A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding glowing shell of ionized gas ejected from old red giant stars late in their lives. The word 'nebula' is Latin for mist or cloud and the term 'planetary nebula' is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects appeared to him to resemble the rounded shapes of planets. Herschel's name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years.
A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red giant phase, the outer layers of the star are expelled by strong stellar winds. Eventually, after most of the red giant's atmosphere is dissipated, the exposed hot, luminous core emits ultraviolet radiation to ionize the ejected outer layers of the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, appearing as a bright coloured planetary nebula at several discrete visible wavelengths.
Planetary nebulae may play a crucial role in the chemical evolution of the Milky Way, returning material to the interstellar medium from stars where elements, the products of nucleosynthesis (such as carbon, nitrogen, oxygen and neon), have been created. Planetary nebulae are also observed in more distant galaxies, yielding useful information about their chemical abundances.
In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About one-fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms which produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may play a role.
Planetary nebulae are generally faint objects; none are visible to the naked eye. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula. It was observed by Charles Messier in 1764 and listed as M27 in his catalogue of nebulous objects. To early observers with low-resolution telescopes, M27 and subsequently discovered planetary nebulae somewhat resembled the giant planets like Uranus. William Herschel, discoverer of Uranus, eventually coined the term 'planetary nebula' for them. At first Herschel thought the objects were stars surrounded by material that was condensing into planets rather than what is known to be evidence of dead stars that have incinerated any orbiting planets.
The nature of planetary nebulae was unknown until the first spectroscopic observations were made in the mid-19th century. Using a prism to disperse their light, William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects. On August 29, 1864, Huggins was the first to analyze the spectrum of a planetary nebula when he observed NGC 6543. His observations of stars showed that their spectra consisted of a continuum of radiation with many dark lines superimposed on them. He later found that many nebulous objects such as the Andromeda Nebula (as it was then known) had spectra that were quite similar. These nebulae were later shown to be galaxies.
However, when Huggins looked at the Cat's Eye Nebula, he found a very different spectrum. Rather than a strong continuum with absorption lines superimposed, the Cat's Eye Nebula and other similar objects showed only a small number of emission lines. The brightest of these was at a wavelength of 500.7 nanometres, which did not correspond with a line of any known element. At first it was hypothesized that the line might be due to an unknown element, which was named nebulium. A similar idea had led to the discovery of helium through analysis of the Sun's spectrum in 1868.
While helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. In the early 20th century Henry Norris Russell proposed that rather than being a new element, the line at 500.7 nm was due to a familiar element in unfamiliar conditions.
Physicists showed in the 1920s that in gas at extremely low densities, electrons can populate excited metastable energy levels in atoms and ions which at higher densities are rapidly de-excited by collisions. Electron transitions from these levels in nitrogen and oxygen ions (O+, O2+ or OIII, and N+) give rise to the 500.7 nm emission line and others. These spectral lines, which can only be seen in very low density gases, are called forbidden lines. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas.
The central stars of planetary nebulae are very hot. Only once a star has exhausted most of its nuclear fuel can it collapse to such a small size. Planetary nebulae came to be understood as a final stage of stellar evolution. Spectroscopic observations show that all planetary nebulae are expanding. This led to the idea that planetary nebulae were caused by a star's outer layers being thrown into space at the end of its life.
Towards the end of the 20th century, technological improvements helped to further the study of planetary nebulae. Space telescopes allowed astronomers to study light wavelengths outside those that the earth's atmosphere transmits. Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures, densities and elemental abundances. Charge-coupled device technology allowed much fainter spectral lines to be measured accurately than had previously been possible. The Hubble Space Telescope also showed that while many nebulae appear to have simple and regular structures when observed from the ground, the very high optical resolution achievable by telescopes above the Earth's atmosphere reveals extremely complex structures.
Stars greater than 8 solar masses (M⊙) will likely end their lives in dramatic supernovae explosions, while planetary nebulae seemingly only occur at the end of the lives of intermediate and low mass stars between 0.8 M⊙ to 8.0 M⊙.  Progenitor stars that form planetary nebulae will spend most of their lifetimes converting their hydrogen into helium in the star's core by nuclear fusion at about 15 million K. This generated energy creates outward pressure from fusion reactions in the core, equally balancing the crushing inward pressures of the star's gravity. Hence, all single intermediate to low-mass stars on the main sequence can last for tens of millions to billions of years.
