Planetary Nebulae

[Messier Planetary Nebulae] Click icon to view a planetary nebula from Messier's catalog

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The icon shows the Helix Nebula NGC 7293, the nearest planetary nebula.

Planetary Nebulae are typically small, faint nebulae, found between the stars as comparatively rare objects. Once thought to be strange, odd objects, evidence has been found that the vast majority of stars will produce such an object when they are evolving from their late Red Giant state to their final White Dwarf state, and their rareness is just due to the short lifetime of this state of evolution.

When a star like our Sun comes to age, having longly burned away all the hydrogen to helium in its core in its main sequence phase, and in a later evolutionary state following the red giant stadium (the "Horizontal Branch" state, for their places in the Color-Magnitude Diagram (CMD) or Hertzsprung-Russell Diagram (HRD)), also the helium to carbon and oxygen, its nuclear reactions come to an end in its core, while helium burning goes on in a shell. This process makes the star expanding, and causes its outer layers to pulsate as a long-periodic Mira-type variable, also called AGB star (for "Asymptotic Giant Branch" star, also for their places in the CMD or HRD), which becomes more and more unstable, and loses mass in strong stellar winds. The instability finally causes the ejection of a significant part of the star's mass in an expanding shell. The stellar core remains as an extremely hot, small central star, which emits high energetic radiation. At this stage, the central stars of planetary nebulae (CSPN), or Planetary Nebulae Nuclei (PNN), are among the hottest stars observed in the universe. Observation shows that most of the central stars are within a narrow mass range of 0.5 to 0.7 solar masses.

The expanding gas shell is excited to shine by the high-energy radiation emitted from the central star. These hot stars are also source of fast stellar winds (order 1,000 km/s), outflowing material which is rammed into and colliding and interacting with the material of the shell, which is expanding at the much slower rate of the old wind of the Red Giant phase (order 10 km/s). The shining gas shell is then visible as a Planetary Nebula, occurring in a variety of interesting morphologies formed by the interacting stellar winds (ISW; Kwok 1978). Most of the bright planetary nebulae have masses of 0.1 to 0.2 solar masses. This flexible model can well explain the observed morphological variety of these objects (see e.g., Balick 1987). In deep exposures, the matter ejected in the Mira-variable or AGB state can be detected as an extended halo surrounding many planetary nebulae. A good reference in particular on the physics of these interesting objects is given in Kwok (2000), a more visual and popular review in Kwok (2001).

The first planetary nebula ever seen by a human was the Dumbbell Nebula M27 in Vulpecula, which was discovered by Charles Messier on July 12, 1764. Charles Messier also discovered the second of these objects, the Ring Nebula M57 in Lyra in January 1779; this object was the first to be compared to a "fading" planet by Antoine Darquier in the same year. Following were the subsequent discoveries of the Little Dumbbell Nebula M76 in Perseus in September 1780, and the Owl Nebula M97 in Ursa Major in February 1781 by Pierre Méchain.

These four planetaries are the only ones which found their way into Messier's catalog, and all which were known to summer 1782, before William Herschel entered the scene. Among his early discoveries, even before he really started his comprehensive scanning the of the deep sky with large telescopes, was that of another famous planetary nebula, the Saturn Nebula NGC 7009 (his H IV.1) in Aquarius, in September 1782.

William Herschel eventually invented the name "Planetary Nebula" for these objects in his classification of nebulae in 1784 or 1785, because he found them to resemble the planet newly discovered by him, Uranus. On November 13, 1790, Herschel found the planetary nebula NGC 1514 (his H IV.69), which has a very bright central star; thus he became convinced that the planetary nebulae were nebulous material (gas or dust) associated with a central star, and not unresolved clusters as he and others had thought previously.

