The term "nebula" was originally used by astronomers to refer to any "fuzzy' patch in the sky that could be easily distinguished by a telescope, but was not sharp like stars or planets. Charles Messier, an eighteenth century French astronomer, published his famous catalogue of these objects in 1784 so that he and other comet hunters would not confuse them with comets. We now know that some of the "nebulae" of Messier's catalogue are other galaxies and some are clusters of stars. On the plasma graph, "nebula" refers to clouds of interstellar dust and gas within our own galaxy made visible by their interaction with nearby stars or star remnants.

There are six main types of nebulae:

The HII regions (emission nebulae) are so named because they are composed mostly of a plasma of ionized hydrogen (HII) and free electrons. The hydrogen atoms of the interstellar medium are ionized by the ultraviolet radiation from a nearby star or stars. Only very hot stars, typically young stars, have enough radiation in the ultraviolet region at wavelengths necessary to ionize the hydrogen. The excess energy beyond that needed to ionize the hydrogen goes to kinetic energy of the ejected electrons. Eventually, by collision, this energy is shared by other particles in the gas. An equilibrium is established in a typical emission nebula when the temperature equivalent of this kinetic motion is between 7000 K and 20,000 K. For a typical emission nebulae, the density of ions (and electrons) is 1.0E8 to 1.0E10 particles per m^3.

As the ions de-excite to lower energy levels, in most cases after recombination of ions with electrons, they emit their characteristic spectral lines. The most prominent of these in the visible spectrum is the red line of hydrogen, giving most emission nebulae a characteristic red glow. There also exist "forbidden lines" (ones not normally seen in earth-bound laboratories) in the spectra from nebulae. The most prominent are green lines from doubly ionized oxygen, giving some nebula a green shading.

Interspersed within the glowing gas of nebulae are lanes of dark dust which can give nebulae their dramatic appearance. Some of the most famous and beautiful of the emission nebulae visible from the Northern Hemisphere are the Orion Nebula (M42, the Messier catalog number), the Lagoon Nebula (M8), and the Trifid Nebula (M20). The Orion Nebula, located in the sword of Orion, is illuminated by the stars of the Trapezium. The Lagoon Nebula is a very large nebula which has distinct bright rims and small dark clouds projected onto its brightest parts. The Trifid Nebula in Sagittarius is characterized by dust lanes that divide it into three distinct parts.

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Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both

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When a star like our Sun comes to age, having longly burned away all the hydrogene to helium in its core in its main sequence phase, and also (in the consequent red giant stadium), the helium to carbon and oxygene, its nuclear reactions come to an end in its core, while helium burning goes on in a shell. This process makes the star expanding, and causes its outer layers to pulsate as a long-periodic Mira-type variable, which becomes more and more unstable, and loses mass in strong stellar winds. The instability finally causes the ejection of a significant part of the star's mass in an expanding shell. The stellar core remains as an exremely hot, small central star, which emits high energetic radiation.

The expanding gas shell is excited to shine by the high-energy radiation emitted from the central star; the material in the shell is moreover accelerated so that the expansion gets faster by the time. The shining gas shell is then visible as a planetary nebula. In deep exposures, the matter ejected in the Mira-variable state can be detected as an extended halo surrounding many planetary nebulae.

The first planetary nebula ever seen by a human was the Dumbbell Nebula M27 in Vulpecula, which was discovered by Charles Messier on July 12, 1764. The comparison to a "fading" planet was first pronounced by Antoine Darquier, the discoverer of the second of these objects, the Ring Nebula M57 in Lyra; he found it shortly before Messier when both were tracing the same comet in January, 1779. Following were the subsequent discoveries of the Little Dumbbell Nebula M76 in Perseus in September 1780, and the Owl Nebula M97 in Ursa Major in February 1781 by Pierre Mechain.

These four planetaries are the only ones which found their way into Messier's catalog, and all which where known to summer 1782, before William Herschel started his comprehensive scanning the of the deep sky with large telescopes. One of his first findings within this survey was that of another famous planetary nebula, the Saturn Nebula NGC 7009 (his H IV.1) in Aquarius, in September 1782.

