WHEN I LOOK UP AT the night sky, the stars appear immutable. They never seem to change or age. But when I observe the array of stars across the heavens-even with the naked eye-I can see signs that they are slowly evolving from young stellar objects toward old, dim concentrations of matter.

Recently I invited a group of amateur astronomers to observe stars at the end of their life cycles. Using our eyes and various telescopes, we scanned the sky for the senior citizens of the cosmos. In particular, we searched for the remnants of some of the oldest stars, known as planetary nebulae [see "Planetary Nebulae," by Noam Soker, page 78]. These shells of gas and dust are among the most beautiful objects in the universe.

Let me begin the tour with a well-known, middle-aged star: the sun. Like most others, it is powered by thermonuclear fusion, releasing energy as hydrogen atoms fuse to form helium. After several billion years, the core of the sun will have exhausted its supply of hydrogen, and our star will begin to die. The core will shrink and heat as the fusion of hydrogen continues around it. Soon after, the sun will stabilize as the helium residue in the core begins to fuse. At that point, our star will be many times larger in diameter, it will appear 100 times brighter and it will turn orange or even red. The sun will become what astronomers call a red giant.

Fortunately, none of us will ever live to see the sun become a giant. But we have all seen such elderly stars. Most of the orange and red stars that can be viewed with the naked eye are giants. In spring, for instance, you should be able to find the bright, orange star Arcturus in the eastern part of the sky, and in winter you may see Aldebaran in the constellation Taurus.

As giants age, they burn up the helium in the core, leaving a central cinder of carbon and oxygen and outer shells of fusing helium and hydrogen. In the process the stars become even brighter, bigger and redder than before. When the sun reaches this stage-known simply as AGB-it will likely envelop the orbit of the earth.

AGB stars are not as easy to find as ordinary giants. Still, a few can be seen with the naked eye, and with a small telescope many come into view. As an AGB star approaches its maxirnum size and brightness, it becomes unstable and begins to fluctuate in brightness. A fme example is the star Mira, "the amazing." Most of the time it is invisible, but every 11 months or so it appears as one of the brighter stars in the constellation Cetus. Mira is easy to locate with a good constellation map.

Because the outer region of an AGB star is tenuous and turbulent, it ejects matter at a high rate, as much as three times the mass of the earth a year. After tens of thousands of years, nothing will be left of the star but the hot, dense core and an enormous cloud of gas. The core will still have enough energy to generate a strong stellar wmd that compresses some of the discarded matter into a shell. The central star will generate ultraviolet radiation that illuminates the shell. This event marks the beginning of the life of a planetary nebula.

After some 50,000 years, the nebula expands to interstellar dimensions, and its gases mix with those in space. At the same time, the central star starts to dim and becomes a dead, dense remnant called a white dwarf. Such will be the fate of most stars and the sun.

Unfortunately, no nebula is visible to the naked eye. A good pair of binoculars, however, will bring a few mto view. For instance, the Dumbbell Nebula glows relatively brightly among the stars of the obscure constellation Vulpecula near the foot of the Northern Cross [see illustration at right].

To view dozens of other objects, you will need a telescope, a good set of star maps and a working knowledge of the sky. Before you dust off your telescope or borrow one from a friend, you might benefit from the experience of eight amateur astronomers who volunteered to search for planetary nebulae.

The amateurs were recruited by Wayne Wyrick, director of the Kirkpatrick Planetarium in Oklahoma City. They made most of their observations during the Okie-Tex Star Party, an event organized last fall by the Oklahoma City Astronomy Club. Together the eight volunteers had a variety of telescopes ranging from four to 24 inches in diameter. In preparation I had compiled a list of targets: some of my choices, I thought, would be rather easy to observe, whereas others, I hoped, would test the limits of the instruments and the talents of the observers [see table in Figure 3]. To my amazement, all the nebulae on my list were viewed in some detail.

Young planetary nebulae are bright but small. As they age, they grow larger and easier to examine. The central star also heats up the gaseous cloud, which brightens and consequently obscures the star. Eventually the star cools and dims while the nebula continues to grow. An elderly nebula can be so large that it can be difficult to see against the background of the sky.

The youngest nebulae on my list were objects known as NGC 40 and IC 418. Larry Mitchell of Houston, Tex., and Phil Kuebler of Columbus, Ohio, searched for NGC 40 with a telescope 10 inches in diameter. They found the nebula easily and saw a slightly elongated halo and a prominent central star. With his 24inch telescope, Mike Dennis of Oklahoma City could distinguish "a hint of a greenish cast." IC 418 is difficult to observe. At first, Kuebler could make out only an odd-looking star, but with persistence he found definite nebulosity.

We next took a look at NGC 6543, an extraordinary nebula, both visually and historically. In 1864 William Huggins observed the spectrum of light from NGC 6543 and discovered that it has mysterious features known as the nebulium lines. They were identified some 64 years later by Ira S. Bowen of the California Institute of Technology as doubly ionized oxygen.

