A planetary nebula is an astronomical unit that acts as a shell of gas. Its formation occurs at the central luminary, belonging to the category of white dwarfs. It is formed during the discharge of the outer "tiers" of a gigantic cosmic body with a mass of 0.8-8 Suns (this happens, as a rule, at the final stage of its development).
Considering the issue from the point of view of astronomy, we can conclude that planetary nebulae are ephemeral elements. Their lifespan barely reaches a couple of tens of millennia, provided that the average lifespan of the "ancestor" is billions of years. The peculiarity of these objects is the lack of relation to the planets. They have this name because, when viewed through a telescope, they look like a planet.
Traditionally, the extent that a planetary nebula has is average. As a rule, it equates to one light year. Its composition is dominated by a rarefied gas, its density is 1000 particles per cubic centimeter on average. This is small when compared with the density index of the earth's atmosphere , but a lot when compared with interplanetary space. The maximum value is observed for newly formed bodies - 10 ^ 6 particles or even more. With "age" the density decreases due to expansion.
Most of the studied bodies are dim, and it is unrealistic to detect them without special devices. The first planetary nebula that was discovered is Dumbbell (a group of stars - Chanterelles ). She was found by Charles Messier, who was looking for a comet . It happened in 1764. The object appears in the catalog as M27.
Twenty years later, William Herschel discovered a whole class of nebulae - 4. According to their characteristics, they practically do not differ in any way from the planet's disk, but the properties, as you might guess, are different.
The first astronomer who managed to obtain spectra was William Huggins. When he observed them (namely, NGC 6543, M27, M57 and others), he was able to find out that the spectrum differs from the spectra of stars: everything that was obtained by that time was related to absorption spectra. As for planetary nebulae, their spectra were emission and contained few lines. This meant that they were different in nature from stars.
The issue related to the chemical characteristics of the objects under consideration also deserved special attention. Huggins succeeded in achieving the maximum identification of the nitrogen and hydrogen lines. Unfortunately, it was not observed in the spectra of substances known at that time. Therefore, the scientist suggested that it corresponds to an unknown substance. It was called nebulium according to the same principle as in the case of helium in 1868 (when the spectral analysis of the sun was carried out).
As a result, the versions and hypotheses associated with its discovery have not been officially confirmed. However, at the very beginning of the 20th century, another scientist - Henry Russell - created new theories. At the same time, it was possible to note that atoms and ions are capable of passing into a state of excessive excitation, which, in turn, does not exist for long in the case of high densities.
In 1927, an expert with the surname Bowen identified this line again. He said that it can arise during the transition of an oxygen atom from one state to another - from metastable to ground. Despite the fact that many answers have not yet been found, spectroscopic studies have made it possible to estimate the upper limit of the gas density. Despite the fact that specialists managed to obtain full information about such important issues as structure, structural composition, mechanism, origin problems, they continued to remain unresolved.
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