When the hydrogen source in the core starts to diminish, gravity starts compressing the core, causing a rise in temperature to about 100 million K. Such higher core temperatures then make the star's cooler outer layers expand to create much larger red giant stars. This end phase causes a dramatic rise in stellar luminosity, where the released energy is distributed over a much larger surface area, even though the average surface temperature is lower. In stellar evolution terms, stars undergoing such increases in luminosity are known as asymptotic giant branch stars (AGB).
For the more massive asymptotic giant branch stars that form planetary nebulae, whose progenitors exceed about 3M⊙, their cores will continue to contract. When temperatures reach about 100 million K, the available helium nuclei fuse into carbon and oxygen, so that the star again resumes to radiate energy, temporarily stopping the core's contraction. This new helium burning phase (fusion of helium nuclei) forms a growing inner core of inert carbon and oxygen. Above it is a thin helium-burning shell, surrounded in turn by a hydrogen-burning shell. However, this new phase lasts only 20,000 years or so, a short period compared to the entire lifetime of the star.
In either scenario, the venting of atmosphere continues unabated into interstellar space, but when the outer surface of the exposed core reaches temperatures exceeding about 30,000 K, there are enough emitted ultraviolet photons to ionize the ejected atmosphere, causing the gas to shine as a planetary nebula.
After a star passes through the asymptotic giant branch (AGB) phase, the short planetary nebula phase of stellar evolution begins as gases blown away from the central star at speeds of a few kilometers per second. The central star is the remnant of its AGB progenitor, an electron-degenerate carbon-oxygen core that has lost most of its hydrogen envelope due to mass loss on the AGB. As the gases expand, the central star undergoes a two-stage evolution, first growing hotter as it continues to contract and hydrogen fusion reactions occur in the shell around the core and then slowly cooling once the hydrogen shell is exhausted through fusion and mass loss. In the second phase, it radiates away its energy and fusion reactions cease, as the central star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. During the first phase, the central star maintains constant luminosity, while at the same time it grows ever hotter, eventually reaching temperatures around 100,000 K. In the second phase, it cools so much that it does not give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf, and the expanding gas cloud becomes invisible to us, ending the planetary nebula phase of evolution. For a typical planetary nebula, about 10,000 years passes between its formation and recombination of the star.
Planetary nebulae play a very important role in galactic evolution. The early universe consisted almost entirely of hydrogen and helium, but stars create heavier elements via nuclear fusion. The gases of planetary nebulae thus contain a large proportion of elements such as carbon, nitrogen and oxygen, and as they expand and merge into the interstellar medium, they enrich it with these heavy elements, collectively known as metals by astronomers.
Subsequent generations of stars which form from the nebular remains will then have a higher initial content of heavier elements. Even though the heavy elements will still be a very small component of the star, they have a marked effect on its evolution. Stars which formed very early in the universe and contain small quantities of heavy elements are known as Population II stars, while younger stars with higher heavy element content are known as Population I stars (see stellar population).
A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally from 100 to 10,000 particles per cm3. (The Earth's atmosphere, by comparison, contains 2.5×1019 particles per cm3.) Young planetary nebulae have the highest densities, sometimes as high as 106 particles per cm3. As nebulae age, their expansion causes their density to decrease. The masses of planetary nebulae range from 0.1 to 1 solar masses.
Radiation from the central star heats the gases to temperatures of about 10,000 K. The gas temperature in central regions is usually much higher than at the periphery reaching 16,000–25,000 K. The volume in the vicinity of the central star is often filled with a very hot (coronal) gas having the temperature of about 1,000,000 K. This gas originates from the surface of the central star in the form of the fast stellar wind.
Nebulae may be described as matter bounded or radiation bounded. In the former case, there is not enough matter in the nebula to absorb all the UV photons emitted by the star, and the visible nebula is fully ionized. In the latter case, there are not enough UV photons being emitted by the central star to ionize all the surrounding gas, and an ionization front propagates outward into the circumstellar envelope of neutral atoms.
Numbers and distribution
About 3000 planetary nebulae are now known to exist in our galaxy, out of 200 billion stars. Their very short lifetime compared to total stellar lifetime accounts for their rarity. They are found mostly near the plane of the Milky Way, with the greatest concentration near the galactic center.
