English astronomer W. Herschel discovered planetary nebulae in about 1783. Nebulae received the name "planetary" on account of their external similarity with the images of distant planets, light wheels of which are different from the point images of the stars. “A planetary nebula is a beautiful object created during the final stages of the life of a star whose birth mass was between 1 and 8 solar masses” (“Planetary Nebulae”). A planetary nebula is an astronomical object consisting of an ionized gas shell and the central star, a white dwarf.
All planetary nebulae originate from stars that are in the final stages of their existence. A star with a mass comparable to the mass of the Sun, after birth has a long stage of stability, during which it melts the hydrogen nuclei, giving rise to the nuclei of helium. When hydrogen, contained in the central part of the star, ends up, this part is heated and reaches a temperature of 100 mln °C. As a result, the outer layers expand, and then cool down: the star becomes a red giant. At this point, it loses stability, and its outer layers may be thrown out. Exactly they form a spherical shell around of what remains after the star – around a white dwarf. The shell surrounding the star expands at a rate of several tens of kilometers per second and forms a planetary nebula with a characteristic spherical shape. The star itself is usually located in the center of the nebula, and is visible as a bright spot – it is called the core of the nebula. Planetary nebulae belong to the bright emission nebulae, although their nature is different. Currently, more than 2,000 planetary nebulae are known in our Galaxy. However, a fairly quick end awaits planetary nebulae: they dilute in the process of extension in space, and as a result become indistinguishable in the heavenly vault. Around 25,000 years is spent on it – quite a short period in the life of any star. Having mastered the stage of planetary nebulae, some stars turn into white dwarfs. Planetary nebulae do not bear any threat to life forms living nearby. Moreover, scientists have found that ultraviolet radiation from the white dwarf in planetary nebulae provokes the destruction of molecules of carbon monoxide (CO) and promotes the formation of ions OH +, which could potentially lead to the formation of water. Astronomers have discovered a high content of OH + in areas with intense ultraviolet radiation near the white dwarfs (Smith, 2014). Scientists are interested in planetary nebulae, not only because of their beauty. They represent one of the main ways in which the elements heavier than hydrogen and helium, are scattered in space after their creation in the thermonuclear heart of the star. Ultimately, the former star ejected material could once again become a part of the new star or planet.
The distribution of colors in the picture of the planetary nebula allows drawing some conclusions about its structure. The inner part of the nebula is filled with very hot gas (more than 40 000 K). Hydrogen in this area is fully ionized. The glow is caused by the emission of atoms of triple ionized oxygen and nitrogen. These atoms emit the so-called "forbidden lines". In these ions, during the collisions with electrons, electrons often falls on the level, the transition to which is "forbidden" (there is a very low probability). Under Earth conditions, such a transition is carried out without radiation, because the ion gives away excess energy during collisions. In the extremely rarefied gas of a planetary nebula, collisions are extremely rare and a "forbidden" transition does take place.
In the yellow areas the temperature is slightly lower, there are few multiply ionized atoms of oxygen and nitrogen, and the glow occurs mainly on the yellow line of ionized helium. Finally, in the red areas the temperature is even lower, hydrogen shines there in the very strong red line Hα. The impression of the ring occurs for the same reason why the edge of the Sun appears sharp to us. A visual ray, which is sent to us from the central star, passes a thin layer, the optical thickness of which is very small. A visual ray, which is directed along the edge of a spherical gas cloud passes a significantly longer path in this layer, the optical thickness in this area appears quite large.
Typical planetary nebula consists of about 70% hydrogen and 28% helium. Carbon, nitrogen, oxygen, and small amounts of other chemical elements complement this composition. The length of such nebulae is an average of about one light year and their density – about 1,000 particles per cubic centimeter. However, the young, the newly formed nebulae are not as sparse. Their density can be up to a million particles per cubic centimeter. Planetary nebulae constitute small gas clouds, measuring no more than 0.1 parsec and the masses about 0,1M☉.
The total amount of energy emitted by the entire planetary nebula is ten times more than the radiation of the nucleus in the visible region of the spectrum. Since the central star is very hot and has a temperature of several tens of thousands of degrees, its maximum radiation is in the invisible ultraviolet region of the spectrum. Rigid core radiation ionizes the rarefied gas of the nebula and heats it to temperatures as high as one or two tens of thousands of degrees. Instead, nebulae atoms emit visible light whose spectrum contains the observed emission lines and weak continuous light.
Planetary nebulae, despite the similar process of formation, can be very different in outline. Perhaps that is why not only the number of directories but also the original names for similarities with a particular object or image, were often assigned to the nebulae. For decades, astronomers thought that many or most planetary nebulae are approximately spherical in shape. However, is now known that most of them are bipolar, i.e. they consist of two circular lobes projecting from opposite sides of the central star. Some planetary nebulae that appear spherical, such as Ring Nebula of Lyre constellation (Fig. 1), in fact, are also bipolar. The point is that the axis passing through the centers of these shares, is aimed at Earth, so they appear to be spherical – like a dumbbell when viewed from the end.
Planetary nebulae are relatively dim objects, so none of them are visible to the naked eye. The observations of the space telescope "Hubble" have made a great contribution to the study of their morphology. As a result of this work, color atlases of the most interesting forms that exist in the world of nebulae are presented on the Internet. High angular resolution of the telescope "Hubble" has allowed revealing the internal structure of nebulae, which was completely indistinguishable from Earth. Bizarre interior intersecting rings, symmetric with respect to "bubbles" center, embedded into each other shells with clear boundaries – all of these items showed a complex picture of the formation of the nebula in the very early stages. As a rule, the details of the nebula have axial symmetry.
We know about 2,000 planetary nebulae, but thousands of them may still be undiscovered and hidden from our sight. Planetary nebulae are formed by the stars that had at the beginning of their lives more than 8 solar masses. This is 95 percent of all the stars, and it is the only criterion that allows thinking that our own sun, eventually dies wrapped in one of these shells. But recent studies suggest that a dying star needs a companion, which helps to create the visible nebula. If it is so, then the individual stars like our Sun will end their lives slowly and gradually sinking into the darkness.
Figure 1 https://www.nasa.gov/mission_pages/hubble/science/ring-nebula.html#.V2ZxefmLS70
Planetary Nebula NGC 7009
The Cat's Eye Nebula
Egg Nebula
Figures 2-4
http://hubblesite.org/gallery/album/nebula/planetary/titles/true/
Works Cited
“Planetary Nebulae”. Williams College. Web. Accessed June 19, 2016 at
<https://web.williams.edu/Astronomy/research/PN/nebulae/nebulaegallery.php>
Smith, Brett. “Planetary Nebulae Observed Releasing Water-Building Molecules”.
RedOrbit.com. June 17, 2014. Web. Accessed June 19, 2016 at <http://www.redorbit.com/news/space/1113171628/water-building-molecules-discovered-planetary-nebulae-061714/>
