Planetary nebula - wikipedia Planetary nebula X-ray / optical composite image of the Cat 's Eye Nebula ( NGC 6543 ) NGC 6326, a planetary nebula with glowing wisps of outpouring gas that are lit up by a binary central star 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 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. After most of the red giant 's atmosphere is dissipated, the ultraviolet radiation of the hot luminous core, called a planetary nebula nucleus ( PNN ), ionizes the outer layers earlier ejected from the star. Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula. Planetary nebulae likely play a crucial role in the chemical evolution of the Milky Way by expelling elements to the interstellar medium from stars where those elements were created. Planetary nebulae are also observed in more distant galaxies, yielding useful information about their chemical abundances. Starting from the 1990s, 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 that 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. Contents 1 Observations 1.1 Discovery 1.2 First spectra 1.3 Nebulium and rarefied gas 1.4 Central stars 1.5 Modern observations 2 Origins 3 Lifetime 4 Role in galactic enrichment 5 Characteristics 5.1 Physical characteristics 5.2 Numbers and distribution 5.3 Morphology 6 Membership in clusters 7 Current issues in planetary nebula studies 8 See also 9 References 10 Cited sources 11 Further reading 12 External links Observations NGC 7293, the Helix Nebula. NGC 2392, the Eskimo Nebula. Discovery 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 resembled the giant planets like Uranus. William Herschel, discoverer of Uranus, eventually coined the term `` planetary nebula ''. At first, Herschel thought the objects were stars surrounded by material that was condensing into planets rather than what is now known to be evidence of dead stars that have incinerated any orbiting planets. First spectra 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 Cat 's Eye Nebula. His observations of stars showed that their spectra consisted of a continuum of radiation with many dark lines superimposed. He later found that many nebulous objects such as the Andromeda Nebula ( as it was then known ) had spectra that were quite similar. Those nebulae were later shown to be collections of stars now called galaxies. Nebulium and rarefied gas 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 a 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 occupy excited metastable energy levels in atoms and ions that would otherwise be de-excited by collisions that would occur at higher densities. Electron transitions from these levels in nitrogen and oxygen ions ( O, O ( a.k.a. O iii ), 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. Planetary nebula NGC 3699 is distinguished by an irregular mottled appearance and a dark rift. Central stars The central stars of planetary nebulae are very hot. Only when 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. Modern observations 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. Under the Morgan - Keenan spectral classification scheme, planetary nebulae are classified as Type - P, although this notation is seldom used in practice. Origins Computer simulation of the formation of a planetary nebula from a star with a warped disk, showing the complexity which can result from a small initial asymmetry. Credit : Vincent Icke 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 radiating 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. Lifetime The Necklace Nebula consists of a bright ring, measuring about two light - years across, dotted with dense, bright knots of gas that resemble diamonds in a necklace. The knots glow brightly due to absorption of ultraviolet light from the central stars. After a star passes through the asymptotic giant branch ( AGB ) phase, the short planetary nebula phase of stellar evolution begins as gases blow 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 when 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. Role in galactic enrichment Planetary nebulae may play a very important role in galactic evolution. Newly born stars consist almost entirely of hydrogen and helium, but as stars evolve through the Asymptotic Giant Branch phase, they create heavier elements via nuclear fusion which are eventually expelled by strong stellar winds. Planetary nebulae usually contain larger proportions of elements such as carbon, nitrogen and oxygen, and these are recycled into the interstellar medium via these powerful winds. In turn, planetary nebulae greatly enrich the Milky Way and their nebulae with these heavier