The Milky Way and Beyond (29 page)

Compared with diffuse nebulae, planetary nebulae are small objects, having a radius typically of 1 light-year and containing a mass of gas of about 0.3 solar mass. One of the largest-known planetary nebulae, the Helix Nebula (NGC 7293) in the constellation Aquarius, subtends an angle of about 20 minutes of arc—two-thirds the angular size of the Moon. Planetary nebulae are considerably denser than most H II regions, typically containing 1,000–10,000 atoms per cubic cm (1 cubic cm = .06 in) within their dense regions, and have a surface brightness 1,000 times larger. Many are so far away that they appear stellar when photographed directly, but the conspicuous examples have an angular size up to 20 minutes of
arc across, with 10–30 seconds of arc being usual. Those that show a bright disk have much more-regular forms than the chaotic H II regions, but there are still usually some brightness fluctuations over the disk. The planetaries generally have regular, sharp outer boundaries; often they have a relatively regular inner boundary as well, giving them the appearance of a ring. Many have two lobes of bright material, resembling arcs of a circle, connected by a bridge, somewhat resembling the letter
Z
.

Most planetaries show a central star, called the nucleus, which provides the ultraviolet radiation required for ionizing the gas in the ring or shell surrounding it. Those stars are among the hottest known and are in a state of comparatively rapid evolution.

As with H II regions, the overall structural regularity conceals large-scale fluctuations in density, temperature, and chemical composition. High-resolution images of a planetary nebula usually reveal tiny knots and filaments down to the resolution limit. The spectrum of the planetary nebula is basically the same as that of the H II region; it contains bright lines from hydrogen and helium recombinations and the bright, collisionally excited forbidden lines and faint recombination lines of other ions. (Recombination is the process by which the higher stage of ionization captures an electron [usually at low energies] into a high level of the ion.) The central stars show a much greater range of temperatures than those in H II regions, ranging from relatively cool (25,000 K) to some of the hottest known (200,000 K).

In the nebulae with hot stars, most of the helium is doubly ionized, and appreciable amounts of five-times-ionized oxygen and argon and four-times-ionized neon exist. In H II regions helium is mainly once ionized and neon and argon only once or twice. This difference in the states of the atoms results from the temperature of the planetary nucleus (up to about 150,000 K), which is much higher than that of the exciting star of the H II regions (less than 60,000 K for an O star, the hottest). High stages of ionization are found close to the central star. The rare heavy ions, rather than hydrogen, absorb the photons of several hundred electron volt energies. Beyond a certain distance from the central star, all the photons of energy sufficient to ionize a given species of ion have been absorbed, and that species therefore cannot exist farther out. Detailed theoretical calculations have rather successfully predicted the spectra of the best-observed nebulae.

The spectra of planetary nebulae reveal another interesting fact: they are expanding from the central star at 24–56 km (15–35 miles) per second. The gravitational pull of the star is quite small at the distance of the shell from the star, so the shell will continue its expansion until it finally merges with the interstellar gas around it. The expansion is proportional to the distance from the central star, consistent with the entire mass of gas having been ejected at one brief period from the star in some sort of instability.

Estimating the distance to any particular planetary nebula is challenging because of the variety of shapes and masses of the ionized gas. There is uncertainty about the amount of ionizing radiation from the central star that escapes from the nebula and the amount of hot low-density material that fills part of the volume but does not emit appreciable radiation. Thus, planetary nebulae are not a homogeneous class of objects.

Distances are estimated by obtaining measurements for about 40 objects that happen to have especially favourable properties. The favourable properties involve association with other objects whose distance can be estimated independently, such as membership in a stellar cluster or association with a star of known properties. Statistical methods, calibrated by these objects, provide rough estimates (about 30 percent errors) of distances for all others. The statistical method involves assuming that all shells have similar masses when all of the shell is ionized and correcting for the fraction that is neutral for the rest.

From the best available distance determination, the true size of any nebula can be found from its angular size. Typically, planetary nebulae are a few tenths of a light-year in radius. If this distance is divided by the expansion speed, the age of the nebula since ejection is obtained. Values range up to roughly 30,000 years, after which the nebula is so tenuous that it cannot be distinguished from the surrounding interstellar gas. This lifetime is much shorter than the lifetimes of the parent stars, so the nebular phase is relatively brief.

Planetary nebulae are chemically enriched in elements produced by nuclear processing within the central star. Some are carbon-rich, with twice as much carbon as oxygen, while there is more oxygen than carbon in the Sun. Others are overabundant in nitrogen; the most luminous ones, observed in external galaxies, are conspicuous examples. Helium is modestly enhanced in many. There are objects that contain almost no hydrogen; it is as if the gas had been ejected from these object at the very end of the nuclear-burning process. Planetary nebulae also show a clear indication of the general heavy-element abundance gradient in the Galaxy, presumably a reflection of the original composition of the stars that gave rise to the present nebulae.

As in the case of H II regions, planetary nebulae show discrepancies between the determinations of abundances of heavy elements from faint recombination lines as opposed to those determined from collisionally excited lines, but in a much more severe form. There are some nebulae for which the two methods give the same abundances. However, the most extreme discrepancies are factors of 30 or
more in the oxygen abundances. Perhaps this wild variation in planetary nebulae is not surprising, since they surely have regions of material that are strongly enriched in heavy elements and deficient in hydrogen. These regions originate in the complicated nuclear processing of the expelled material ejected from the evolved central star. They would have strong cooling from the heavy-element emissions and thus much lower temperatures than the regions of normal composition. These regions would contribute very little to the hydrogen emission lines because they are hydrogen-poor.