Only about 20% of planetary nebulae are spherically symmetric (for example, see Abell 39). A wide variety of shapes exist with some very complex forms seen. Planetary nebulae are classified by different authors into: stellar, disk, ring, irregular, helical, bipolar, quadrupolar, and other types, although the majority of them belong to just three types: spherical, elliptical and bipolar. Bipolar nebulae are concentrated in the galactic plane, likely produced by relatively young massive progenitor stars; and bipolars in the galactic bulge appear to prefer orienting their orbital axes parallel to the galactic plane. On the other hand, spherical nebulae are likely produced by the old stars similar to the Sun.
The huge variety of the shapes is partially the projection effect—the same nebula when viewed under different angles will appear different. Nevertheless, the reason for the huge variety of physical shapes is not fully understood. Gravitational interactions with companion stars if the central stars are double stars may be one cause. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. It has been determined that the more massive stars produce more irregularly shaped nebulae. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesized that the fields might be partly or wholly responsible for their remarkable shapes.
Membership in clusters
Planetary nebulae have been detected as members in four globular clusters: Messier 15, Messier 22, NGC 6441 and Palomar 6. However, there is currently only one established case of a planetary nebula discovered in an open cluster. The cases of NGC 2348 in Messier 46, and NGC 2818, are often cited as bona fide instances, however, they are instead line-of-sight coincidences.
Partly because of their small total mass, open clusters have relatively poor gravitational cohesion. Consequently, open clusters tend to disperse after a relatively short time, typically from 100 to 600 million years.
Theoretical models predict that planetary nebulae can form from main-sequence stars of between one and eight solar masses, which puts the progenitor star's age at greater than 40 million years. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a planetary nebula within. For one reason, the planetary nebula phase for more massive stars is on the order of thousands of years—a blink of the eye in cosmic terms.
Current issues in planetary nebula studies
The distances to planetary nebulae are generally poorly determined. It is possible to determine distances to the nearest planetary nebula by measuring their expansion rates. High resolution observations taken several years apart will show the expansion of the nebula perpendicular to the line of sight, while spectroscopic observations of the Doppler shift will reveal the velocity of expansion in the line of sight. Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula.
The issue of how such a diverse range of nebular shapes can be produced is a debatable topic. It is theorised that interactions between material moving away from the star at different speeds gives rise to most observed shapes. However, some astronomers postulate that close binary central stars might be responsible for the more complex and extreme planetary nebulae. Several have been shown to exhibit strong magnetic fields, and their interactions with ionized gas could explain some planetary nebulae shapes.
There are two main methods of determining metal abundances in nebulae. These rely on recombination lines and collisionally excited lines. Large discrepancies are sometimes seen between the results derived from the two methods. This may be explained by the presence of small temperature fluctuations within planetary nebulae. The discrepancies may be too large to be caused by temperature effects, and some hypothesize the existence of cold knots containing very little hydrogen to explain the observations. However, such knots have yet to be observed.
Preceding evolutionary stage:
Subsequent evolutionary stage:
- Miszalski et al. 2011
- Frankowski & Soker 2009, pp. 654–8
- SEDS 2013
- Hubblesite.org 1997
- Kwok 2000, pp. 1–7
- Moore 2007, pp. 279–80
- Malin, David (1993), A View of the Universe, Cambridge, Massachusetts: Sky Publishing Corporation, p. 168, ISBN 0876541015
- Huggins & Miller 1864, pp. 437–44
- Bowen 1927, pp. 295–7
- Gurzadyan 1997
- Kwok 2005, pp. 271–8
- Hora et al. 2004, pp. 296–301
- Kwok et al. 2006, pp. 445–6
- Reed et al. 1999, pp. 2430–41
- Aller & Hyung 2003, p. 15
- Krause 1961, p. 187
- Maciel, Costa & Idiart 2009, pp. 127–37
- Harpaz 1994, pp. 55–80
- Harpaz 1994, pp. 99–112
- "Hubble Offers a Dazzling Necklace". Picture of the Week. ESA/Hubble. Retrieved 18 August 2011.