elements -- collectively known by astronomers as metals and specifically referred to by the metallicity parameter Z. Subsequent generations of stars formed from such nebulae also tend to have higher metallicities. Although these metals are present in stars in relatively tiny amounts, they have marked effects on stellar evolution and fusion reactions. When stars formed earlier in the universe they theoretically contained smaller quantities of heavier elements. Known examples are the metal poor Population II stars. ( See Stellar population ). Identification of stellar metallicity content is found by spectroscopy. Characteristics Physical characteristics NGC 6720, The Ring Nebula Credit : STScI / AURA Lemon slice nebula ( IC 3568 ). 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 cm. ( The Earth 's atmosphere, by comparison, contains 2.5 × 10 particles per cm. ) Young planetary nebulae have the highest densities, sometimes as high as 10 particles per cm. 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. Morphology Play media This animation shows how the two stars at the heart of a planetary nebula like Fleming 1 can control the creation of the spectacular jets of material ejected from the object. 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 binary 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 Abell 78, 24 inch telescope on Mt. Lemmon, AZ. Courtesy of Joseph D. Schulman. Planetary nebulae have been detected as members in four Galactic globular clusters : Messier 15, Messier 22, NGC 6441 and Palomar 6. Evidence also points to the potential discovery of planetary nebulae in globular clusters in the galaxy M31. However, there is currently only one case of a planetary nebula discovered in an open cluster that is agreed upon by independent researchers. That case pertains to the planetary nebula PHR 1315 - 6555 and the open cluster Andrews - Lindsay 1. Indeed, through cluster membership PHR 1315 - 6555 possesses among the most precise distances established for a planetary nebula ( i.e., a 4 % distance solution ). The cases of NGC 2818 and NGC 2348 in Messier 46, exhibit mismatched velocities between the planetary nebulae and the clusters, which indicates they are line - of - sight coincidences. A subsample of tentative cases that may potentially be cluster / PN pairs includes Abell 8 and Bica 6, and He 2 - 86 and NGC 4463. 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, which is a blink of the eye in cosmic terms. Also, partly because of their small total mass, open clusters have relatively poor gravitational cohesion and tend to disperse after a relatively short time, typically from 100 to 600 million years. Current issues in planetary nebula studies Odd pair of aging stars sculpt spectacular shape of planetary nebula. Tiny planetary nebula NGC 6886. 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. See also Star portal Preceding evolutionary stage : Asymptotic giant branch Protoplanetary nebula Subsequent evolutionary stage : Fast Low - Ionization Emission Region PG 1159 star ( predegenerates ) White dwarf General topics : List of planetary nebulae Otherwise related : Cosmic distance ladder Alternative developments : Nova remnant Supernova remnant References Jump up ^ Miszalski et al. 2011 ^ Jump up to : Frankowski & Soker 2009, pp. 654 -- 8 Jump up ^ SEDS 2013 Jump up ^ Hubblesite.org 1997 Jump up ^ Frew & Parker 2010, pp. 129 -- 148 ^ Jump up to : Kwok 2000, pp. 1 -- 7 ^ Jump up to : Moore 2007, pp. 279 -- 80 Jump up ^ Malin, David ( 1993 ), A View of the Universe, Cambridge, Massachusetts : Sky Publishing Corporation, p. 168, ISBN 0876541015 Jump up ^ Huggins & Miller 1864, pp. 437 -- 44 Jump up ^ Bowen 1927, pp. 295 -- 7 Jump up ^ Gurzadyan 1997 Jump up ^ `` A Planetary Nebula Divided ''. 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( 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 ), Barlow, M.J. ; Méndez, R.H., eds., `` Planetary nebulae in the GLIMPSE survey '', 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. 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( 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. 413.1835 P, 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... 118.2430 R, doi : 10.1086 / 301091 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 '', Monthly Notices of the Royal Astronomical Society, 435 ( 2 ) : 975 -- 991, arXiv : 1307.5711, Bibcode : 2013MNRAS. 435... 975R, doi : 10.1093 / mnras / stt1300 Planetary Nebulae, SEDS, September 9, 2013, retrieved 2013 - 11 - 10 Further reading Iliadis, Christian ( 2007 ), Nuclear physics of stars. Physics textbook, Wiley - VCH, pp. 18, 439 -- 42, ISBN 978 - 3 - 527 - 40602 - 9 Renzini, A. ( 1987 ), S. Torres - Peimbert, ed., `` Thermal pulses and the formation of planetary nebula shells '', Proceedings of the 131st symposium of the IAU, 131 : 391 -- 400, Bibcode : 1989IAUS... 131... 