Sky & Telescope February 1987 [antikvár]

The Shaping of Planetary NebulaeBruce Balick, University of WashingtonPLANETARY NEBULAE adorn the galactic plane like Christmas tree ornaments. Generally tiny but bright compared to other diffuse nebulae, they are a delight when viewed through even a small telescope. The Ring nebula in Lyra may be seen by more observatory visitors than any other attraction in the summer sky. What makes planetaries so appealing is their striking symmetry. Many appear shaped like rings or doughnuts, but most are far more intricately structured, as we shall soon see.Planetaries have a fascinating history as objects of astrophysical interest. Many bright emission lines that cannot be duplicated under laboratory conditions were first discovered in their spectra. These "forbidden" lines arise from ordinary elements like oxygen and nitrogen. Early 20th-century theories of how electrons orbit atomic nuclei were judged in part by whether they could explain the wavelengths and strengths of the forbidden lines in planetary nebulae.Today planetaries are seen as a key to understanding how ordinary stars evolve near the end of their lives. Astronomers think that all stars except the rare, very massive ones pass through the planetary-nebula phase. This occurs sometime during the complex, often very rapid transformation of red giants into white dwarfs, when violent disruptions in the outer parts of stars should be common.For nearly a third of a century astronomers have known that a planetary nebula is actually the former outer layer of a red giant. This "envelope" is ejected after the nuclear fires that heat the stellar core die out and the star is forced to undergo major readjustments. One consequence is mass-ejection, which can attain rates as high as one ten-thousandth of a solar mass per year and can last for tens of thousands of years. One way or another, the star banishes more than half its material to interstellar space before eventually settling down as a white dwarf.We would better understand this phase of stellar evolution if we could monitor changes deep in the interiors of red giants. Unfortunately, this is impossible. Nor can we crack open an evolved stellar core to examine its contents. But there is another way to find out what happens.The ejected red-giant envelope, so easily observed as a planetary nebula, is in some ways like the edible part of a peach. Imagine, for example, that you want to examine the life cycle of peaches but can't open the pit to get to the seed. Even so, analyzing the outer layers of the fruit would be very revealing. You might develop a precise model for the genes in the seed. From this and a knowledge of biology, you might conceive a theory to account not only for the evolution of the peach, but perhaps for the evolution of the apricot and nectarine as well."Color images produced at observatories around the world are more than just beautiful curios; they reveal the fundamental processes shaping the end of stars' lives."Similarly, the envelope of a planetary nebula once surrounded the core of the red giant. By observing the ejected gas, astronomers can study changes in the chemical composition of the outer parts of the star. These observations, interpreted in the context of current theories of stellar evolution and the ejection of the red-giant envelope, can help us understand how various layers within stars are affected by nuclear fusion processes. With this approach, we can fill some big gaps in our knowledge of how stars evolve beyond the red-giant phase.Astronomers continue to make great strides in tracing the late stages of stellar evolution, yet many outstanding questions remain. What phenomena determine the shapes of planetary nebulae? How do their often remarkable and complex symmetries arise? Written into the shape of each nebula is the history of how its gas was ejected. The photographs displayed on the next few pages and more fully described below show that at least for some planetaries the history was very rich indeed.BLOWING BUBBLESSuppose that the red-giant envelope is a perfect sphere surrounding the star that ejected it. Astronomers now recognize that after the bulk of the gas is expelled, the star blows a low-mass "fast wind." The drawings on the left side of page 126 show what is thought to happen as a result. The fast stellar wind sweeps into the envelope from the inside and compresses the inner part into a thin, dense shell. Aptly named the "snowplow," this shell becomes visible as a dense (and bright) rim advancing into the sparser (and faint, or even invisible) envelope. Inside this rim lies a "bubble" of tenuous, hot gas which does not emit detectable radiation.Once the snowplow reaches the edge of the red-giant envelope, where there is no gas left to constrain it, the nebula bursts open and the hot gas inside the bubble escapes into the interstellar medium. Theoretical studies of this process were described in the May, 1982, issue, page 449.This spherically symmetric model is now well established theoretically. Nearly all of the very round planetary nebulae give testimony to its success (an example is the Eskimo nebula, NGC2392, shown on page 127). However, the nonspherical shape of most planetaries implies that this model is too simplistic. Therefore, we must ask whether more complex shapes result from nonuniform ejection of the red-giant envelope or from some mechanism that focuses the fast wind.