|Sun Kwok (2007), Scholarpedia, 2(9):4338.||doi:10.4249/scholarpedia.4338||revision #138051 [link to/cite this article]|
Planetary nebulae are astronomical objects made up primarily of gaseous materials. They are extended in size and fuzzy in appearance, and generally show some degree of symmetry. The nebula is illuminated by a central star, which sometimes is too faint to be seen. Although initially grouped with galaxies and star clusters under the class of “nebulae”, we now know that galaxies and star clusters are made up of stars, whereas planetary nebulae are gaseous.
Planetary nebulae were discovered by astronomers as early as the 18th century, with four planetary nebulae being included in the catalogue of nebulae by Charles Messier in 1784. The most well known planetary nebula is the Ring Nebula in the constellation of Lyra (Figure 1), which can easily be observed with a small telescope in summer from the Northern hemisphere. The term “planetary nebulae” was coined by William Herschel for their apparent resemblance to the greenish disks of planets such as Uranus and Neptune. This turned out to be an unfortunate misnomer as planetary nebulae have nothing to do with planets.
Physical and spectral properties of planetary nebulae
Planetary nebulae are typically one light year across and are expanding at a rate of about 20-50 km per second. The density in the nebulae is very low, ranging from several hundred to a million atoms per cubic centimeter. Such conditions are better than any vacuum one can achieve on Earth. The temperature of the gas in the nebula is about 10,000 degrees Celsius, and the central stars of planetary nebulae are among the hottest stars in the Universe, with temperature in the range of 25,000 to over 200,000 degrees Celsius. The central stars are also very luminous, usually hundreds to thousands of times more luminous than the Sun. However, because of their high temperatures, they radiate primarily in the ultraviolet and are often faint in visible light.
The spectra of planetary nebulae are fundamentally different from those of stars. Instead of a continuous color from red to blue as in the case of the Sun, the spectra of planetary nebulae are dominated by discrete emission lines emitted by atoms and ions. Unlike stars, whose continuous spectra give them a composite white appearance, planetary nebulae have a rich variety of colors. Some examples of strong emission lines are the red line of hydrogen and the green line of doubly ionized oxygen (O++). These bright emission lines are powered by the central star, which is the source of energy for the entire nebula. Ultraviolet light emitted by the central star is intercepted by atoms in the nebula and converted to visible line radiation. First the ultraviolet light removes electrons from the atom (in a process called photoionization). The freed electrons then either recombine with the ion and emit a recombination line, or collide with other atoms and ions to cause the emission of a collisionally excited line. Because of the low density conditions, atomic lines that are generally suppressed under high density conditions as in the laboratory on earth but which can be produced in the low density conditions of planetary nebulae. These "forbidden lines" (of which the oxygen green line is an example) are very prominent in planetary nebulae, making them ideal laboratories to study atomic physics (Aller 1991).
Planetary nebulae are among the very few classes of celestial objects that radiate strongly throughout the electromagnetic spectrum from radio to X-ray. Radio continuum radiation is emitted by the ionized gas component of the nebulae. The molecular and solid-state components contribute to radiations in the infrared and submillimeter-wave regions (see section below). The optical region is dominated by atomic line emissions from ionized gas. A million-degree bubble of extremely low-density gas created by the interacting winds process produces emissions in the X-ray.
Planetary nebulae as a phase of stellar evolution
Although the existence of planetary nebulae has been known for over 200 years, their origin was not understood until relatively recently. In 1956, the Russian astronomer Josif Shklovsky suggested that planetary nebulae represent objects in the late stages of stellar evolution and are descendants of red giants and precursors of white dwarfs (Shklovsky 1978). This hypothesis was supported by U.S. astronomers George Abell and Peter Goldreich who in 1966 suggested that the nebulae represent the ejected atmospheres of red giants and that the central stars are remnants of red giant cores. However, the physical reason for the ejection was not known. In 1970, Polish astronomer Bohdan Paczynski established that the central stars of planetary nebulae are cores of asymptotic giant branch stars (a type of very old red giants) and that they generate energy by nuclear burning of hydrogen in a shell above the core (Paczynski 1970). The evolutionary tracks calculated by Paczynski, extended by further calculations by German astronomer Detlef Schönberner (Schönberner 1981), define the precise path of planetary nebulae evolution in the scheme of the late stages of stellar evolution.
The problem of nebula ejection was solved in 1978 when Canadian astronomers Sun Kwok, Chris Purton and Pim FitzGerald traced the origin of the nebulae to the stellar winds from asymptotic giant branch stars and showed that the shell-like structure of the nebulae is the result of a “snow-plow” effect of the collision of two stellar winds. This “interacting winds model” has been widely used to model the morphological structure of planetary nebulae (Balick & Frank 2002). Our current understanding is that stars born with masses in the range of 1 to 8 times the mass of the Sun will evolve through the planetary nebulae stage. Since theses stars constitute about 95% of the entire galactic stellar population, planetary nebulae, not supernovae, are the eventual fate of most stars. A more extensive description of our modern understanding of the origin and evolution of planetary nebulae can be found in this book.