William Herschel eventually invented the name "Planetary Nebula" for these objects in his classification of nebulae in 1784 or 1785, because he found them to resemble the planet newly discovered by him, Uranus. On November 13, 1790, Herschel found the planetary nebula NGC 1514 (his H IV.69), which has a very bright central star; thus he became convinced that the planetary nebulae were nebulous material (gas or dust) associated with a central star, and not unresolved clusters as he and others had thought previously.

The radiation emitted by the planetary nebula is remarkable because of its peculiar spectrum, as was discovered for the planetary nebula NGC 6543 (also known as Cat Eye Nebula, not in Messier's catalog) by the English amateur astronomer and pioneer of astronomical spectroscopy, William Huggins, on August 29, 1864 and published in the Philosophical Transactions of the Royal Society of 1864 and later in the Nineteenth Century Review of June 1897 (according to Hynes):

As expected for gaseous emission nebulae, the spectra of planetaries consist of emission lines, but 90 to 95 % of the visible light are emitted in one single emission line only ! This `Chief Nebular Line' occurs at 500.7 nm (5007 Angstrom), in the green part of the spectrum. It is this circumstance that planetary nebula brightnesses differ significantly if determined with various methods: These objects are often considerably brighter (up to 2 magnitudes, a factor of more than 6) visually than photographically, because the 5007 Angstrom line lies close to the highest sensitivity of the human eye. Also, as films are often less sensitive in the green part of the spectrum, it is difficult to get a good "true color" image of planetary nebulae. As this spectral line at 5007 Angstrom could not be assigned to a known element at the time of its discovery, Huggins suspected it must be emitted from a previously unknown substance, which was called "nebulium". It was not before 60 years later that the "nebulium" spectrum was identified (by the American astro-physicist Ira S. Bowen) to be caused by forbidden lines of double ionized "normal" oxygene, "[O III]" (with the square brackets).

Besides the "nebulium" [OIII] lines, other emission lines occur in the planetary nebula spectra in weaker intensity. These include more forbidden lines of ionized oxygene, neon, nitrogene, and other abundant elements, as well as permitted lines of hydrogene and helium, as well as fluorescence O III lines in case of strong He II emission. Also, a very week continuum underlies the line spectrum, which is due to interactions of electrons with ions.

Our Sun will probably reach this state of evolution at an age of about 10-13 billion years; as it is now only about 4.7 billion years old, we have probably some time left until this event happens.

The planetary nebula has only a short life compared to the time scales in stellar evolution, being visible only a few thousands or 10,000s of years, and then fading out as its matter is spread in the cosmic environment, enriching the interstellar matter with carbon, oxygene, and other elements. Its central star cools down to a white dwarf. This is the reason that, although there are very many sunlike stars among the hundreds of billions in our Milky Way galaxy, which now come into age (especially in the globular clusters), there are only about 10,000 planetary nebula (of which only about 1,500 could yet be detected, the other being hidden behind obscuring interstellar dust); of the 150 globular clusters with each several 100,000 stars, planetary nebulae have been discovered only in 4 (or perhaps 5) of them, namely Pease 1 in M15 (which may even contain a second one according to Peterson, 1976, but this one was never confirmed since), IRAS 18333-2357 in M22, the probable member Peterson 1 lying 3 arc minutes from globular cluster NGC 6401 (Peterson 1977), the recently discovered planetary in NGC 6441 (Jacoby and Fullerton 1997; also see George Jacoby's Planetary Nebula gallery), and a recently found planetary nebula in the faint globular cluster Palomar 6.

As planetary nebulae occur only late in the life of a star, they are usually absent in open star clusters, because these stellar swarms tend to dissolve in times much shorter than that needed for a star to evolve in a planetary nebula: Only few open clusters live longer than a billion years, while planetary nebulae occur only for stars of less than 3 solar masses (the more massive explode as supernovae). Those low-mass stars however have considerably more than 1 billion years of lifetime on the mean sequence alone while they burn up their hydrogene. These arguments are however questionable, as a number of white dwarf stars has been discovered in young clusters, as the Pleiades, M45: These stars must have started their life with a high mass so that they evolved rapidly, but lost a significant portion of their mass during their lifes, probably in the form of strong stellar winds, and must have gone through a planetary nebula stage.