William Herschel classified 79 of his objects as planetary nebulae, but only 20 of them actually are of this type, together with 13 others which he had classified differently, bringing his total to 33 to 1794. The next couple of discoveries was made in the 1820s, when Karl Ludwig Harding and William Herschel's son John Herschel discovered each one, Friedrich Georg Wilhelm Struve found two, and James Dunlop five of these objects; John Herschel added another 16 southern planetaries between 1834 and 1837, so that his General Catalogue (GC) of 1864 contains a total of 62 planetary nebulae - two of them having two entry numbers each (M76 = GC 385/386 = NGC 650/651, and H II.316/317 = GC 1519/1520 = NGC 2371/2372). J.L.E. Dreyer's 1877 addendum to the GC lists an additional 6, plus one rediscovery (duplication). His NGC catalog of 1888 contains 94 (again with two entry numbers for each of the two objects listed above, plus a total of two duplications), and the IC catalog of 1895 and 1907 has an additional 35 IC objects; the IC also contains 3 more duplicate entry numbers for planetary nebulae (plus one for a knot in another one, IC 4677 in NGC 6543). Together with the only one discovered but not included into either of these to the time of IC II (BD+30 3639, "Campbell's Hydrogen Star"), this was a total of 130 objects to 1907. During the 1910s and 1920s, only few new planetary were found, a recent count of the present author brought up a number of 153 planetary nebulae known to 1930 (not all recognized as such objects at that time). A notable summary, the first catalog devoted solely to planetary nebulae and a photographic atlas, was published by Heber D. Curtis in 1918 (Curtis 1918). Eventually, Steven Hynes' book (Hynes 1991) lists a total of 1340 planetaries known to about 1991. Today, databases like SIMBAD contain over 3,000 planetary nebulae (early 2014).

The radiation emitted by the planetary nebula is remarkable because of its peculiar spectrum, as was discovered for the planetary nebula NGC 6543 (also known as Cat Eye Nebula) by the English amateur astronomer and pioneer of astronomical spectroscopy, William Huggins, on August 29, 1864 and published in the Philosophical Transactions of the Royal Society of 1864 and later in the Nineteenth Century Review of June 1897 - according to Hynes (1991):

As expected for gaseous emission nebulae, the spectra of planetaries consist of emission lines, but 90 to 95 % of the visible light are emitted in one single emission line only ! This `Chief Nebular Line' occurs at 500.7 nm (5007 Angstrom), in the green part of the spectrum. It is this circumstance that planetary nebula brightnesses differ significantly if determined with various methods: These objects are often considerably brighter (up to 2 magnitudes, a factor of more than 6) visually than photographically, because the 5007 Angstrom line lies close to the highest sensitivity of the human eye. Also, as films are often less sensitive in the green part of the spectrum, it is difficult to get a good "true color" image of planetary nebulae. As this spectral line at 5007 Angstrom could not be assigned to a known element at the time of its discovery, Huggins suspected it must be emitted from a previously unknown substance, which was called "nebulium". It was not before 60 years later that the "nebulium" spectrum was identified (by the American astro-physicist Ira S. Bowen) to be caused by forbidden lines of double ionized "normal" oxygen, "[O III]" (with the square brackets).

Besides the "nebulium" [OIII] lines, other emission lines occur in the planetary nebula spectra in weaker intensity. These include more forbidden lines of ionized oxygen, neon, nitrogen, and other abundant elements, as well as permitted lines of hydrogen and helium, as well as fluorescence O III lines in case of strong He II emission. Also, a very week continuum underlies the line spectrum, which is due to interactions of electrons with ions.

Our Sun will probably reach this state of evolution at an age of about 10-13 billion years; as it is now only about 4.7 billion years old, we have probably some time left until this event happens.

The planetary nebula has only a short life compared to the time scales in stellar evolution, being visible only a few thousands or 10,000s of years, and then fading out as its matter is spread in the cosmic environment, enriching the interstellar matter with carbon, oxygen, and other elements. Its central star cools down to a white dwarf. This is the reason that, although there are very many sunlike stars among the hundreds of billions in our Milky Way galaxy, which now come into age (especially in the globular clusters), the number of planetary nebulae in the Galaxy is of order of 10,000s only: Accounting for a number of uncertainties, estimates have been given that there are probably only between 15,000 and 60,000 planetary nebulae (of which only about 3,000 could yet be detected, the other being hidden behind obscuring interstellar dust). Of the about 150 globular clusters with each several 100,000 stars, planetary nebulae have been discovered only in 4 of them, namely Pease 1 in M15, IRAS 18333-2357 in M22, and the two recently discovered planetary nebulae Jafu 1 and Jafu 2 in globular clusters Palomar 6 and NGC 6441, respectively (Jacoby and Fullton 1997; also see George Jacoby's Planetary Nebula gallery).

