A planetary nebula is an astronomical object consisting of a glowing shell of gas formed by certain types of stars at the end of their lives. They are in fact unrelated to planets; the name originates from a supposed similarity in appearance to giant planets. They are a short-lived phenomenon, lasting a few thousand years, compared to a typical stellar lifetime of several billion years. About 1,500 are known to exist in our galaxy.
Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements by nucleosynthesis. In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.
In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a 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.
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Planetary nebulae are generally faint objects, and none are visible to the naked eye. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, 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 gas giants, and William Herschel, discoverer of Uranus, eventually coined the term 'planetary nebula' for them, although, as we now know, they are very different from planets.
The nature of planetary nebulae was unknown until the first spectroscopic observations were made in the mid-19th century. William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects, using a prism to disperse their light. His observations of stars showed that their spectra consisted of a continuum with many dark lines superimposed on them, and he later found that many nebulous objects such as the Andromeda Nebula had spectra which were quite similar to this – these nebulae were later shown to be galaxies.
However, when he 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 hypothesised 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.
However, 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 500.7nm 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 oxygen give rise to the 500.7nm line. 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.
As discussed further below, the central stars of planetary nebulae are very hot. Their luminosity, though, is very low, implying that they must be very small. Only once a star has exhausted all its nuclear fuel can it collapse to such a small size, and so planetary nebulae came to be understood as a final stage of stellar evolution. Spectroscopic observations show that all planetary nebulae are expanding, and so the idea arose 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 emitted beyond the visible spectrum which is not visible from ground-based observatories. Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures, densities and abundances. CCD 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 from the ground, the very high optical resolution achievable by a telescope above the Earth's atmosphere reveals extremely complex morphologies.
Planetary nebulae are the end stage of stellar evolution for most stars. Our Sun is a very average star, and only a small number of stars weigh very much more than it. Stars weighing more than a few solar masses will end their lives in a dramatic supernova explosion, but for the medium and low mass stars, the end involves the creation of a planetary nebula.
A typical star weighing less than about twice the mass of the Sun spends most of its lifetime shining as a result of nuclear fusion reactions converting hydrogen to helium in its core. The energy released in the fusion reactions prevents the star from collapsing under its own gravity, and the star is stable.
After several billion years, the star runs out of hydrogen, and there is no longer enough energy flowing out from the core to support the outer layers of the star. The core thus contracts and heats up. Currently the sun's core has a temperature of approximately 15 million K, but when it runs out of hydrogen, the contraction of the core will cause the temperature to rise to about 100 million K.
The outer layers of the star expand enormously because of the very high temperature of the core, and become much cooler. The star becomes a red giant. The core continues to contract and heat up, and when its temperature reaches 100 million K, helium nuclei begin to fuse into carbon and oxygen. The resumption of fusion reactions stops the core's contraction. Helium burning soon forms an inert core of carbon and oxygen, with a helium-burning shell surrounding it.
Helium fusion reactions are extremely temperature sensitive, with reaction rates being proportional to T40. This means that just a 2% rise in temperature more than doubles the reaction rate. This makes the star very unstable – a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space .
The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30,000K, there are enough ultraviolet photons being emitted to ionise the ejected atmosphere, making it glow. The cloud has then become a planetary nebula.
The gases of the planetary nebula drift away from the central star at speeds of a few kilometres per second. At the same time as the gases are expanding, the central star is cooling as it radiates away its energy – fusion reactions have ceased, as the star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. Eventually it will cool down so much that it doesn't give off enough ultraviolet radiation to ionise the increasingly distant gas cloud. The star becomes a white dwarf, and the gas cloud recombines, becoming invisible. For a typical planetary nebula, about 10,000 years will pass between its formation and recombination.
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 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.
A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally around 1000 particles per cm³ – which is about a million billion billion times less dense than the earth's atmosphere. 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.
Radiation from the central star heats the gases to temperatures of about 10,000K. Counterintuitively, the gas temperature is often seen to rise at increasing distances from the central star. This is because the more energetic a photon, the less likely it is to be absorbed, and so the less energetic photons tend to be the first to be absorbed. In the outer regions of the nebula, most lower energy photons have already been absorbed, and the high energy photons remaining give rise to higher temperatures.
Nebulae may be described as matter bounded or radiation bounded. In the former case, there is so much matter around the star that all the UV photons emitted are absorbed, and the visible nebula is surrounded by a shell of un-ionised gas—hence the radiation is "bounded" by the matter. In the latter case there are enough UV photons being emitted by the central star to ionise all the surrounding gas.
Numbers and distribution
About 1500 planetary nebulae are 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 centre. They are only very rarely seen in star clusters, with only one or two known cases.
While CCDs have almost entirely superceded photographic film in modern astronomy, a recent survey which greatly increased the number of known planetary nebulae used Kodak Technical Pan film together with a very high quality filter isolating the brightest emission line of hydrogen, which is strongly emitted by almost all planetary nebulae .
Generally speaking, planetary nebulae are symmetrical and approximately spherical, but a wide variety of shapes exist with some very complex forms seen. Approximately 10% of planetary nebulae are strongly bipolar, and a small number are asymmetric. One is even rectangular. The reason for the huge variety of shapes is not fully understood, but may be caused by gravitational interactions with companion stars if the central stars are double stars. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms. In January 2005, astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and hypothesised that the fields might be partly or wholly responsible for their remarkable shapes .
Current issues in planetary nebula studies
A long standing problem in the study of planetary nebulae is that in most cases, their distances are very poorly determined. For a very few nearby planetary nebulae, it is possible to determine distances by measuring their expansion parallax: 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 controversial topic. Broadly, it is believed that interactions between material moving away from the star at different speeds gives rise to most shapes observed. However, some astronomers believe that double central stars must be responsible for at least the more complex and extreme planetary nebulae . One recent study has found that several planetary nebulae contain strong magnetic fields, something which has long been hypothesised. Magnetic interactions with ionised gas could be responsible for shaping at least some planetary nebulae .
There are two different ways of determining metal abundances in nebulae, which rely on different types of spectral lines, and large discrepancies are sometimes seen between the results derived from the two methods. Some astronomers put this down to the presence of small temperature fluctuations within planetary nebulae; others claim that the discrepancies are too large to be explained by temperature effects, and hypothesise the existence of cold knots containing very little hydrogen to explain the observations. However, no such knots have yet been observed .
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- The first detection of magnetic fields in the central stars of four planetary nebulae