Some, but not all, planetary nebulae contain internal dust. In general, this dust cannot be seen directly but can be detected from the infrared radiation it emits after being heated by nebular and stellar radiation. The presence of dust implies that planetary nebulae are even richer in heavy elements than gas-phase abundance studies suggest.

Among nebulae so far discovered, two are particularly deviant in chemical composition: one is in the globular cluster M15 and the other in the halo (tenuous outer regions) of the Galaxy. Both have very low heavy-element content (down from normal by factors of about 50) but normal helium. Both objects are very old, suggesting that the primeval gas in the Galaxy had a low heavy-element content but an almost normal amount of helium. The origin of most helium in the Galaxy was the big bang, the initial explosion of the universe itself.

One of the best indicators of the average age of astronomical objects is their position and motion in the Galaxy. The youngest are in the spiral arms, near the gas from which they have formed; the oldest are not concentrated in the plane of the Galaxy, nor are they found within the spiral arms. By these criteria, the planetaries reveal themselves to be rather middle-aged; they are moderately but not strongly concentrated in the plane; rather, they are concentrated toward the galactic centre, as the older objects are. Their motions in the Galaxy follow elliptical paths, whereas circular orbits are characteristic of younger stars. They belong to the type of distribution often called a “disk population,” to distinguish them from the Population II (very old) and Population I (young) objects proposed by the German American astronomer Walter Baade. There is a wide variation in the ages of planetaries, and some are very young objects.

A description of the evolution of a planetary nebula begins before the ejection of the nebula itself. As will be discussed below, the central star is a red giant before the ejection. In such a phase it experiences a rapid loss of mass, up to 0.01 Earth mass per day, in the form of a comparatively slowly expanding stellar wind.
At this stage the red giant might be heavily obscured by dust that forms from the heavy elements in the wind. Eventually the nature of both the star and its wind changes. The star becomes hotter because its hot core is exposed by the loss of the overlying atmosphere. The inner gas is ionized by radiation from the hot star. The ionization zone moves steadily outward through the slowly moving material of what was formerly the stellar wind. The expansion speed of the gas is typically 30 km (19 miles) per second. Nebulae in this stage are bright but have starlike images as seen from Earth, because they are too small to show a disk. The gas is at a relatively high density—about one million atoms per cubic centimetre (1 cubic cm = .06 cubic in)—but becomes more dilute as the gas expands. During this stage the nebula is surrounded by neutral hydrogen. It appears to expand faster than the individual atoms of gas in it are moving; the ionized shell is “eating into” the neutral material as the density falls.

The middle stage of evolution occurs when the density has dropped to the point at which the entire mass of gas is ionized. After this stage is reached, some of the ultraviolet radiation escapes into space, and the expansion of the nebula is caused entirely by the motion of the gas. Most planetaries are now in this middle stage. Finally the central star becomes less luminous and can no longer provide enough ultraviolet radiation to keep even the dilute nebula ionized. Once again the outer regions of the nebula become neutral and therefore invisible. Eventually the gas is mingled with the general interstellar gas. A curious feature of several planetaries is that faint rings surrounding the bright inner nebula can be observed; they are the remnants of a previous shell ejected earlier by the star.

Many central stars are known from their spectra to be very hot. A common type of spectrum has very broad emission lines of carbon or nitrogen, as well as of ionized helium, superimposed upon a bluish continuum. These spectra are indistinguishable from those from the very bright rare stars known as Wolf-Rayet stars, but the planetary nuclei are about 100 times fainter than true Wolf-Rayet objects. The stars appear to be losing some mass at the present time, though evidently not enough to contribute appreciably to the shell.

The presence of the nebula allows a fairly precise determination of the central star's evolution. The temperature of the star can be estimated from the nebula from the amounts of emission of ionized helium and hydrogen by a method devised by the Dutch astronomer H. Zanstra. The amount of ionized-helium radiation is determined by the number of photons with energy of more than 54 electron volts, while hydrogen is ionized by photons in excess of 13.6 electron volts. The relative numbers of photons in the two groups depend strongly on temperature, since the spectrum shifts dramatically to higher energies as the temperature of the star increases. Hence,
the temperature can be found from the observed strengths of the hydrogen and helium lines. The rate of evolution of the stars can be determined from the sizes of their nebulae, as the time since ejection of the shell is the radius of the nebula divided by the expansion rate. The energy output, or luminosity, of the central star can be estimated from the brightness of the nebula, because the nebula is converting the star's invisible ultraviolet radiation (which contains the greater part of the star's luminous energy) into visible radiation.

The resulting theoretical description of the star's evolution is quite interesting. While there seem to be real differences in stars at a given stage, the trends are quite clear. The central stars in young planetary nebulae are about as hot as the massive O and B stars—35,000–40,000 K—but roughly 10 times fainter. They have half the diameter of the Sun but are 1,000 times as luminous. As the nebula expands, the star increases its brightness and temperature, but its radius decreases steadily. It reaches a maximum energy output when it is roughly 10,000 times as luminous as the Sun, about 5,000 years after the initial expansion. This is a very small fraction of the star's age of several billion years; it represents a period equivalent to about half an hour in a human life. From this point on, the star becomes fainter, but for some time the temperature continues to increase while the shrinkage of the star continues. At its hottest the star is perhaps 200,000 K, almost five times hotter than the hottest of most of the stars. It then cools and after about 10,000 years becomes a very dense white dwarf star, scarcely larger than Earth but with a density of thousands of kilograms per cubic centimetre (1 cubic cm = .06 cubic in). From this point it cools very slowly, becoming redder and fainter indefinitely.