- Kwok 2000, pp. 199–207
- Marochnik, Shukurov & Yastrzhembsky 1996, pp. 6–10
- Osterbrock & Ferland 2005, p. 10
- Gurzadyan 1997, p. 238
- Gurzadyan 1997, pp. 130–7
- Osterbrock & Ferland 2005, pp. 261–2
- Osterbrock & Ferland 2005, p. 207
- Parker et al. 2006, pp. 79–94
- Majaess, Turner & Lane 2007, pp. 1349–60
- Jacoby, Ferland & Korista 2001, pp. 272–86
- Kwok & Su 2005, pp. L49–52
- Kwok 2000, pp. 89–96
- Rees & Zijlstra 2013
- Morris 1990, pp. 526–30
- SpaceDaily Express 2005
- Jordan, Werner & O'Toole 2005, pp. 273–9
- Parker 2011, pp. 1835–1844
- Kiss et al. 2008, pp. 399–404
- Mermilliod et al. 2001, pp. 30–9
- Allison 2006, pp. 56–8
- "Cosmic Sprinklers Explained". ESO Press Release. Retrieved 13 February 2013.
- R. Gathier. "Distances to Planetary Nebulae". ESO Messanger. Retrieved 31 May 2014.
- Soker 2002, pp. 481–6
- Gurzadyan 1997, p. 424
- Liu et al. 2000, pp. 585–587
- Aller, Lawrence H.; Hyung, Siek (19–23 November 2001) , "Historical Remarks on the Spectroscopic Analysis of Planetary Nebulae (invited review)", in Kwok, Sun; Dopita, Michael; Sutherland, Ralph, Proceedings of the 209th Symposium of the International Astronomical Union, Planetary Nebulae: Their Evolution and Role in the Universe (Canberra, Australia: Astronomical Society of the Pacific) 209: 15, Bibcode:2003IAUS..209...15A
- Allison, Mark (2006), Star clusters and how to observe them, Birkhäuser, pp. 56–8, ISBN 978-1-84628-190-7
- Bowen, I. S. (October 1927), "The Origin of the Chief Nebular Lines", Publications of the Astronomical Society of the Pacific 39: 295–7, Bibcode:1927PASP...39..295B, doi:10.1086/123745
- Frankowski, Adam; Soker, Noam (November 2009), "Very late thermal pulses influenced by accretion in planetary nebulae", New Astronomy 14 (8): 654–8, arXiv:0903.3364, Bibcode:2009NewA...14..654F, doi:10.1016/j.newast.2009.03.006, "A planetary nebula (PN) is an expanding ionized circumstellar cloud that was ejected during the asymptotic giant branch (AGB) phase of the stellar progenitor."
- Gurzadyan, Grigor A. (1997), The Physics and dynamics of planetary nebulae, Springer, ISBN 978-3-540-60965-0
- Harpaz, Amos (1994), Stellar Evolution, A K Peters, Ltd., ISBN 978-1-56881-012-6
- Hora, Joseph L.; Latter, William B.; Allen, Lori E.; Marengo, Massimo; Deutsch, Lynne K.; Pipher, Judith L. (September 2004), "Infrared Array Camera (IRAC) Observations of Planetary Nebulae", Astrophysical Journal Supplement Series 154 (1): 296–301, Bibcode:2004ApJS..154..296H, doi:10.1086/422820
- Hubble Witnesses the Final Blaze of Glory of Sun-Like Stars, Hubblesite.org, December 17, 1997, retrieved 2008-08-09
- Huggins, W.; Miller, W. A. (1864), "On the Spectra of some of the Nebulae", Philosophical Transactions of the Royal Society of London 154: 437–44, Bibcode:1864RSPT..154..437H, doi:10.1098/rstl.1864.0013
- Iliadis, Christian (2007), Nuclear physics of stars. Physics textbook, Wiley-VCH, pp. 18, 439–42, ISBN 978-3-527-40602-9
- Jacoby, George. H.; Ferland, Gary. J.; Korista, Kirk T. (2001), "The Planetary Nebula A39: An Observational Benchmark for Numerical Modeling of Photoionized Plasmas", The Astrophysical Journal 560 (1): 272–86, Bibcode:2001ApJ...560..272J, doi:10.1086/322489
- Jordan, S.; Werner, K.; O'Toole, S. J. (March 2005), "Discovery of magnetic fields in central stars of planetary nebulae", Astronomy & Astrophysics 432 (1): 273–9, arXiv:astro-ph/0501040, Bibcode:2005A&A...432..273J, doi:10.1051/0004-6361:20041993
- Kiss, L. L.; Szabó, Gy. M.; Balog, Z.; Parker, Q. A.; Frew, D. J. (November 2008), "AAOmega radial velocities rule out current membership of the planetary nebula NGC 2438 in the open cluster M46", Monthly Notices of the Royal Astronomical Society 391 (1): 399–404, arXiv:0809.0327, Bibcode:2008MNRAS.391..399K, doi:10.1111/j.1365-2966.2008.13899.x
- Krause, Arthur (1961), Astronomy, Oliver and Boyd, p. 187
- Kwok, Sun (2000), The origin and evolution of planetary nebulae, Cambridge University Press, ISBN 0-521-62313-8 (Chapter 1 can be downloaded here.)
