A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelated to planets. The term originates from the planet-like round shape of these nebulae observed by astronomers through early telescopes. 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". Though the modern interpretation is different, the old term is still used.
All planetary nebulae form at the end of the life of a star of intermediate mass, about 1-8 solar masses. It is expected that the Sun will form a planetary nebula at the end of its life cycle. They are relatively short-lived phenomena, lasting perhaps a few tens of millennia, compared to considerably longer phases of stellar evolution.They are created after the red giant phase, when most of the outer layers of the star have been expelled by strong stellar winds 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 (P.N.N.), ionizes the ejected material. Absorbed ultraviolet light then energizes the shell of nebulous gas around the central star, causing it to appear as a brightly colored planetary nebula.
Planetary nebulae probably 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 in 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.
Observations
Discovery
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 on July 12, 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. 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".
The nature of these objects remained unclear. In 1782, William Herschel, discoverer of Uranus, found the Saturn Nebula (NGC 7009) and described it as "A curious nebula, or what else to call it I do not know". He later described these objects as seeming to be planets "of the starry kind". As noted by Darquier before him, Herschel found that the disk resembled a planet, but it was too faint to be one. In 1785, Herschel wrote to Jérôme Lalande:
He assigned these to Class IV of his catalogue of "nebulae", eventually listing 78 "planetary nebulae", most of which are in fact galaxies.p. 16 in
Herschel used the term "planetary nebulae" for these objects. The origin of this term is not known. 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.
Spectra
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. 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 (, (a.k.a. O ), and ) 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.
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 a small size. Planetary nebulae are 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. The first UV observations of PNe (IC 2149) were performed from space, with the Orion 2 Space Observatory (see Orion 1 and Orion 2 Space Observatories) on board the Soyuz 13 spacecraft in December 1973, two photon emission from nebulae was detected for the first time. 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
Stars greater than 8 solar masses () will probably 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 to . 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 generates energy in the core, which creates outward pressure that balances the crushing inward pressures of 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 in the core starts to run out, nuclear fusion generates less energy and gravity starts compressing the core, causing a rise in temperature to about 100 million K while the hydrogen burning shifts to a thin shell around the core. To maintain thermal equilibrium, the star's outer layers expand and cool. This phase causes a dramatic rise in stellar luminosity, but because the released energy is distributed over a much larger surface area, it in fact causes a decrease of the surface temperature. This phase of stellar evolution is known as the red giant branch (RGB). In stars more massive than , the core, now consisting of mostly helium, is ignited again; helium is then converted into carbon and oxygen, slowly consuming the core's helium.
As the helium in the core runs out, the core's temperature starts rising again in a manner similar to the RGB. The luminosity increases again and the star expands once more, decreasing the surface temperature. Stars in this phase of their lives are known as asymptotic giant branch stars (AGB). During this phase, the star can lose 50–70% of its total mass from its stellar wind.
The venting of atmosphere continues unabated into interstellar space. 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
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 pass between its formation and recombination of the resulting plasma.
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 this way, 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.