The term "planetary nebula" is arguably a misnomer because they are unrelated to planets or exoplanets. The true origin of the term was likely derived from the planet-like round shape of these nebulae as observed by astronomers through early telescopes, and although the terminology is inaccurate, it is still used by astronomers today. The first usage may have occurred during the 1780s with the English astronomer William Herschel who described these nebulae as resembling planets; however, as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "... very dim but perfectly outlined; it is as large as Jupiter and resembles a fading planet."
All planetary nebulae form at the end of intermediate massed star's lifetimes. They are a relatively short-lived phenomenon, lasting perhaps a few tens of thousands of years, compared to a considerably longer phases of stellar evolution. Once all of the red giant's atmosphere has been dissipated, energetic ultraviolet radiation from the exposed hot luminous core, called a planetary nebula nucleus (PNN), ionizes the ejected material. Absorbed ultraviolet light then 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 into the interstellar medium from stars where those elements were created. Planetary nebulae are observed in more distant galaxies, yielding useful information about their chemical abundances.
Starting from the 1990s, Hubble Space Telescope images revealed that many planetary nebulae 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.
The first planetary nebula discovered (though not yet termed as such) 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, perhaps coined the term "planetary nebula". However, in as early as January 1779, the French astronomer Antoine Darquier de Pellepoix described in his observations of the Ring Nebula, "a very dull nebula, but perfectly outlined; as large as Jupiter and looks like a fading planet."" Whatever the true origin of the term, the label "planetary nebula" became ingrained in the terminology used by astronomers to categorize these types of nebulae, and is still in use by astronomers today.
The true nature of these objects was uncertain, and Herschel first 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. In 1782, William Herschel had discovered the object now known as NGC 7009 (the "Saturn Nebula"), upon which he used the term "planetary nebula".
In 1785, Herschel wrote to Jerome Lalande:
"These are celestial bodies of which as yet we have no clear idea and which are perhaps of a type quite different from those that we are familiar with in the heavens. I have already found four that have a visible diameter of between 15 and 30 seconds. These bodies appear to have a disk that is rather like a planet, that is to say, of equal brightness all over, round or somewhat oval, and about as well defined in outline as the disk of the planets, of a light strong enough to be visible with an ordinary telescope of only one foot, yet they have only the appearance of a star of about ninth magnitude."
Herschel assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies.
The nature of planetary nebulae remained 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 had shown that their spectra consisted of a continuum of radiation with many dark lines superimposed. He found that many nebulous objects such as the Andromeda Nebula (as it was then known) had spectra that were quite similar. 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. 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+, O2+ (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.
The central stars of planetary nebulae are very hot. Only when a star has exhausted most of its nuclear fuel can it collapse to 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, balancing the crushing inward pressures of the star's gravity. This state of equilibrium is known as the main sequence, which can last for tens of millions to billions of years, depending on the mass.
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). During this phase, the star can lose 50 to 70% of its total mass from its stellar wind.
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.
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 resulting plasma.
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.
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.
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 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.
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.
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.
A planetary nebula (PN) is an expanding ionized circumstellar cloud that was ejected during the asymptotic giant branch (AGB) phase of the stellar progenitor.
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.
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.
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.
Source: Journal Astronomy & Astrophysics
The Cat's Eye Nebula or NGC 6543, is a relatively bright planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot planetary nebula nucleus (PNN).
It is a well-studied object that has been observed from radio to X-ray wavelengths.Dumbbell Nebula
The Dumbbell Nebula (also known as Apple Core Nebula, Messier 27, M 27, or NGC 6853) is a planetary nebula in the constellation Vulpecula, at a distance of about 1227 light-years. This object was the first planetary nebula to be discovered; by Charles Messier in 1764. At its brightness of visual magnitude 7.5 and its diameter of about 8 arcminutes, it is easily visible in binoculars, and a popular observing target in amateur telescopes.IC 2149
IC 2149 is a planetary nebula in the constellation of Auriga. It is a small, bright planetary nebula with something to offer in telescopes of most sizes.Little Dumbbell Nebula
The Little Dumbbell Nebula, also known as Messier 76, NGC 650/651, the Barbell Nebula, or the Cork Nebula, is a planetary nebula in the constellation Perseus. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects as number 76. It was first recognised as a planetary nebula in 1918 by the astronomer Heber Doust Curtis. However, there is some contention to this claim, as Isaac Roberts in 1891 did suggest that M76 might be similar to the Ring Nebula (M57), being instead as seen from the side view. The structure is now classed as a bipolar planetary nebula (BPNe).