- Kwok, Sun (June 2005), "Planetary Nebulae: New Challenges in the 21st Century", Journal of the Korean Astronomical Society 38 (2): 271–8, Bibcode:2005JKAS...38..271K, doi:10.5303/JKAS.2005.38.2.271
- Kwok, Sun; Su, Kate Y. L. (December 2005), "Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881", The Astrophysical Journal 635 (1): L49–52, Bibcode:2005ApJ...635L..49K, doi:10.1086/499332, "We report the discovery of multiple two-dimensional rings in the quadrupolar planetary nebula NGC 6881. As many as four pairs of rings are seen in the bipolar lobes, and three rings are seen in the central torus. While the rings in the lobes have the same axis as one pair of the bipolar lobes, the inner rings are aligned with the other pair. The two pairs of bipolar lobes are likely to be carved out by two separate high-velocity outflows from the circumstellar material left over from the asymptotic giant branch (AGB) wind. The two-dimensional rings could be the results of dynamical instabilities or the consequence of a fast outflow interacting with remnants of discrete AGB circumstellar shells."
- Kwok, Sun; Koning, Nico; Huang, Hsiu-Hui; Churchwell, Edward (2006), "Planetary nebulae in the GLIMPSE survey", in Barlow, Michael J.; Méndez, Proceedings of the International Astronomical Union, Symposium #234, Planetary Nebulae in our Galaxy and Beyond (Cambridge: Cambridge University Press) 2 (S234): 445–6, Bibcode:2006IAUS..234..445K, doi:10.1017/S1743921306003668, "Planetary nebulae (PNs) have high dust content and radiate strongly in the infrared. For young PNs, the dust component accounts for ∼1/3 of the total energy output of the nebulae (Zhang & Kwok 1991). The typical color temperatures of PNs are between 100 and 200 K, and at λ >5 μm, dust begins to dominate over bound-free emission from the ionized component. Although PNs are traditionally discovered through examination of photographic plates or Hα surveys, PNs can also be identified in infrared surveys by searching for red objects with a rising spectrum between 4–10 μm."
- Liu, X.-W.; Storey, P. J.; Barlow, M. J.; Danziger, I. J.; Cohen, M.; Bryce, M. (March 2000), "NGC 6153: a super–metal–rich planetary nebula?", Monthly Notices of the Royal Astronomical Society 312 (3): 585–628, Bibcode:2000MNRAS.312..585L, doi:10.1046/j.1365-8711.2000.03167.x
- Maciel, W. J.; Costa, R. D. D.; Idiart, T. E. P. (October 2009), "Planetary nebulae and the chemical evolution of the Magellanic Clouds", Revista Mexicana de Astronomía y Astrofísica 45: 127–37, arXiv:0904.2549, Bibcode:2009RMxAA..45..127M, "These objects are produced by low and intermediate mass stars, with main sequence masses roughly between 0.8 and 8 M⊙, and present a reasonably large age and metallicity spread."
