These mysterious objects, looking at people from the depths of space, have long ago attracted the attention of those for whom observing the sky has become a part of life. Even in the catalog of the ancient Greek scientist Hipparchus, several foggy objects were noted in the starry sky. And his colleague, Ptolemy, added five more nebulae to his catalog to those already known. Before Galileo's invention of the telescope, not many objects of this type could be seen with the naked eye. But already in 1610, a primitive telescope designed by Galileo aimed at the sky discovered the Orion Nebula there. Two years later, the Andromeda nebula was discovered. And since then, as telescopes improved, more and more new discoveries began, which eventually led to the identification of a special class of stellar objects - nebulae.
After some time, there were enough known nebulae that they began to interfere with the search for new objects, such as comets. And so, in 1784, the French astronomer Charles Messier, who was engaged in the search for comets, compiled the world's first catalog of cosmic nebulae, which was published in several parts. In total, there were 110 known objects of this class at that time.
When compiling the catalogue, Messier gave them numbers M1, M2 and so on, up to M110. Many objects in this catalog still have this designation.
However, at that time it was not known that the nature of various nebulae was completely different from each other. To astronomers, they were simply nebulous spots, different from ordinary stars.
Now, thanks to the achievements of astronomy, we know incomparably more about nebulae. What are these mysterious objects, and how do they differ from each other?
First of all, many will probably be surprised to learn that there are not only light nebulae. Today there are many known objects called dark nebulae. They are dense clouds of interstellar dust and gas that are opaque to light due to absorption by the dust contained in the nebula. Such nebulae stand out clearly against the background of the starry sky or against the background of light nebulae. A classic example of such a nebula is the Coalsack Nebula in the constellation of the Southern Cross. It often happens that such a nebula serves as material for the formation of new stars in its region due to the large amount of interstellar matter.
As for light nebulae, they also contain gas and dust. However, several factors may be responsible for the glow of such a nebula. Firstly, this is the presence of a star inside such a nebula or next to it. In this case, if the star is not too hot, then the nebula glows due to the light reflected and scattered by the cosmic dust included in its composition. This nebula is called a reflection nebula. A classic example of such an object is the perhaps well-known Pleiades cluster.
Another type of light nebula is ionized nebula. Such nebulae are formed as a result of strong ionization of the interstellar gas included in their composition. The reason for this is the radiation of a nearby hot star or other object that is a source of powerful radiation, including ultraviolet and x-rays. Thus, bright ionized nebulae are found in the cores of active galaxies and quasars. A number of these nebulae, also known as Region H II, are sites of active star formation. The hot young stars forming inside it ionize the nebula with powerful ultraviolet radiation.
Another type of cosmic nebula is planetary nebula. These objects arise as a result of the shedding of the outer shell by a giant star with a mass of 2.5 to 8 solar masses. This process occurs during a nova explosion (not to be confused with a supernova explosion, these are different things!), when part of the stellar matter is ejected into outer space. Such nebulae have the shape of a ring or disk, as well as a sphere (for novae).
A Supernova explosion also leaves behind a luminous nebula, heated up to several million degrees during the explosion. These are much brighter light-colored nebulae than ordinary planetary nebulae. Their lifespan is very short by cosmic standards - no more than 10 thousand years, after which they merge with the surrounding interstellar space. 
A rarer and more exotic type of nebula is the nebula around Wolf-Rayet stars. These are stars with a very high temperature and luminosity, with powerful radiation and the speed of outflow of stellar matter from their surface (over 1000 kilometers per second). Such stars ionize interstellar gas within a radius of several parsecs. However, very few stars of this type are known (there are just over 230 in our Galaxy), therefore there are correspondingly few nebulae of this type.
As you can see, our knowledge about cosmic nebulae today is quite extensive, although, of course, there is still a lot of uncertainty about the processes of their formation and life. However, this does not at all prevent us from admiring their beauty in the same way as our less knowledgeable ancestors did.
Some of my favorite objects)). And it’s even more surprising that such beauties are not included in the album. Therefore, I’ll make up for it (especially since I promised to continue about nebulae).
What is a planetary nebula? This is a star, called the core of the nebula, and a glowing shell of gas surrounding it. Planetary nebulae were discovered by W. Herschel around 1783. The name reflects their certain similarity with the disks of the outer planets - Uranus and Neptune. There are approximately 1,500 known planetary nebulae. With the development of observation technology, it became possible to see similar objects in the Magellanic Clouds, in the Andromeda Nebula and in a number of other galaxies.
During their lives, stars continuously lose matter in the form of the so-called. stellar wind. Depending on the mass of the star and the evolutionary stage it is in, the rate of mass loss may be greater or less. Our Sun, for example, is now losing matter very slowly, this is typical for not very massive main sequence stars. However, even a weak solar wind leads to some consequences, for example, it turns out to be the cause of such a beautiful phenomenon as the aurora. In the future, the Sun will lose matter much more actively. The shedding of a red giant's shell corresponds to the loss of quite a lot of mass in the form of a slow stellar wind. It is this substance that will make up the future nebula, and the appearance of the nebula depends on its structure. However, the ejected shell itself will not shine brightly: for the birth of a planetary nebula, a collision of two winds is necessary.
