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Planetary Nebula in the majority are a faint objects, and usually not visible to the naked eye. First open a planetary nebula was Dumbbell Nebula in the constellation chanterelles: Charles Messier, to search for comets, in compiling its directory nebulae (fixed object, similar in monitoring the sky for a comet) in 1764 brought it to the catalog number M27. In 1784 William Gershel, pioneer of Uranus, in compiling its directory identified them as a separate class of nebulae (class IV nebulae) * and suggested to them the term Planetary because of their apparent similarity with the disk of Uranus.
Unusual nature of planetary nebulae discovered in the middle of the XIX century, with the introduction of the observational method of spectroscopy. William Huggins was the first astronomer to win the spectra of planetary nebulae - objects are distinguished by their unusual: One of the most mysterious of these remarkable objects are those with a telescopic observation has the form of round or slightly oval-shaped discs. ... Remarkable, and their greenish-blue color is extremely rare for single stars. In addition, the nebula has no signs of a central thickening. On these grounds the planetary nebula has identified as an object which has properties quite different from the properties of the Sun and the fixed stars. From these considerations, as well as their brightness, I chose these nebulae are most appropriate for the spectroscopic study*.
In studying the spectra Huggins nebulae NGC 6543 (Cat's Eye), M27 (Dumbbell), M57 (the Ring Nebula in Lira) and several others, proved that their range is extremely different from the spectra of stars: all comments received by the time the spectra of stars were the absorption spectra ( continuous spectrum with a large number of dark lines), while the spectra of planetary nebulae have emission spectra with a small number of emission lines, indicating their nature, fundamentally different from the nature of the stars:
Undoubtedly, the 37 nebula H IV (NGC 3242), Struve 6 (NGC 6572), 73 H IV (NGC 6826), 1 H IV (NGC 7009), 57 M, 18 H. IV (NGC 7662) and 27 M could not be more than clusters of stars of the same type, which are still the stars and our Sun. <...> Of these objects have a special and distinct from the structure of <...> We are likely to have to take these objects are huge masses of glowing gas or steam *.
Another problem was the chemical composition of planetary nebulae: Huggins comparison with reference spectra was able identifitsiirovat lines of nitrogen and hydrogen, but the brightest of the lines with a wavelength of 500.7 nm is not observed in the spectra of chemical elements known at that time. It was suggested that this line corresponds to an unknown element. He previously gave the name nebuly - by analogy with the idea that led to the discovery of helium in the spectral analysis of the Sun in 1868.
Assumptions about the opening of a new element nebuliya not been confirmed. At the beginning of XX century, Henry Russell put forward a hypothesis that the line at 500.7 nm corresponds to no new element, and the old items in unknown circumstances.
In 20-ies of the XX century, it was shown that in very diluted gases of atoms and ions can move in the excited metastable state, which at higher densities due to collisions of particles can not exist long enough. In 1927, Bowen identified nebuliya 500.7 nm line as arising in the transition from the metastable state to the ground twice ionized atoms of oxygen (OIII) *. Spectral lines of this type, observed only at extremely low densities, are called forbidden lines. Thus, spectroscopic observations made it possible to estimate the upper limit of the density of gas nebulae. However, the spectra of planetary nebulae, obtained at the gap spectrometry showed izlomannos and splitting of lines due to Doppler shifts emitting nebula regions moving with different velocities, thus the speed of the expansion of planetary nebulae in the 20-40 km / sec.
Despite a detailed understanding of the structure, composition and the mechanism of radiation of planetary nebulae, their origin remained open until the mid 50-ies of the XX century, while J. S. Shklovskii not notice that if proekstrapolirovat parameters of planetary nebulae at the time of the beginning of their expansion , then the current set of parameters consistent with the properties of the atmospheres of red giants, and the properties of their nuclei - with the properties of hot white dwarfs, * *. Currently, this theory of origin of planetary nebulae confirmed by numerous observations and calculations.
By the end of XX century, improved technology has allowed a more detailed examination of the planetary nebula. Space telescopes have allowed them to explore the spectra outside the visible range, which could not have been done before, through observation of the Earth's surface. Observations in the infrared and ultraviolet wavelengths have a much more accurate assessment of the temperature, density and chemical composition of planetary nebulae. Application of CCD-matrix allowed for much less clear-cut analysis of spectral lines. Using the Space Telescope Hubble revealed an extremely complex structure of planetary nebulae, previously thought to be simple and homogeneous.
It is generally accepted that planetary nebulae have a spectral class of P, although this designation is rarely used in practice.