- Majaess, D. J.; Turner, D.; Lane, D. (December 2007), "In Search of Possible Associations between Planetary Nebulae and Open Clusters", Publications of the Astronomical Society of the Pacific 119 (862): 1349–60, arXiv:0710.2900, Bibcode:2007PASP..119.1349M, doi:10.1086/524414
- Marochnik, L.S.; Shukurov, Anwar; Yastrzhembsky, Igor (1996), "Chapter 19: Chemical abundances", The Milky Way galaxy, Taylor & Francis, pp. 6–10, ISBN 978-2-88124-931-0
- Mermilliod, J.-C.; Clariá, J. J.; Andersen, J.; Piatti, A. E.; Mayor, M. (August 2001), "Red giants in open clusters. IX. NGC 2324, 2818, 3960 and 6259", Astronomy and Astrophysics 375 (1): 30–9, Bibcode:2001A&A...375...30M, doi:10.1051/0004-6361:20010845
- Miszalski, B.; Jones, D.; Rodríguez-Gil, P.; Boffin, H. M. J.; Corradi, R. L. M.; Santander-García, M. (2011), "Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778", Astronomy and Astrophysics 531: A158, arXiv:1105.5731, Bibcode:2011A&A...531A.158M, doi:10.1051/0004-6361/201117084
- Moore, S. L. (October 2007), "Observing the Cat's Eye Nebula", Journal of the British Astronomical Association 117 (5): 279–80, Bibcode:2007JBAA..117R.279M
- Morris, M. (1990), "Bipolar asymmetry in the mass outflows of stars in transition", in Mennessier, M.O.; Omont, Alain, From Miras to planetary nebulae: which path for stellar evolution?, Montpellier, France, September 4–7, 1989 IAP astrophysics meeting: Atlantica Séguier Frontières, pp. 526–30, ISBN 978-2-86332-077-8
- Osterbrock, Donald E.; Ferland, G. J. (2005), Ferland, G. J., ed., Astrophysics of gaseous nebulae and active galactic nuclei, University Science Books, ISBN 978-1-891389-34-4
- Parker, Quentin A.; Acker, A.; Frew, D. J.; Hartley, M.; Peyaud, A. E. J.; Ochsenbein, F.; Phillipps, S.; Russeil, D.; Beaulieu, S. F.; Cohen, M.; Köppen, J.; Miszalski, B.; Morgan, D. H.; Morris, R. A. H.; Pierce, M. J.; Vaughan, A. E. (November 2006), "The Macquarie/AAO/Strasbourg Hα Planetary Nebula Catalogue: MASH", Monthly Notices of the Royal Astronomical Society 373 (1): 79–94, Bibcode:2006MNRAS.373...79P, doi:10.1111/j.1365-2966.2006.10950.x
- Parker, Quentin A.; Frew, David J.; Miszalski, B.; Kovacevic, Anna V.; Frinchaboy, Peter.; Dobbie, Paul D.; Köppen, J. (May 2011), "PHR 1315–6555: A bipolar planetary nebula in the compact Hyades-age open cluster ESO 96-SC04", Monthly Notices of the Royal Astronomical Society 413 (3): 1835–1844, arXiv:1101.3814, Bibcode:2011MNRAS.413.1835P, doi:10.1111/j.1365-2966.2011.18259.x
- Reed, Darren S.; Balick, Bruce; Hajian, Arsen R.; Klayton, Tracy L.; Giovanardi, Stefano; Casertano, Stefano; Panagia, Nino; Terzian, Yervant (November 1999), "Hubble Space Telescope Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution", Astronomical Journal 118 (5): 2430–41, arXiv:astro-ph/9907313, Bibcode:1999AJ....118.2430R, doi:10.1086/301091
- Renzini, A. (1987), "Thermal pulses and the formation of planetary nebula shells", in S. Torres-Peimbert, Proceedings of the 131st symposium of the IAU 131: 391–400, Bibcode:1989IAUS..131..391R
- Soker, Noam (February 2002), "Why every bipolar planetary nebula is 'unique'", Monthly Notices of the Royal Astronomical Society 330 (2): 481–6, arXiv:astro-ph/0107554, Bibcode:2002MNRAS.330..481S, doi:10.1046/j.1365-8711.2002.05105.x
- The first detection of magnetic fields in the central stars of four planetary nebulae, SpaceDaily Express, January 6, 2005, retrieved October 18, 2009, "Source: Journal Astronomy & Astrophysics"
- Rees, B.; Zijlstra, A.A. (July 2013), "Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge", Mon. Not. R. Astron. Soc (SpaceTelescope.org)
- Planetary Nebulae, SEDS, September 9, 2013, retrieved 2013-11-10
|Wikimedia Commons has media related to Planetary nebulae.|
- Entry in the Encyclopedia of Astrobiology, Astronomy, and Spaceflight
- Press release on recent observations of the Cat's Eye Nebula
- Planetary Nebulae, SEDS Messier Pages
- The first detection of magnetic fields in the central stars of four planetary nebulae
- Planetary Nebulae—Information and amateur observations
- Planetary nebula on arxiv.org