There are also only few, if any, planetary nebulae found in open star clusters. The reason for this fact is again the short lifetime of these objects. Once it had been thought that, as planetary nebulae occur only late in the life of a star (for stars of an initial mass of 1 solar mass, after about 10 billion years), and these stellar swarms tend to dissolve in times shorter than that needed for a star to evolve in a planetary nebula (typically, less than 1 billion years); low-mass stars of less than about 3 solar masses have considerably more than 1 billion years of lifetime in their hydrogen-burning phase. With the discovery of a number of white dwarf stars in young clusters, such as the Pleiades, M45, of only about 100 million years of age, it became clear that stars of initial masses of at least up to 6 solar masses will develop to White Dwarfs: These stars must have started their life with a high mass so that they evolved rapidly, but lost a significant portion of their mass during their lifes, probably in the form of strong stellar winds, and must have gone through a planetary nebula stage. The upper limit for the initial mass of a star to develop to White Dwarf is now thought to be at about 8 to 9 solar masses (more massive stars are thought to end up in a supernova explosion).

It seems that because of the short lifetime of this stage, there is only one faint planetary nebula, PHR J1315-6555 (PN G305.3-03.1), which was discovered to be a member of open cluster ESO 96-SC04 (cluster discovered 1967, nebula around 2000). The more wellknown cases of the planetary nebula NGC 2438 which is observed in the same direction as M46, and NGC 2818. projected on the inconspicuous, rather old open cluster, NGC 2818A, are apparently chance alignments.

The cooling process of the white dwarf goes on until all thermal energy is radiated, and the star approaches a stable "end state" as "black dwarf" after many billion years - the universe is probably still much too young to contain any "cooled-out" black dwarf.

Planetary nebula are often typized for their appearance, according to the Vorontsov-Velyaminov scheme:

1 Stellar Image
2 Smooth disk (a, brighter toward center; b, uniform brightness; c, traces of a ring structure)
3 Irregular disk (a, very irregular brightness distribution; b, traces of ring structure)
4 Ring structure
5 Irregular form, similar to a diffuse nebula
6 Anomalous form
More complex structures are characterized by combinations such as "4+2" (ring and disk), or "4+4" (two rings). The four Messier planetary nebulae are classified as of "VV" types "4+3a" (M27), "4+3" (M57), "3+6" (M76), and "3a" (M97), respectively.

Another interesting, though rarely used classification scheme had been proposed by Stuart R. Pottasch (Pottasch 1984), the "excitation classes:" Running from excitation class 1 to 10, with increasing degree of ionization of the various chenical elements in the nebula. For example, class I has considerably stronger lines from O II (single ionized O) than from O III (double ionized O), He II comes in with class VI, and Ne V with class 8. The excitation class of the nebula is related to the properties of the central star, in particular surface gravity and surface effective temperature. Messier's planetary nebulae are classified by Pottasch in excitation classes as follows: M27, class 7p to 8p; M57, class 6p to 8p; M76, class 8p; and M97, unclassified. Grigor Gurzadyan (Gurzadyan 1991) uses a slightly modified classification, running from class 1 to 12, with increasing degree of ionization. He classifies the Messier planetary nebulae as: M27, M57, M76, all class 10; and M97, class 8.

All individual planetary nebulae mentioned in this page, including the four Messier objects, are members of our Milky Way Galaxy. Planetary nebulae have also been discovered in other galaxies with large telescopes, including the Large and the Small Magellanic Cloud, the Andromeda Galaxy M31, M33, M32, and NGC 6822 as well as other galaxies in the Local Group and beyond. It is very probable that they are common in all galaxies.

Messier's Planetary Nebulae: M27, M57, M76, M97.


Find more information and beautiful image collections of planetary nebulae, including many which are not in Messier's catalogs, at the following sites: For "Nebulium": (notify me if you know further cool pages)


Further Reading and Reference:

Diffuse Nebulae

Supernova Remnants

Dark Nebulae

Hartmut Frommert
Christine Kronberg

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Last Modification: October 22, 2018