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Planetary nebula

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   NGC 6543, the Cat's Eye Nebula
   Enlarge
   NGC 6543, the Cat's Eye Nebula

   A planetary nebula is an astronomical object consisting of a glowing
   shell of gas and plasma 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 tens of thousands of years,
   compared to a typical stellar lifetime of several billion years. About
   1,500 are known to exist in the Milky Way 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 and other products of nucleosynthesis (such as carbon,
   nitrogen, oxygen and calcium). 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, but binary central
   stars, stellar winds and magnetic fields may all play a role.

Observations

   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 detectable from ground-based observatories (because only radio
   waves and visible light penetrate the earth's atmosphere). 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.

   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.
   Enlarge
   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.

   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 T^40. 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.

Lifetime

   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.

Galactic recyclers

   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 (see stellar population).

Characteristics

Physical characteristics

   Planetary nebula NGC 7009
   Enlarge
   Planetary nebula NGC 7009

   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
   (10^24) times less dense than the earth's atmosphere. Young planetary
   nebulae have the highest densities, sometimes as high as 10^6 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 10,000 K. 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.
   According to this rather counterintuitive terminology, 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 ionise all the surrounding gas, and an ionization
   front propagates outward into the circumstellar neutral envelope.

   Because most of the gas in a typical planetary nebula is ionised (i.e.
   a plasma), the effects of magnetic fields can be significant, giving
   rise to phenomena such as filamentation and plasma instabilities.

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 superseded 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 .

Morphology

   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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