It seems that because of the short lifetime of this stage, there is only one planetary nebula, NGC 2818, which was discovered to be a member of an inconspicuous, rather old open cluster, NGC 2818A. The more wellknown case of the planetary nebula NGC 2438 which is observed in the same direction as M46 is apparently a chance alignment.

The cooling process of the white dwarf goes on until all thermal energy is radiated, and the star approaches a stable "end state" as "black dwarf" after many billion years - the universe is probably still much too young to contain any "cooled-out" black dwarf.

Planetary nebula are often typized for their appearance, according to the Vorontsov-Velyaminov scheme:

1	Stellar Image
2	Smooth disk (a, brighter toward center; b, uniform brightness; c, traces of a ring structure)
3	Irregular disk (a, very irregular brightness distribution; b, traces of ring structure)
4	Ring structure
5	Irregular form, similar to a diffuse nebula
6	Anomalous form

More complex structures are characterized by combinations such as "4+2" (ring and disk), or "4+4" (two rings).

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When a star explodes in a supernova explosion, it depends on its type what exactly remains. But anyway, the offbursted gaseous remainders will form a rapidly expanding and slowly fading cloud, a domain of an extreme kind of physics. These nebulae are called supernova remnants. Depending on the type of the supernova, there may also be a central compact remnant in the form of a neutron star.

According to current theory, two different mechanisms produce supernovae: First, stars considerably more massive than our Sun can most probably not evolve quitely into an end state as a white dwarf. When coming to age, these massive stars explode in a most violent detonation which flashes up at a luminosity of up to 10 billion times that of the sun, called supernova (of type II), and ejecting the very greatest part of the stellar matter in a violently expanding shell. Aternatively, infalling matter on a white dwarf star can cause it to explode as a supernova of type I.

Although the Crab nebula is the only Messier SNR, and one of few historical supernovae observed in our Milky Way galaxy, other supernovae have appeared in Messier galaxies (see our table), and produced SNRs. These special kind of nebulae can be observed in some cases, e.g. the  of the Supernova 1993J in M81.

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Diffuse nebulae, sometimes inacurately referred to as gaseous nebulae, are clouds of interstellar matter, namely thin but widespread agglomerations of gas and dust. If they are large and massive enough they are frequently places of star formation, thus generating big associations or clusters of stars. Some of the young stars are often very massive and so hot that their high energy radiation can excite the gas of the nebula (mostly hydrogene) to shine; such nebula is called emission nebula. If the stars are not hot enough, their light is reflected by the dust and can be seen as white or bluish reflection nebula. Note that many emission nebulae also have an additional reflection nebula component; a most impressive example for this is the Trifid Nebula M20.

Diffuse emission nebulae are often called H II regions because they are mainly consisted of ionized hydrogene, H II - the roman number after the element symbol (here H) designating the ionization level: `I' would stand for neutral atoms, the `II' here means first ionization, i.e. the hydrogene atoms have lost their single electron, and for other elements higher numbers (ionization levels, or numbers of lost electrons) would be possible (e.g., He III, O III or Fe V).

After some million years, the gas and dust of the nebula will have been used up for forming stars (and planets), or blown away by the stellar winds of the young hot stars. A newly born open star cluster will remain.

The first diffuse nebula discovered was the Orion Nebula, M42, observed telescopically in 1610 by N. Pereisc. The diffuse nebulae were longly be considered as distant, unresolved star clusters, or star clouds, until in the 1860s spectroscopy revealed their gaseous nature. Eventually, in 1912, V.M. Slipher discovered that the nebulae in the Pleiades, M45, had the same spectra as the stars illuminating them, thus proving their nature as reflection nebulae. Of Messier's nebulae, M78 is the only pure reflection nebula, and the first one to be discovered

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Besides the luminous bright nebulae of various types, consisting of light emitting gas or illuminated dust masses, there are dark nebulae which can be seen because they obscure the light coming from stars or bright nebulae behind them

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Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the Horsehead Nebula)

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