Science Matters (21 page)

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 supernova—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.

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.

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.

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:

1. Early in its life, Mars had liquid water on its surface for extended periods of time.
   
2. There is at present no evidence for life on Mars, although many scientists feel that life may have developed there billions of years ago.
   
3. 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.
   

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 observed 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 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.

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.

In general, the planets we have found so far do not look like Earth. They tend to be very large (larger than Jupiter) and close to their star (closer than Mercury). They also seem to have highly elliptical orbits, rather than orbits that are nearly circular, as in our own system. Astronomers think that these so-called hot Jupiters formed far out in their solar systems and moved in to their present orbits.

Stars are not scattered at random throughout the universe; all stars are gathered into clumps called galaxies. Our own sun, for
example, is one of a group of about 100 billion stars called the Milky Way galaxy. About 120,000 light-years across, the Milky Way, like about three fourths of all galaxies, is a flattened rotating disk with bright spiral arms. On a good night, your unaided eye can see about 2,500 stars, all of them in the Milky Way. You can also see a few fuzzy patches of light called nebulae (clouds). Seemingly unimportant in the grand celestial display, they are actually other galaxies, with billions of stars, planets, and possibly life like our own.

The discovery that there were other “island universes” besides the Milky Way was made by the American astronomer Edwin Hubble in 1923, using the then new 100-inch telescope on Mount Wilson, near Los Angeles. Until this telescope was built, astronomers, like someone peering at fine print without glasses, had been trying to understand nebulae without much success. The advent of the Mount Wilson telescope changed all that. Hubble could pick out types of stars that astronomers used to establish distances within the Milky Way. Using measurements of these stars, Hubble showed that the Andromeda nebula is some 2 million light-years away—far outside the confines of the Milky Way. Because of his work, we now realize that our galaxy is only one among billions, each with billions of stars, in the universe.

The M 81 spiral galaxy, which contains billions of stars, is one of the closest galaxies to our own Milky Way. Viewed from 4 million light-years away, our home cluster of stars would look similar to this
. PHOTO COURTESY OF THE CARNEGIE INSTITUTION OF WASHINGTON.

Most galaxies, like the Milky Way, are relatively sedate, homey places where the slow process of the stellar life cycle goes on quietly. A small number, however, seem to house a kind of violence unknown in our own peaceful neighborhood. Cataclysmic explosions rip through the cores of the galaxies, spewing huge jets of material hundreds of thousands of light-years into space. These active galaxies typically emit large amounts of energy as radio waves, and hence shine brightly in the radio sky.