A planetary nebula is a nebula associated with and deriving from a star, in principle having a disk-like appearance, similar to the telescopic view of a planet. About 1000 planetary nebula are catalogued, perhaps one-tenth the number in our Galaxy. Their radii range from that of our own Solar System to about 0.5 to 1 light year.

Morhpology

The name was coined in 1785 by Herschel, echoing remarks by Darquier about the Ring Nebula. Herschel thought at first that planetary nebula were unresolved groups of stars like distant globular clusters, but in 1790 the observed NGC 1514, a planetary nebula which looked like a “star about the eight magnitude with a faint luminous atmosphere, of a circular form … The star is perfectly at the center …” The particularly simple for of the nebula, and the strikingly central position of the very bright star, so much brighter than others nearby, convinced Herschel that true nebulosity existed, which, no matter how powerful his telescopes, could never be resolved into stars.

Astronomers are attracted to the circularly shaped planetary nebula, for the very good reason that they have a simple symmetry for which it looks relatively easy to construct a mathematical model – how can one explain the complicated if one cannot explain the simple? However, only 10 % of planetary nebula have a circular shape. About 70% have a bipolar structure and have two lobes. The shapes suggest a wide variety names, Butterfly, Dumbbell, Eskimo, Helix, Owl, Ring, Saturn.

Even the simpler shapes are open to interpretations. Is the Ring Nebula a hollow ellipsoidal shell completely surrounding the star at its center, or is I a torus (circular doughnut) seen at a slightly oblique angle? The strong contrast between the brightness of the ring and the hollow space within the suggests that the hollow space is truly empty, and that there are no front and read faces of a shell to contribute to the light from this line of sight. Minkowski and Osterbrock concluded therefore that the Ring Nebula was toroidal. If viewed exactly in the pane of the toroid the Ring Nebula would have a twin lobed shape typical of many planetary nebula. Suppose the height of the toroid were exaggerated so that it became a squat cylinder centered on its star? If such a cylinder were viewed obliquely, then the open ends could give the hollow eye-like appearance of the Owl Nebula.

Physical Characteristics

A planetary nebula shines because of the ultraviolet light emitted by its central star. The atoms of the nebular gas may be ionized by ultraviolet photons of sufficient energy; hydrogen atoms, for instance, can be ionized by photons with wavelength of 912 angstroms or shorter. When the ion recombines with its electrons it emits photons of light from a structured ladder of energy steps. Each ultraviolet photon input into the nebula produces of spectrum of photons out: on this sense planetary nebula are photon counters. Their symmetry and fact that the central stars are identifiable means that planetary nebula have been laboratories for atomic astrophysics.

Each central star has the output to ionize a certain volume of planetary nebula. There may be gas beyond the visible boundary of the nebula, but ultraviolet photons don’t reach far enough to make it visible. In fact within each planetary nebula numerous sub-nebula exist, each corresponding to the ionization of a given type of ion or atom. Thus pictures taken in different optical spectral lines show stratification – the images of the planetary nebula are nested like a set of Russian matroshka dolls made up of colored glass. The Ring Nebula (M57) is a good example of this: blue, green, and red images are successively larger because each is dominated by a particular spectral emission form a different atom r ion. Because it is relatively simply structured, the celestial “laboratory” of M57 can be analyzed to determine what it contains. The nebula itself is in total about 0.2 times the mass of the Sun. Its density is about 10,000 ions of hydrogen per cubic centimeter (air at sea level has 30 million, million, million atoms per cubic centimeter). The ratios are: hydrogen 10,000, helium 800, oxygen 8, nitrogen 3, and neon 1 (ions/cc).

The input of energy from the central star of the nebula warms the gas: the electrons which are split from the constituent ions have temperatures of about 12,000K. Radio telescopes detect the thermal emissions from planetary nebula, and in fact give a more complete view of the structure of some nebula that are obscured by dust.

Life

All the planetary nebulae are expanding. Two techniques demonstrate this to be true. The Doppler Shift of the optical spectral lines provides a measurement of the expansion velocity along the line of sight: values of 20 km/s are typical The increase in the size of a planetary nebula taken over the years shows its expansion across the line of sight. One of the closest planetary nebula, the Dumbbell, has been measured to grow on angular size at 0.068 arc sec per year – the actual growth rate of the trunk of a tree at 100km whose rings grow at 1 mm per year. Optical techniques for measuring the growth of planetary nebula are likely quickly to be superseded by measurements using large interferometer systems.

The central stars of planetary nebula are very hot and blue (typical temperatures from 30,000 to 400,000K), and form a range of stars somewhat fainter than the usual main sequence of stars (absolute magnitude 0 to +10). They form a missing link between the Red Giants and the White Dwarfs, a transitional stage at the end of stellar evolution. This is also indicated by the fact that planetary nebula form a population in our Galaxy which lies neither in the galactic plane of young stars nor in the halo of very old stars: they form a so-called “disk population” of intermediate age. The precise evolutionary status of only one planetary nebula is known that lies within the globular cluster M15 in Pegasus.

It is clear that the planetary nebula stage of a star’s life marks a change from advanced middle age to senility and is associated with a form of loss of material from the star. It is, as yet, unclear how the mass loss takes place. Such planetary nebula such as the Eskimo or the Saturn nebula surrounded by fainter extensions, give the impression that a series of explosions occurred in the central star, ejecting successive shells of gas into space. This conceptualization of planetary nebula is based on analogies with the ejection of shells in nova, such as the one that was produced by nova Persei 1901. It is an idea that is traceable back to Herschel’s papers of the 1780’s which first recognized planetary nebula. However the ejection velocities of novae are thousands of times faster and, in nova, the amount of matters in hundreds of thousands of times smaller.

The modern view relates the planetary nebula to the stellar wind material blown out from a red giant star, identifying the central star as the red giant’s core, as it evolves to become a white dwarf. The gravity at the surface of a red giant is very low, because the star is so big, and storms on its surface akin to solar prominences can throw off material easily. A star of up to about eight solar masses can lose about ¾ of its mass into space before it turns into a white dwarf of mass about 1.4 solar mass. Such a process could readily provide enough material to make a planetary nebula. The visible nebula is simply the central region of a much larger object, the only part which the limited ultraviolet light from the central star is capable of illuminating.

The idea that the outflow is connected to stellar winds and magnetic storms leads to a connection between the curious shapes of planetary nebula and the magnetic field and rotation properties of the star. The magnetic lines of flow might control the outflow, channeling it into particular directions, and the rotation of the star can add spiral twists. The bipolar structure of most planetary nebula must be in general terms connected with the existence of a preferred direction, such as a spin or a magnetic axis.

The fact that the nebula are expanding as the central stars are fade means that planetary nebula are relatively transitory objects, with lifetimes measured in tens of thousands of years. Given the estimated number of planetary nebula in our Galaxy (10,000), this means that several per year are forming. One possible event that astronomers believe may represent the birth of a planetary nebula has occurred in the star FG Saggitta. Its brightness rose steadily from 13.6 in 1894 to 8.9 on 1970. In 1960 it was noticed that the star had acquired a nebula of radius 18 arc seconds; there were indications that the old photographs of the star showed a fuzzy image as if the nebula had formed in the 19th century. The atmosphere of the star is expanding and it appeared to be ejecting a second shell. This shell contains spectral lines of unusual chemical elements such as yttrium, zirconium, cerium, neodymium, and samarium. The significance of these elements is that they are formed by one nuclear process called the S-process: the elements have risen to the star’s surface and become visible in its ejected shell.