>> Messier's Planetary Nebulae; Links
The icon shows the Helix Nebula NGC 7293,
the nearest planetary nebula.
The expanding gas shell is excited to shine by the high-energy radiation emitted from the central star; the material in the shell is moreover accelerated so that the expansion gets faster by the time. The shining gas shell is then visible as a planetary nebula. In deep exposures, the matter ejected in the Mira-variable state can be detected as an extended halo surrounding many planetary nebulae.
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. The comparison to a "fading" planet was first pronounced by Antoine Darquier, the discoverer of the second of these objects, the Ring Nebula M57 in Lyra; he found it shortly before Messier when both were tracing the same comet in January, 1779. 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 Mechain.
These four planetaries are the only ones which found their way into Messier's catalog, and all which where known to summer 1782, before William Herschel started his comprehensive scanning the of the deep sky with large telescopes. One of his first findings within this survey 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.
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, not in Messier's catalog) 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):
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" oxygene, "[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 oxygene, neon, nitrogene, and other abundant elements, as well as permitted lines of hydrogene 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, oxygene, 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), there are only about 10,000 planetary nebula (of which only about 1,500 could yet be detected, the other being hidden behind obscuring interstellar dust); of the 150 globular clusters with each several 100,000 stars, planetary nebulae have been discovered only in 4 (or perhaps 5) of them, namely Pease 1 in M15 (which may even contain a second one according to Peterson, 1976, but this one was never confirmed since), IRAS 18333-2357 in M22, the probable member Peterson 1 lying 3 arc minutes from globular cluster NGC 6401 (Peterson 1977), the recently discovered planetary in NGC 6441 (Jacoby and Fullerton 1997; also see George Jacoby's Planetary Nebula gallery), and a recently found planetary nebula in the faint globular cluster Palomar 6.
As planetary nebulae occur only late in the life of a star, they are usually absent in open star clusters, because these stellar swarms tend to dissolve in times much shorter than that needed for a star to evolve in a planetary nebula: Only few open clusters live longer than a billion years, while planetary nebulae occur only for stars of less than 3 solar masses (the more massive explode as supernovae). Those low-mass stars however have considerably more than 1 billion years of lifetime on the mean sequence alone while they burn up their hydrogene. These arguments are however questionable, as a number of white dwarf stars has been discovered in young clusters, as the Pleiades, M45: 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.
It seems that because of the short lifetime of this stage, there is only one planetary nebula, NGC 2818, which was discovered to be a member of an inconspicuous, rather old open cluster, NGC 2818A. The more wellknown case of the planetary nebula NGC 2438 which is observed in the same direction as M46 is apparently a chance alignment.
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:
More complex structures are characterized by combinations such as "4+2" (ring and disk), or "4+4" (two rings).
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
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.
Last Modification: 13 May 1999, 10:00 MET