NGC 6543, The Cat's Eye Nebula

NGC 6853, The Dumbbell 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. The Cat's Eye Nebula ( NGC 6543) is a Planetary nebula in the constellation of Draco. The Dumbbell Nebula (also known as Messier 27, M 27, or NGC 6853) is a Planetary nebula (PN in the Constellation Vulpecula Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. This page is about the physical properties of gas as a state of matter In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The name originates from a similarity in appearance to giant planets when viewed through a small optical telescope and is unrelated to planets of the solar system. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large Planet that is not primarily A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is 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.

Many years after the red giant phase, the area around the core blows away, leaving just the core and a shell around it. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 The hydrogen fusion stops in the core. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus The core then illuminates the shell making the shell look like a planet. A planet, as defined by the International Astronomical Union (IAU is a celestial body Orbiting a Star or stellar remnant that is Now there is a planetary nebula.

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). Chemistry (from Egyptian kēme (chem meaning "earth") is the Science concerned with the composition structure and properties The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning the formation of the first galaxies the way Nucleosynthesis is the process of creating new atomic nuclei from preexisting Nucleons (protons and neutrons 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. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into About a fifth are roughly spherical, but the majority are not spherically symmetric. "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe 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. A binary star is a Star system consisting of two Stars orbiting around their Center of mass. A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a Star. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

Observations

NGC 7293, The Helix Nebula

Planetary nebulae are generally faint objects, and none are visible to the naked eye. The Helix Nebula, also known as The helix or NGC 7293, is a large Planetary nebula (PN in the sky and is placed the zodiac Constellation The naked eye is a Figure of speech referring to human Visual perception that is unaided by enhancing equipment such as a Telescope or 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. The Dumbbell Nebula (also known as Messier 27, M 27, or NGC 6853) is a Planetary nebula (PN in the Constellation Vulpecula Vulpecula (vəlˈpɛkjʊlə Latin for "little Fox " is a faint northern Constellation located in the middle of the Summer Triangle Charles Messier ( June 26, 1730 &ndash April 12, 1817) was a French astronomer most notable for publishing an The Messier objects are a set of Astronomical objects first listed by French astronomer Charles Messier in his "Catalogue des Nébuleuses 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. Sir Frederick William Herschel FRS KH ( 15 November 1738 – 25 August 1822) was a German -born British

The nature of planetary nebulae was unknown until the first spectroscopic observations were made in the mid-19th century. Astronomical spectroscopy is the technique of Spectroscopy used in Astronomy. William Huggins was one of the earliest astronomers to study the optical spectra of astronomical objects, using a prism to disperse their light. Sir William Huggins, OM, FRS ( February 7 1824 &ndash May 12 1910) was an English Astronomer In Optics, a prism is a transparent optical element with flat polished surfaces that refract 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 (as it was then known) had spectra which were quite similar to this – these nebulae were later shown to be galaxies. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter

However, when he looked at the Cat's Eye Nebula, he found a very different spectrum. The Cat's Eye Nebula ( NGC 6543) is a Planetary nebula in the constellation of Draco. 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. A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared The brightest of these was at a wavelength of 500. 7 nanometres, which did not correspond with a line of any known element. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a ^[1] 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. Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical The Sun (Sol is the Star at the center of the Solar System.

NGC 2392, The Eskimo Nebula

However, while helium was isolated on earth soon after its discovery in the spectrum of the sun, nebulium was not. The Eskimo Nebula ( NGC 2392) also known as the Clownface Nebula, is a bipolar Planetary nebula (PN In the early 20th century Henry Norris Russell proposed that rather than being a new element, the line at 500. Henry Norris Russell ( October 25, 1877 &ndash February 18, 1957) was an American Astronomer who along with Ejnar 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 populate excited metastable energy levels in atoms and ions which at higher densities are rapidly de-excited by collisions. The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Excitation is an elevation in energy level above an arbitrary baseline energy state Metastability is a general scientific concept which describes states of delicate equilibrium A quantum mechanical system or particle that is bound, confined spacially can only take on certain discrete values of energy as opposed to classical particles which ^[2] Electron transitions from these levels in oxygen ion (O²⁺ or OIII) give rise to the 500. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the 7 nm line. These spectral lines, which can only be seen in very low density gases, are called forbidden lines. A forbidden mechanism or forbidden line is a concept in Physics / Chemistry. Spectroscopic observations thus showed that nebulae were made of extremely rarefied gas. ^[3]

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. Luminosity has different meanings in several different fields of science 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). A space observatory is any instrument in Outer space which is used for observation of distant planets galaxies and other outer space objects Infrared and ultraviolet studies of planetary nebulae allowed much more accurate determinations of nebular temperatures, densities and abundances. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature The density of a material is defined as its Mass per unit Volume: \rho = \frac{m}{V} Different materials usually have different CCD technology allowed much fainter spectral lines to be measured accurately than had previously been possible. A charge-coupled device ( CCD) is an analog Shift register, that enables the transportation of analog signals (electric charges through successive stages (capacitors 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. The Hubble Space Telescope ( HST; also known colloquially as "the Hubble" or just "Hubble" is a space telescope that was carried into Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five

Under the Morgan-Keenan spectral classification scheme, planetary nebulae are classified as Type-P, although this notation is seldom used in practice. In Astronomy, stellar classification is a classification of Stars based initially on photospheric temperature and its associated Spectral characteristics

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.