The scenario for the formation of a planetary nebula is as follows. Initially, the star must lose significant mass in the form of a slow stellar wind. This could be, for example, an ejected shell of a red giant (another option is associated with evolution in a binary system). After the shell is shed, a hot core remains from the star. It becomes a source of very fast stellar wind, the flow speed is about 1000 km per second. A fast wind catches up with a powerful slow stream, and their collision makes the substance glow, as if revealing an already “woven” intricate pattern.
Will our Sun ever present such a picture? Helix Nebula- a very close example of a planetary nebula that appears at the end of the life of a star similar to our Sun. The gas ejected into the surrounding space by the star gives the impression that we are looking at the curl of a spiral. The stellar core remaining in the center should eventually turn into a white dwarf. The central star emits intense radiation that causes the ejected gas to glow. The Helix Nebula is located in the constellation Aquarius and is designated in the catalog as NGC 7293. This nebula is located at a distance of 650 light years from us, its dimensions are 2.5 light years. The photo montage you see is based on the latest images from the ACS (Advanced Camera for Surveys) camera on board the Hubble Space Telescope and wide-angle images from the Mosaic Camera installed on the 0.9-m telescope at the Whale Peak Observatory. A close-up view of the inner edge of the Helix Nebula reveals a complex structure of gas formations of unknown origin. 
Planetary Hourglass Nebula
This is an image of the young planetary nebula MyCn18, located approximately 8 thousand light years away. years was obtained by the Wide Field Planetary Camera 2 on board the space telescope. The image was synthesized from three different images taken in the red line of ionized nitrogen, the green line of hydrogen, and the blue line of doubly ionized oxygen.
Previous images from Earth show two intersecting rings but lack detail. According to one theory, the formation of this shape is associated with a fast stellar wind inside a slowly expanding cloud, which has a density greater at the poles than at the equator. The space telescope also discovered other new unexpected properties in the structure of this nebula. For example, there are a couple of intersecting rings in the central area and numerous arcs. These features can be satisfactorily explained by the presence of an invisible companion star.
The planetary nebula pictured here is called Shapley 1 in honor of the famous astronomer Harlow Shapley, it has a pronounced ring structure.
The very fact of the existence of one of the largest spheres in our Galaxy is a source of valuable information about the chemical composition of stars. Planetary nebula Abell 39, which is now six light years across, represents the outer layers of the atmosphere of a solar-type star, ejected by it several thousand years ago. The almost perfect spherical shape of Abell 39 allowed astronomers to accurately estimate the ratio of absorbing and emitting matter in it. According to observational data, the oxygen content of Abell 39 is about half that of the Sun - a very interesting, although not surprising result, confirming the differences in the chemical composition of the two stars. The reason for the off-central position of the nebula's central star (it is displaced by 0.1 light years) has not yet been established. The distance to Abell 39 is about 7,000 light-years, and the galaxies visible up close and through the nebula are millions of light-years away.
This planetary nebula with two bubbles, imaged by the Space Telescope. Hubble, it “boils” beautifully. Designated Hubble-5 this bipolar planetary nebula was formed by a hot wind of particles escaping from the central star system. The hot gas expands into the surrounding interstellar medium in the form of inflating hot gas balls. A supersonic shock wave is formed at the boundary, which excites the gas. The gas glows when electrons recombine with atoms. In the picture, the colors correspond to the energy of recombination radiation. This nebula is located 2,200 light years from Earth. At the center of the nebula there is most likely a Sun-like star that is slowly turning into a white dwarf.
Why does this “ant” look so different from a ball? After all, planetary nebula Mz3- this is the shell thrown off by a star like our Sun, that is, an object, without a doubt, spherical. Why then does the gas flowing from the star give rise to an ant-shaped nebula, the shape of which has nothing in common with a ball? The reasons for this may be the extremely high - up to 1000 kilometers per second - speed of the emitted gas; the gigantic size of the structure, reaching one light year; or the presence of a strong magnetic field in a star located above the center of the nebula. Another star of lower luminosity may also be hidden in the depths of Mz3, which orbits the bright star at a very close distance from the latter. According to another hypothesis, gas flows owe their direction to the rotation of the central star and its magnetic field. Astronomers hope that, thanks to the central star's resemblance to the Sun, studying the history of this giant cosmic ant will provide a glimpse into the future of the Sun and our Earth.
This planetary nebula is formed by a dying star that sheds its shells of glowing gas. The nebula is located at a distance of three thousand light years. In today's picture taken by the space telescope. Hubble, shows how complex the structure of the nebula is cat's eye. Because of the complex structure visible in this image, astronomers believe the bright central object is a double star.