Distance to M76 is currently estimated as 780 parsecs or 2,500 light years, making the average dimensions about 0.378 pc. (1.23 ly.) across.The total nebula shines at the apparent magnitude of +10.1 with its central star or planetary nebula nucleus (PNN) at +15.9v (16.1B) magnitude. The UV-light from the PNN is expanding outer layers that form the present nebula, and has the surface temperature of about 88,400 K. The whole planetary nebula is approaching us at 19.1 km/s.The Little Dumbbell Nebula derives its common name from its resemblance to the Dumbbell Nebula (M27) in Vulpecula. It was originally thought to consist of two separate emission nebulae and was thus given two catalog numbers in the NGC 650 and 651. Some consider this object to be one of the faintest and hardest to see objects in Messier's list.NGC 2242
NGC 2242 is a planetary nebula in the constellation Auriga. It was discovered by Lewis A. Swift on November 24, 1886, and was thought to be a galaxy until a study published in 1987 showed it to be a planetary nebula. The nebula is located about 6,500 light-years away, and about 1,600 light-years above the galactic plane.NGC 2346
NGC 2346 (also known as the Butterfly Nebula) is a planetary nebula near the celestial equator in the constellation Monoceros. It is bright and conspicuous and has been extensively studied. Among its most remarkable characteristics is its unusually cool central star, which is a spectroscopic binary, and its unusual shape.
The binary star, which has a period of about 16 days, is also variable, probably due to dust in orbit around it. The dust itself is heated by the central star and so NGC 2346 is unusually bright in the infrared part of the spectrum. When one of the two stars evolved into a red giant, it engulfed its companion, which stripped away a ring of material from the larger star's atmosphere. When the red giant's core was exposed, a fast stellar wind inflated two ‘bubbles’ from either side of the ring.NGC 2438
NGC 2438 is a planetary nebula about 3,000 light years away in the constellation Puppis. It was discovered by William Herschel on March 19, 1786.
NGC 2438 appears to lie within the cluster M46, but it is most likely unrelated since it does not share the cluster's radial velocity. The case is yet another example of a superposed pair, joining the famed case of NGC 2818.Long exposures have shown that this planetary nebula has an extended double halo, while the more easily visible portion probably dates to the death of the red giant in its center.NGC 3132
NGC 3132, also known as the Eight-Burst Nebula, the Southern Ring Nebula, is a bright and extensively studied planetary nebula in the constellation Vela. Its distance from Earth is estimated at about 613 pc. or 2,000 light-years.NGC 3195
NGC 3195 is a planetary nebula located in the constellation Chamaeleon. It is the most southern of all the bright sizable planetary nebula in the sky, and remains invisible to all northern observers. Discovered by Sir John Herschel in 1835, this 11.6 apparent magnitude planetary nebula is slightly oval in shape, with dimensions of 40×35 arc seconds, and can be seen visually in telescopic apertures of 10.5 centimetres (4.1 in) at low magnifications.
Spectroscopy reveals that NGC 3195 is approaching Earth at 17 kilometres per second (11 mi/s), while the nebulosity is expanding at around 40 kilometres per second (25 mi/s). The central star is listed as >15.3V or 16.1B magnitude. Distance is estimated at about 1.7 kpc.NGC 4361
NGC 4361 is a planetary nebula in the constellation of Corvus. It is included in the Astronomical League's Herschel 400 Observing Program.NGC 5189
NGC 5189 (Gum 47, IC 4274, nicknamed Spiral Planetary Nebula) is a planetary nebula in the constellation Musca. It was discovered by James Dunlop on 1 July 1826, who catalogued it as Δ252. For many years, well into the 1960s, it was thought to be a bright emission nebula. It was Karl Gordon Henize in 1967 who first described NGC 5189 as quasi-planetary based on its spectral emissions.
Seen through the telescope it seems to have an S shape, reminiscent of a barred spiral galaxy. The S shape, together with point-symmetric knots in the nebula, have for a long time hinted to astronomers that a binary central star is present.