Stars weighing more than 8 solar masses will likely end their lives in a dramatic supernova explosion, but for medium and low mass stars on the order of a solar mass, such as our Sun, the end may involve the creation of a planetary nebula. The solar mass is a standard way to express Mass in Astronomy, used to describe the masses of other Stars and galaxies. A supernova (plural supernovae or supernovas) is a stellar Explosion.

Stars that inevitably become planetary nebulae spend most of their lifetime shining as a result of nuclear fusion reactions converting hydrogen to helium in its core. In Physics and Nuclear chemistry, nuclear fusion is the process by which multiple- like charged atomic nuclei join together to form a heavier nucleus Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical 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 kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic

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. A red giant is a luminous Giant star of low or intermediate mass (roughly 0 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. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the The resumption of fusion reactions stops the core's contraction. Helium burning soon forms an inert core of carbon and oxygen, with both a helium-burning shell and a hydrogen-burning shell surrounding it. In this last stage the star will observationally be a red giant and structurally an asymptotic giant branch star. A red giant is a luminous Giant star of low or intermediate mass (roughly 0

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. ^[4]

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 ionize the ejected atmosphere, making it glow. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays In Physics, the photon is the Elementary particle responsible for electromagnetic phenomena Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons 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 kilometers per second. At the same time as the gases are expanding, the central star undergoes a two stage evolution first growing hotter as it continues to contract and hydrogen fusion reactions are occurring in a shell around the core of carbon and oxygen and then cooling as it radiates away its energy and fusion reactions have ceased, as the star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. During the first phase the central star gets hotter eventually reaching temperatures around 100,000K. Eventually it will cool down so much that it doesn't give off enough ultraviolet radiation to ionize the increasingly distant gas cloud. The star becomes a white dwarf, and the gas cloud recombines, becoming invisible. A white dwarf, also called a degenerate dwarf, is a small Star composed mostly of Electron-degenerate matter. Plasma recombination is a process by which ions of a plasma capture the free energetic electrons to form new neutral atoms. For a typical planetary nebula, about 10,000 years will pass between its formation and recombination of the star.

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 Universe is defined as everything that Physically Exists: the entirety of Space and Time, all forms of Matter, Energy Hydrogen (ˈhaɪdrədʒən is the Chemical element with Atomic number 1 Helium ( He) is a colorless odorless tasteless non-toxic Inert Monatomic Chemical 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. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the

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). In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen

Characteristics

Physical characteristics

NGC 6720, The Ring Nebula

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. The famously named " Ring Nebula " is located in the northern Constellation of Lyra, and also catalogued as Messier 57, M57 or A light-year or light year (symbol ly) is a unit of Length, equal to just under ten trillion Kilometres As defined by (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.

Radiation from the central star heats the gases to temperatures of about 10,000 K. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic 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 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. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges Instability in systems is generally characterized by some of the Outputs or internal states growing without Bounds.

Numbers and distribution

About 3000 planetary nebulae are now known to exist in our galaxy. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter ^[5], 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. The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The Galactic Center is the rotational center of the Milky Way Galaxy. ^[6] Planetary nebulae have been detected as members in only four globular clusters: M 15, M 22, NGC 6441 and Palomar 6. Globular Cluster M15 (also known as Messier Object 15 or NGC 7078) is a Globular cluster in the Constellation Messier 22 (also known as M22 or NGC 6656) is an elliptical Globular cluster in the Constellation Sagittarius near the Galactic However, there has yet to be an established case of a planetary nebula discovered in an open cluster. ^[7]

Morphology

Only about 20% of planetary nebulae are spherically symmetric. A wide variety of shapes exist with some very complex forms seen. 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. Double Star is a Science fiction Novel by Robert A Heinlein, first serialized in Astounding Science Fiction 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 [1]. In Physics, a magnetic field is a Vector field that permeates space and which can exert a magnetic force on moving Electric charges

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. Parallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between The Doppler effect (or Doppler shift) named after Christian Doppler, is the change in Frequency and Wavelength of a Wave for Comparing the angular expansion with the derived velocity of expansion will reveal the distance to the nebula. ^[8]

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. ^[9] One recent study has found that several planetary nebulae contain strong magnetic fields, something which has been hypothesised by Grigor Gurzadyan already in 1960s (see e. g. ref. [3]). Magnetic interactions with ionised gas could be responsible for shaping at least some planetary nebulae. ^[10]

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. ^[11]

See also

• Asymptotic Giant Branch
• Bipolar nebula
• Interstellar medium
• List of planetary nebulae
• Nebula
• Nova remnant
• Protoplanetary nebula
• Stellar evolution
• Supernova remnant
• White dwarf
• Extragalactic Distance Scale

References

External links

• 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
• WWW Search for Galactic Planetary Nebulae
• Planetary Nebulae - Information and amateur observations