild free-for-all, then blown into space. For a brief few days, the star can emit more energy than an entire galaxy. This event is a supern
ova—the most spectacular stellar cataclysm known. When the dust has cleared in a supernova, the end product may be a neutron star or a black hole—we don’t know enough to predict which with certainty.
On February 23, 1987, a star exploded into a visible supernova in a neighboring region of our galaxy, giving astronomers a ringside seat during the spectacle and, not incidentally, giving them a chance to verify their theories of how stars live and die. The theories came through with flying colors.
When the fireworks of a supernova are over, all that’s left of the original large star is a neutron core—a sphere of solid neutrons about ten miles across. The neutron star is usually rotating very fast, typically turning on its axis thirty to fifty times a second, because the collapse speeds up the original slow rotation of the star (remember the ice skater). The star’s original magnetic field has also been concentrated by the collapse, and a field many trillion times that at the Earth’s surface exists on the neutron star.
Electrons spiraling in toward the north and south magnetic poles of the rotating star give off energy, mostly in the form of radio waves. This radiation moves out into space in a narrow beam focused at the pole of the star. You can think of it as being something like the rotating beam of a lighthouse. As the beam sweeps by us, we see a pulse of radio waves, then darkness, then another pulse. When these signals were first seen in the radio sky, they were called LGMs (for “little green men”) because they looked like coded signals. Now we realize that they result from rotating neutron stars, which astronomers call pulsars. More than a thousand pulsars have been discovered and thousands more undoubtedly wait to be found.
If the star is very massive, then even the force exerted by the neutrons will not be enough to overcome gravity, and the collapse will continue down to a black hole. Black holes represent the ultimate triumph of gravity, the ultimate defeat of the star.
Our picture of the life of stars, then, is that early in the history of the universe large stars formed, lived their brief life, and became supernovae. In the last moments of their existence, these stars synthesized all the known chemical elements, then returned them to space. There, these elements in turn are incorporated into new generations of stars as the concentration of heavy elements increases throughout the universe.
All elements heavier than helium, including the iron in your blood and the calcium in your bones, are made in stars. We are, all of us, made of star stuff.
THE SOLAR SYSTEM
Our sun formed from a slowly rotating cloud of interstellar dust. Almost all of that cloud’s debris was pulled into the proto-sun, but a tiny fraction of the mass concentrated instead into the planets, plus a diverse collection of asteroids and moons, which adopted stable orbits around the sun. This collection of small, relatively cool objects is called the solar system.
The fact that the planets formed out of a contracting, heating, spinning ball of gas explains a number of the regularities we see when we look at them. For one thing, all of the planetary orbits lie in the plane of the sun’s equator, and all of the planets move in the same direction around the sun. This systematic behavior occurred because the rotation of the collapsing cloud tended to throw material outward in the plane of the rotation, and it was in that plane that the planets eventually formed. You can think of the nascent solar system as something like a tennis ball (the sun) stuck in the middle of a large pancake, with the planets eventually forming in the latter. The process of formation itself is thought to be similar to the gravitational bunching described for the formation of a star.
Close to the sun, the temperatures in the cloud were high enough to vaporize substances like methane and ammonia. Particles streaming out from the sun blew these and other gases into space, leaving only solids behind to form the planets. This is why the inner solar system is populated by small, rocky planets. Farther out, however, methane, water, and ammonia were frozen solid and the original stock of hydrogen and helium was not greatly affected by the early sun. In the outer reaches of the solar system, then, we find the so-called gas giants—large planets formed primarily of frozen hydrogen, helium, methane, and ammonia.
Littered throughout the planetary systems are the remains of the building process—material that for one reason or another never got incorporated into larger bodies. The asteroid belt, between the orbits of Mars and Jupiter, contains the rocky remains of a planet that never fully formed, probably because of the gravitational influence of Jupiter. Outside the orbit of Pluto are two structures that can be thought of as more remains of the early solar system. First is a flat disk called the Kuiper belt, and farther out a spherical cloud of comets called the Oort cloud. (The two structures are named for the Dutch-American astronomer Gerard Kuiper and the Dutch astronomer Jan Oort, respectively.) Occasionally collisions or other disturbances in the Oort cloud send new comets into the inner solar system, where some (like Halley’s comet) are captured by gravity into sedate, predictable orbits.