The Hubble Space Telescope imaging analysis showed that this S shape structure is indeed two dense low-ionization regions: one moving toward the north-east and another one moving toward the south-west of the nebula, which could be a result of a recent outburst from the central star. Observations with the Southern African Large Telescope have finally found a white dwarf companion in a 4.04 day orbit around the rare low-mass Wolf-Rayet type central star of NGC 5189. NGC 5189 is estimated to be 546 parsecs or 1,780 light years away from Earth. Other measurements have yielded results up to 900 parsecs (~3000 light-years)NGC 5307
NGC 5307 is a planetary nebula located in the constellation Centaurus.NGC 6881
NGC 6881 is a planetary nebula, located in the constellation of Cygnus. It is formed of an inner nebula, estimated to be about one fifth of a light-year across, and symmetrical structure that spread out about one light-year from one tip to the other. The symmetry could be due to a binary star at the nebula's centre.NGC 6884
NGC 6884 is a planetary nebula located in the constellation Cygnus.NGC 7027
NGC 7027 is a very young and dense planetary nebula located around 3,000 light-years (920 parsecs) from Earth in the constellation Cygnus. Discovered in 1878 by Édouard Stephan using the 800 mm (31 in) reflector at Marseille Observatory, it is one of the smallest planetary nebulae and by far the most extensively studied. Helium hydride was detected in the nebula in 2019, the first discovery of that molecule in space.NGC 7048
NGC 7048 is a planetary nebula in the constellation of Cygnus. The bright star to the lower left of the nebula is a magnitude 10.5 star, designated TYC 3589-4652-1. The nebula is slightly brighter along the west and east sides. This planetary nebula has an apparent magnitude of 12.1. NGC 7048 was discovered by Édouard Stephan on 19 October 1878 using a 31.5-inch reflector.The central star of NGC 7048 is though to be a white dwarf. The planetary nebula itself has an elliptical shape; from its low surface brightness it is thought to be highly evolved.NGC 7662
NGC 7662, also known as the Blue Snowball Nebula or Snowball Nebula, is a planetary nebula located in the constellation Andromeda.
The distance to this nebula is not known with any real accuracy. According to the Skalnate Pleso Catalogue (1951) the distance of NGC 7662 is about 1,800 light years, the actual diameter about 20,000 AU. In a more recent survey of the brighter planetaries, C.R.O'Dell (1963) derived a distance of 1,740 parsecs or about 5,600 light years, increasing the actual size to 0.8 light year, or nearly 50,000 AU. It has a faint central star that is variable, with a magnitude range of 12 to 16. The central star is a bluish dwarf with a continuous spectrum and a computed temperature of about 75,000K. The nuclei of the planetary nebulae are among the hottest stars known.NGC 7662 is a popular planetary nebula for casual observers. A small telescope will reveal a star-like object with slight nebulosity. A 6" telescope with a magnification around 100x will reveal a slightly bluish disk, while telescopes with a primary mirror at least 16" in diameter may reveal slight color and brightness variations in the interior.Owl Nebula
The Owl Nebula (also known as Messier 97, M97 or NGC 3587) is a planetary nebula located approximately 2,030 light years away in the constellation Ursa Major. It was discovered by French astronomer Pierre Méchain on February 16, 1781. When William Parsons, 3rd Earl of Rosse, observed the nebula in 1848, his hand-drawn illustration resembled an owl's head. It has been known as the Owl Nebula ever since.The nebula is approximately 8,000 years old. It is approximately circular in cross-section with a little visible internal structure. It was formed from the outflow of material from the stellar wind of the central star as it evolved along the asymptotic giant branch. The nebula is arranged in three concentric shells, with the outermost shell being about 20–30% larger than the inner shell. The owl-like appearance of the nebula is the result of an inner shell that is not circularly symmetric, but instead forms a barrel-like structure aligned at an angle of 45° to the line of sight.The nebula holds about 0.13 solar masses of matter, including hydrogen, helium, nitrogen, oxygen, and sulfur; all with a density of less than 100 particles per cubic centimeter. Its outer radius is around 0.91 ly (0.28 pc) and it is expanding with velocities in the range of 27–39 km/s into the surrounding interstellar medium.The 14th magnitude central star has since reached the turning point of its evolution where it condenses to form a white dwarf. It has 55–60% of the Sun's mass, 41–148 times the brightness of the Sun, and an effective temperature of 123,000 K. The star has been successfully resolved by the Spitzer Space Telescope as a point source that does not show the infrared excess characteristic of a circumstellar disk.Ring Nebula
The Ring Nebula (also catalogued as Messier 57, M57 or NGC 6720) is a planetary nebula in the northern constellation of Lyra. Such objects are formed when a shell of ionized gas is expelled into the surrounding interstellar medium by a red giant star, which was passing through the last stage in its evolution before becoming a white dwarf.
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