Through a telescope, NGC 6543 looks ghostly green. "No mistaking this one," said Clive Cadle of Oklahoma City, peering through his 10-inch telescope. He could glimpse the central star by fixing his gaze at a region slightly to the left or right of the star. (This technique, known as averted vision, works because the outer regions of the retina are more sensitive to light than the center.)

When Barbara Wilson of Houston observed NGC 6543 using a 20-inch telescope, she thought the nebula looked "like a cat's eye, a green oval with a dark ovoid." She was easily able to see the central star.

The archetype of all nebulae is NGC 7009, in the constellation Aquarius. This lovely object is named the Saturn Nebula because of its faint projections, or ansae. Discovered in 1782 by William Herschel, it was the first known planetary nebula.

Through an eight-inch telescope, Beryl Cadle, Clive's wife, saw "a bright, large, bluish, oval nebula, hazy around the edges." Using a 20-inch instrument, Wilson observed its ansae and its double-shell structure. (Each shell may represent a different episode of mass loss.) The ansae may represent jets of material that has been ejected from the central star. No one, incidentally, saw the star itself. The bright nebula hides it from view.

We next examined the Blue Snowball, NGC 7662, in the constellation Andromeda. Using a 10-inch telescope, Kuebler could see "a small, bright blue nebula" with a "perforation" within the disk. Peering through larger telescopes, Wilson clearly observed a ring of gas and a bright clump of material on the southwest side. With a professional 40inch telescope, I can make out two rings and the central star. Astronomers have studied this nebula intensely because of its triple-shell structure and hot nucleus (100,000 kelvins).

In NGC 7027 the star reaches 180,000 kelvins. To observe the star at all, astronomers must use sophisticated imaging systems. The nebula itself is rather compact and looks rather plain. Clive Cadle searched for NGC 7027 with a 10-inch telescope and passed over it several times because it looked like an ordinary star. Using a 24-inch telescope, Dennis clearly saw a "small patch of nebulosity with a greenish color... becoming more diffuse toward the edges." From these modest descriptions, it is hard to believe that NGC 7027 is the planetary nebula most frequently observed by professionals.

One of the older nebulae has two catalogue numbers, NGC 650 and 651, for its two lobes. Such a system, known as a bipolar planetary nebula, is created as the central star loses mass preferentially in some directions. Using 10-inch telescopes, both Kuebler and Mitchell saw the lobes.

The three best-known nebulae are the Dumbbell Nebula, the Ring Nebula in Lyra and the Helix Nebula in Aquarius. The Dumbbell, NGC 6853, is one of the most beautiful objects in the sky. It looks like a glorious mobile in an art gallery, hanging from a magnificent array of stars in the Milky Way. Using a four-inch telescope in suburban lighting, Linda McArthur of Moore, Okla., could see not only the dumbbell shape but also details along the edges. At 10 inches, Kuebler describes the nebula as blue-green and "an awesome sight."

Herschel did not even classify the Ring Nebula, NGC 6720, as a planetary because of its smoke-ring shape. With a 24-inch telescope, Dennis could see not only faint nebulosity but also the dim central star.

The last of the trio is the Helix Nebula, NGC 7293. The closest known planetary nebula to the earth, it is a mere 500 light-years away. Because it is so old, the nebula is enormous. Furthermore, because it so close, it covers a region of the sky almost half the angular diameter of the full moon. Consequently, it can be best observed with a low-power, wide-field telescope. But even with binoculars, Wyrick picked it out as a "grayish blob" and saw a hint of its ring structure. Using 10-inch telescopes, Clive Cadle and Mitchell clearly saw its ghostly ring set among the stars.

As a final challenge, I asked the group to look for Abell 80, a planetary nebula near the end of its life. Huge and faint, it will disappear from view altogether within the next 10,000 years or so. The only ones who could see the object were Wilson, Mitchell and Kenneth Drake of Houston, using telescopes 20 inches or more in diameter. Even then the feat proved difficult. They spent more than an hour at the eyepiece, applying every trick in the book of amateur astronomy: they covered their heads to keep out unwanted light, used averted vision and employed a contrastenhancing filter that passes the oxygen nebulium lines.

The three described and drew the same image, so we were assured that they were actually viewing Abell 80. Wilson saw two narrow arcs. "At times it seemed that the entire planetary could be seen as a diffuse glow," she explained. "But I really feel it was an illusion, and only the western lobe could be described." Mitchell found a brighter, northwest edge, and Drake saw a faint, round diffuse glow that faded into the background.

Several years ago I observed Abell 80 through a 40-inch telescope, to measure its brightness. I never saw it directly and relied on electronic instrumentation. At the star party I got a second chance. I think I saw it. But afterward I wondered if it was just an illusion. I hope I did. It was an opportunity to see the last throes of a planetary nebula before it disappears forever into the void, before its gases help to give birth to another generation of stars.

Bibliography

PHYSICS OF THERMAL GASEOUS NEBULAE. Lawrence H. Aller. D. Reidel Publishing, 1984.

THE GUIDE TO AMATEUR ASTRONOMY. Jack Newton and Philip Teece. Cambridge University Press, 1988.

ATOMS, STARS, AND NEBULAE. Lawrence H. Aller. Cambridge University Press, 1991.

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