Connecting all these bodies is a thin, wispy web of magnetic field that starts deep inside the sun and extends outward to the galactic magnetic field. The magnetic fields of individual planets are embedded in the interplanetary field like lumps in gravy, and a steady stream of particles from the sun’s surface, called the solar wind, moves out along the field lines.
The Formation of the Moon
Earth’s solitary, lifeless moon provides a striking contrast to our dynamic planet, and its origin has long posed a problem in the theory of Earth’s formation. The moon has roughly the same chemical makeup and density as Earth’s mantle, so it looks like a piece torn out of our own planet. The prevailing theory for the moon’s origin is that some millions of years after Earth formed, one last giant moon-sized asteroid crashed into Earth, throwing a lot of material into orbit. The moon then formed from this loose material by a process similar to the original formation of Earth.
A QUICK TOUR OF THE SOLAR SYSTEM
Terrestrial Planets
The planets Mercury, Venus, Earth, and Mars, together with Earth’s moon, are usually designated the terrestrial (Earth-like) planets. They are relatively small and rocky. Mercury and the moon are too small to hold gases to their surface, but the other three have atmospheres.
Venus is shrouded by clouds, but has been mapped by radar from orbit. Soviet spacecraft have landed on its surface, which is at a temperature of approximately 500°C (850°F). Of all the planets, Venus is the closest to Earth in size.
The diameter of Mars is about half that of Earth. The planet has a thin atmosphere, mainly carbon dioxide, and the red color of its surface reflects the oxidized (rusted) iron in its rocks and soil. There is no evidence for the existence of life or liquid water on Mars, nor are there “canals,” despite a mythology to the contrary. Mars has been the object of many orbiter and lander missions in the past few decades, and we now know a great deal about it. The most important points are that:
Early in its life, Mars had liquid water on its surface for extended periods of time.
There is at present no evidence for life on Mars, although many scientists feel that life may have developed there billions of years ago.
An important goal of NASA is to bring a sample of rock back from Mars within the next decade. The hope is that if there are fossils in the rock, it may show that life is common in the universe.
JOVIAN PLANETS
Jupiter, Saturn, Uranus, and Neptune are called the Jovian planets, after the Roman name for the god Jupiter. The largest Jovian planet, Jupiter, has a mass more than three hundred times that of Earth. These planets probably have a rocky core slightly larger than the size of a terrestrial planet, but the core is buried under thousands of miles of liquid and solid hydrogen, helium, methane, water, and ammonia. All Jovian planets have multiple moons and ring systems, with the rings of Saturn being the most spectacular and best known. They are far from the sun, and therefore cold. Some of their moons are virtually planets in their own right, being larger than Mercury All of the Jovian planets have now been o
bserved close up by the Pioneer and Voyager spacecraft. The Galileo spacecraft, which orbited Jupiter, uncovered evidence that there is liquid water under about a mile of ice on the surface of the moon Europa. The water is kept from freezing by heat generated when the moon is flexed by Jupiter’s gravity. Some scientists suspect that life may have developed in that environment.
Pluto
Pluto is normally the farthest of the traditional planets from the sun, though its elliptical orbit occasionally takes it inside the path of Neptune for part of its year. Pluto is small and rocky with one large moon. Today Pluto is not seen as the last of the planets, but as the first object in the Kuiper belt. Astronomers have discovered other planet-like bodies in the belt, some larger than Pluto. In 2006 the International Astronomical Union decided to refine their definition of a planet to reflect this fact, and Pluto is now referred to as a plutoid, although this redefinition remains controversial among some astronomers.
OTHER SOLAR SYSTEMS
Starting in the late 1980s, astronomers have begun detecting planets circling other stars, and we now know of hundreds of such extrasolar planetary systems. Planets are much too small to be seen directly, but are discovered by seeing the effect their gravitational pull has on the nearby star they orbit. The idea is that when the planet lies between Earth and that star, it pulls the star toward us and we can detect a slight blueshift due to the Doppler effect in the light the star emits. Similarly, when the planet is on the far side of the star, the star is pulled away from us and we see a redshift. In a few instances, where the alignment is just right, we can see the star dim slightly as the planet passes in front of it.
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