Winds of Change

In the mysterious, glowing clouds created by the collision of violently blowing gases, you can read the record of a star's dying days.

By Adam Frank|Wednesday, June 01, 1994

Peering through their still-crude telescopes, eighteenth-century astronomers discovered a new kind of object in the night sky that appeared neither as the pinprick of light from a distant star nor as the clearly defined disk of a planet but rather as a mottled, cloudy disk. They christened these objects planetary nebulas, or planetary clouds.

In the sharper eyes of twentieth-century telescopes, these smudges have resolved into great luminous clouds of gas and dust that appear in a baffling variety of shapes, from huge glowing ellipses to giant gaseous peanuts. Modern astronomers recognize planetary nebulas as the fossil wreckage of dying stars ripped apart by powerful winds. Their vast symmetrical shapes outline hypersonic shock waves produced by colliding shells of stellar gas. Over the past few years, astronomers have learned to read in these glowing fossils the history of the winds, and therefore the history of the stars from which they came. It is a tale of galactic fireworks rivaled only by the apocalypse of supernovas. But while only the occasional star makes its exit as a supernova, most stars in the cosmic census--including our sun--will be blown away by the winds of change, and their gravestones will be planetary nebulas.

Astronomers can't just cook up planetary nebulas to study in their labs, so they let high-speed supercomputers do it for them. The equations describing the collision of stellar winds are big, messy, and elaborately interconnected, and trying to solve them with pencil, paper, stamina, and faith is often impossible. But a supercomputer can do it simply by taking tiny steps over and over again, a million times a second. After a few hours or weeks, depending on how super the supercomputer is, a thousand years of planetary nebula history is waiting in the computer's memory to be studied in detail.

The story the computers tell is based on the "interacting stellar winds" model of planetary formation, and it corresponds startlingly well with the images astronomers see in the sky. According to this model, the violent wind that creates a planetary nebula is also the engine that transforms a bloated red giant into the burnt-out cinder of a white dwarf, a metamorphosis common to all stars of low and intermediate mass--stars up to eight times more massive than the sun. From birth, these stars shine by fusing hydrogen into helium in their cores. When the hydrogen runs out, the star's core collapses in on itself, contracting until it becomes hot enough to burn its own ashes--the core now fuses helium into the heavier elements carbon and oxygen. But burning helium releases energy in the core faster than it can be radiated away at the surface. The outer layers of the star-- the stellar "atmosphere"--absorb this excess and swell outward into the characteristic distended figure of a red giant.

As the bloated star ages, this extended outer atmosphere cools and contracts, then soaks up more energy from the star and again puffs out: with each successive cycle of expansion and contraction the atmosphere puffs out a little farther. Like a massive piston, these pulsations drive the red giant's atmosphere into space in a dense wind that blows with speeds up to 15 miles per second. In as little as 10,000 years some red giants lose an entire sun's worth of matter this way. Eventually this slow wind strips the star down close to its fusion core. In a few thousand years, it will be nothing but carbon and oxygen ash--a dead white dwarf.

In the meantime, however, the exposed core becomes a violent scene of fusion reactions among remaining hydrogen and helium nuclei, which release a torrent of energetic photons, mostly in the form of ultraviolet rays. The photons punch into space whatever atmosphere is left, creating a tenuous high-velocity wind. This "fast" wind, with speeds up to 3,000 miles per second, quickly overtakes the slow wind and slams into it with the force of a trillion one-megaton H-bombs. That's when the fireworks begin.

Anytime a gas is pushed faster than it can react by getting out of the way, a shock wave occurs; as the wave moves through the medium, it quickly and violently smashes together the gas molecules like cars in a highway pileup. And in this case, the collision of the stellar winds produces two powerful shock waves. When the fast wind slams into the slow wind, a shock wave moves outward, accelerating and compressing the slow wind as it sweeps through it, squeezing it into a dense shell of gas ions. At the same time another shock wave rebounds off the slow wind, back through the fast wind, toward the star. This rebound shock jerks the fast wind to a near stop, and the violent deceleration heats the fast wind to more than 10 million degrees, creating a hot bubble of gas. Ultimately, the result is a kind of shock wave layer cake. The inner shock wave is closest to the star, surrounded by the hot bubble, which in turn is surrounded by the dense shell and its outer boundary--the outer shock wave.

As they heat and compress the gas, the shock waves emit light. The dense shell glows intensely because it is crammed with excited hydrogen atoms. The gas in the hot bubble is too rarefied to produce much light in spite of its enormous temperatures. Therefore, the shell and the outer shock wave are the glowing forms we see when we view a planetary nebula from Earth. When the shape of the outer shock changes, the shape of the planetary nebula changes as well.

Only collisions between spherical winds create spherical shocks and therefore round planetary nebulas. But most planetary nebulas are not round. Nebulas of other shapes can still be explained by the collision of stellar winds, however, if we assume the slow wind itself is not perfectly symmetrical. This situation could occur if, for example, the amount of mass leaving a star was not the same at every point on the surface of that star- -say, if more mass was driven off from the equator of the star than from its poles. In that case the slow wind would assume a flat, disklike shape.

Astronomers do not know yet how nature actually makes aspheric winds, but they have some plausible ideas. For example, if the red giant is part of a binary star system and is therefore orbiting a companion star, then the gravity of the other star could possibly pull the slow wind into the shape of a disk. The outer shock forming behind this flattened cloud could then quickly blow out the tenuous poles because relatively little matter would stand in the way--like a strong gust of wind blowing out a weak spot in a sail. Along the equator the shock would plow slowly through the densest parts of the disk. After just a few thousand years, the shock wave layer cake would be distorted into a peanut or elliptical shape, depending on the shape of the slow wind. The more matter spewed out along the equator rather than the poles, the more peanut-shaped, or "bipolar," the final planetary nebula.

Last year astronomers using Cray supercomputers in the United States and the Netherlands performed hundreds of planetary nebula simulations, which revealed a rich array of behavior in the shock wave layer cake. In some simulations huge rolling vortices appeared between the star and the inner shock in the hot bubble. Other simulations showed narrow supersonic jets being squeezed from the top of a disklike slow wind. Most satisfying of all, the menagerie of simulated planetary nebula shapes looked a lot like the creatures in the real planetary nebula zoo. The interacting stellar winds model had passed the test. When theory and observation agree, astronomers experience a brief moment of feeling they understand something.

While astronomers now believe they know how stellar winds can sculpt a planetary nebula, much still escapes them. A number of planetaries refuse to fit anywhere in the catalog of shapes produced by the model. Many of these planetary nebulas have a funny kind of inverted mirror symmetry, with their tops and bottoms reflected and then reversed, as in the letter s. These planetaries might be explained as the result of binaries if the combined orbit of the two stars precessed like a top. At the moment, though, that is just a guess.

The cloud of unanswered questions surrounding planetaries should not obscure the real insight astronomers have recently gained into the extraordinary death of ordinary stars. In a particularly happy marriage of theory and observation, astronomers have discovered our own sun's fate. With the interacting stellar winds model, they can confidently predict the weather about 5 billion years from now: very hot, with really strong gusts from the east.

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