391R External links 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 Star Formation Accretion Molecular cloud Bok globule Young stellar object Protostar Pre-main - sequence star Herbig Ae / Be Orion T Tauri FU Orionis Herbig -- Haro object Hayashi track Henyey track Evolution Main sequence Red giant branch Horizontal branch Red clump Asymptotic giant branch Protoplanetary nebula Planetary nebula PG1159 star Dredge - up Instability strip Luminous blue variable Blue straggler Stellar population Supernova Supernova impostor Hypernova Hertzsprung -- Russell diagram Color -- color diagram Luminosity class Subdwarf Dwarf Blue Red White Yellow Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Spectral classification O WR Be OB Subdwarf O Subdwarf B Late - type Chemically peculiar Am Ap / Bp Barium Carbon CH CN Extreme helium Lambda Boötis Lead HgMn Technetium Shell Remnants White dwarf Helium planet Neutron star Radio - quiet Pulsar Binary X-ray Magnetar Stellar black hole X-ray binary Burster Theoretical stars Black dwarf Exotic Quark Strange Preon Planck Electroweak star Dark - matter star Dark - energy star Black star Gravastar Frozen star Q star Quasi-star Thorne -- Żytkow object Iron star Blitzar Nucleosynthesis Deuterium burning Lithium burning Proton -- proton chain CNO cycle Helium flash Triple - alpha process Alpha process Carbon burning Neon burning Oxygen burning Silicon burning S - process R - process Fusor Nova Symbiotic Remnant Luminous red nova Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Bipolar outflow Accretion disk Asteroseismology Helioseismology Eddington luminosity Kelvin -- Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Absolute magnitude Mass Metallicity Rotation UBV color Variable star Mira variable Star systems Binary Contact Common envelope Eclipsing Multiple Star cluster Open cluster Globular cluster Super star cluster Planetary system Earth 's Solar System Earth - centric observation of Pole star Circumpolar star Constellation Asterism Magnitude Apparent Extinction Photographic Radial velocity Proper motion Parallax Photometric - standard star Lists Star names Arabic Chinese Extremes Most massive Highest temperature Largest volume Smallest volume Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs White dwarfs Milky Way novae Notable supernovae Supernova candidates Supernova remnants Planetary nebulae Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Galactic year Galaxy Supercluster Guest star Gravity Intergalactic star Infrared dark cloud Starfield Category : Stars Star portal Nebula List Visible nebula Dark nebula Diffuse nebula Emission nebula Planetary nebula Supernova remnant Nova remnant H II region Reflection nebula Variable nebula Preplanetary nebula Pre-stellar nebula Giant molecular cloud Bok globule Evaporating gaseous globule Solar nebula Stellar nebula Nova remnant Protoplanetary nebula Wolf -- Rayet nebula Post-stellar nebula Planetary nebula Supernova remnant Pulsar wind nebula Supershell Clouds Interstellar cloud Molecular cloud Infrared cirrus High - velocity cloud HI region Morphology Bipolar nebula Pinwheel nebula Lists List Diffuse Planetary ( PNe ) Protoplanetary ( PPNe ) Supernova renmants ( SNRs ) Commons Wiktionary LCCN : sh85102649 GND : 4174792 - 6 BNF : cb12002958q ( data ) Retrieved from `` https://en.wikipedia.org/w/index.php?title=Planetary_nebula&oldid=804096390 '' Categories : Planetary nebulae Stellar evolution Hidden categories : Wikipedia articles with LCCN identifiers Wikipedia articles with GND identifiers Wikipedia articles with BNF identifiers Articles containing video clips Talk Contents About Wikipedia Afrikaans Azərbaycanca বাংলা Беларуская Беларуская ( тарашкевіца ) ‎ Български Bosanski Català Čeština Dansk Deutsch Eesti Ελληνικά Español Esperanto Euskara فارسی Français Gaeilge Galego 한국어 हिन्दी Hrvatski Bahasa Indonesia Italiano עברית ქართული Latina Latviešu Lëtzebuergesch Lietuvių Magyar Македонски മലയാളം Bahasa Melayu Nederlands 日本 語 Norsk Norsk nynorsk Occitan Oʻzbekcha / ўзбекча Polski Português Română Русский සිංහල Simple English Slovenčina Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska ไทย Türkçe Українська Tiếng Việt 粵語 中文 Edit links This page was last edited on 6 October 2017, at 18 : 24. 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Question: what is at the center of a planetary nebula
Answer: the hot luminous core, called a planetary nebula nucleus (PNN)