Planetary nebulae are rapidly evolving objects. From the time the star leaves the asymptotic giant branch to the time it burns out the available hydrogen fuel and gradually fades to become a white dwarf, the total time is several tens of thousands of years. Since typical lifetimes of stars are measured in billion of years, planetary nebulae therefore represent a short phase of glory near the end of a star’s life. The transition from the asymptotic giant branch to the beginning of photoionization, i.e. when the temperature of the central star reaches 25,000 degrees, is about several thousand years. During this phase, the nebula does not shine by line emission, but only through reflected light from the central star. Objects in this transition period, known as “proto-planetary nebulae” (Figure 2), represented a missing link in our understanding of planetary nebulae evolution. Proto-planetary nebulae were only discovered in the 1980s and the observations of these objects provide much needed information on the morphological, dynamical, and chemical evolution of planetary nebulae.
Morphology of planetary nebulae
Planetary nebulae have a variety of morphological structures, making them not only beautiful to look at but also challenging to understand. The high sensitivity and resolving power provided by the Hubble Space Telescope have greatly expanded our views of planetary nebulae (see pictures in Kwok 2001). Although many planetary nebulae have shell-like structures similar to that of the Ring Nebula, some show butterfly-like structures with a pair of bipolar lobes (Figure 3). Other well-known bipolar planetary nebulae include NGC 6302 in Scorpius, Hubble 5 in Sagittarius, NGC 6537 in Sagittarius, etc. Current thinking is that the bipolar lobes are created by a high-speed, collimated stellar wind, although the physical origin of the directional nature of this wind is not understood. Astronomers now believe that transformation from a spherical to bipolar form takes place very rapidly, probably within a period of several hundred years.
Observations by the Hubble Space Telescope have revealed that many planetary nebulae have multiple layers, and these are labeled as “shells”, “crowns” and “haloes”. Computer modeling (Steffen & Schoenberner 2006) has demonstrated that these multiple shell structures are the dynamical consequence of interacting winds (see previous section), as well as the changing photoionization effects of the evolving central star. Other minor morphological structures include arcs, rings, jets, ansaes, and multiple lobes and they probably reflect the episodic and/or direction-changing nature of the stellar winds (Figure 4).
The rich morphological structures of planetary nebulae suggest that there are complex dynamical processes at work, involving, e.g. ejection, collimation, and precession. An improved understanding of the physical mechanisms behind these morphological structures will help astronomers understand more distant phenomena such as active galactic nuclei.
Discovery and distribution of planetary nebulae
Planetary nebulae are usually identified by their emission-line spectrum. Most recent discoveries of new planetary nebulae are the result of imaging surveys of the Galaxy using a narrow-band filter around the Hα line of hydrogen (Parker et al. 2006). This allows emission nebulae to be easily separated from stars. There are approximately 2,500 planetary nebulae catalogued in the Milky Way Galaxy, but because of obscuration of galactic dust and incompleteness of surveys, the total population is expected to be about ten times this number. Due to spectral similarities, planetary nebulae can be confused with other emission-line objects such as HII regions (nebulae associated with young stars), symbiotic stars or novae (both are results of binary star evolution). Most planetary nebulae in the Milky Way Galaxy are distributed around the Galactic plane, as their progenitors descend from an intermediate-mass stellar population.
Since the light from planetary nebulae is concentrated in emission lines, they can be easily distinguished from stars even in galaxies far away. Thousands of planetary nebulae have now been catalogued in external galaxies as far away as 100 million light years away. Planetary nebulae have been extensively used as standard candles to determine the age and size of the Universe (Jacoby 1989). By tracking the velocity patterns of planetary nebulae in galaxies, astronomers can also map out the distribution of dark matter in galaxies.
Chemistry of planetary nebulae
The optical spectra of planetary nebulae show emission lines of many heavy elements, many of which recently synthesized by nuclear processes during the preceding asymptotic giant branch phase. Planetary nebulae therefore are regarded as important agents in the spread of heavy elements in the Galaxy. Recent observations by infrared and millimeter-wave telescopes have found that planetary nebulae contain, in addition to atoms, molecules and solid-state particles. In fact, some planetary nebulae emit most of their energy from their solid-state component in the form of far infrared radiation. Gas-phase molecules can be identified through their rotational or vibrational transitions and solid-state particles through their lattice vibrational modes. Most interestingly, planetary nebulae are found to contain complex organic compounds of aromatic and aliphatic structures ( Figure 5). Comparison of the spectra of planetary nebulae at different stages of evolution show that these compounds are synthesized quickly over timescales of the order of hundreds of years (Kwok 2004). How such organic matter is made and what effect it has (e.g., on the solar system) from being distributed throughout the Galaxy are topics of high current interest.
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