Eskimo Nebula
This planetary nebula, first discovered by Herschel in 1787, was nicknamed "Eskimo" because it resembled a face surrounded by a fur hood from ground-based telescopes. In the Hubble image, the "fur hood" appears as a disk of gas decorated with comet-like objects (see also the Helix Nebula) - elongated tails from the star.
"Face" also contains interesting details. The bright central region is nothing more than a bubble blown into space by the intense wind of fast particles from the star.
The Eskimo Nebula began to form about 10,000 years ago. It consists of two elongated bubbles of material flowing in opposite directions. In the picture, one of the bubbles lies above the other, overlapping it. The origin of the comet-like features remains mysterious.
The Eskimo Nebula is located 5,000 light-years from Earth in the constellation Geminga. The colors correspond to luminous gases: nitrogen (red), hydrogen (green), oxygen (blue) and helium (purple).
This beautiful planetary nebula, cataloged as NGC 6369, was discovered by the 18th century astronomer William Herschel when he explored the constellation Ophiuchus with a telescope. Round and planet-like, this relatively faint nebula has received the popular name Nebula Little Ghost. The amazingly intricate details of NGC 6369's structure are revealed in this remarkable color image from data from the Hubble Space Telescope. The nebula's main ring is about a light year in diameter. Emission from ionized oxygen, hydrogen, and nitrogen atoms is shown in blue, green, and red, respectively. The Little Ghost Nebula, more than 2,000 light years away, reveals the future fate of our Sun, which is also about to form its own beautiful planetary nebula, but not before? than in about five billion years.
Planetary nebula IC 418, nicknamed The Spirograph Nebula for its similarity to the drawing instrument of the same name, it is distinguished by a very unusual structure, the origin of which still remains largely unsolved. The nebula's bizarre shape may be due to the chaotic wind emanating from the central variable star, whose brightness varies unpredictably over time intervals of just a few hours. Moreover, according to available data, just a few million years ago, IC 418 was apparently a simple star similar to our Sun. Just a few thousand years ago, IC 418 was an ordinary red giant. However, after exhausting its nuclear fuel reserves, the outer shell of the star began to expand, leaving behind a hot core, which fate destined to turn into the white dwarf star located in the center of the image. Radiation from the central core excites atoms in the nebula, causing them to glow. IC 418 is located about 2000 light years away from us, and its diameter is 0.3 light years. This false-color image taken recently by the Hubble Space Telescope shows unusual details in the nebula's structure.
In the center NGC 3132, an unusual and beautiful planetary nebula, is home to a double star. By its origin this nebula, also called Nebula of eight flashes or Southern Annular Nebula, is due not to a bright, but to a faint star. The source of the glowing gas is the outer layers of a star similar to our Sun. The energy for the hot blue glow around the binary that you see in the picture comes from high temperatures on the surface of the faint star. The planetary nebula initially became an object of research due to its unusual symmetrical shape. She subsequently attracted attention when she was revealed to have asymmetrical details. So far, neither the strange shape of the cooler envelope nor the structure and origin of the cold dust lanes crossing the nebula NGC 3132 have been explained.
Is it true that stars look more beautiful when they die? Planetary nebula M2-9, Butterfly Nebula, is located at a distance of 2100 light years from Earth. The nebula's wings may tell us an unusual, unfinished story. At the center of the nebula is a double star system. The stars of this system move inside a gas disk 10 times the diameter of Pluto's orbit. The ejected shell of a dying star bursts out of the disk, forming bipolar structures. Much remains unclear about the physical processes that result in the formation of a planetary nebula.
How could a square nebula form around a round star? A study of a planetary nebula like IC 4406. There is reason to believe that the nebula IC 4406 has the shape of a hollow cylinder, and the square shape is explained by the fact that we are looking at this cylinder from the side. If we were to look at IC 4406 from the end, it might well look like the Ring Nebula. This color image is a combination of images taken by the Hubble Space Telescope. Hot gas flows from the ends of the cylinder, and filaments of dark dust and molecular gas line its walls. The star that is responsible for this piece of interstellar sculpture lies at the center of a planetary nebula. In a few million years, all that will remain of IC 4406 is a fading white dwarf.
Rapidly expanding clouds of gas spell the end of the nebula's central star Rotten Egg. Once there was a normal star, it used up its reserves of nuclear fuel, as a result of which its central part collapsed, forming a white dwarf. Part of the released energy causes the outer shell of the star to expand. In this case, the result is a photogenic protoplanetary nebula. When the gas, moving at millions of kilometers per hour, hits the surrounding interstellar gas, it creates a supersonic shock wave in which ionized hydrogen and nitrogen glow blue. Previously, there were hypotheses about the complex structure of the shock front, but until now such clear images had not been obtained. Thick layers of gas and dust hide the dying central star. The Rotten Egg Nebula, also known as the Pumpkin Nebula and OH231.8+4.2, is likely to develop into a bipolar planetary nebula within 1000 years. The nebula shown above is about 1.4 light years in size and located 5000 light years away in the constellation Puppis
You can show the pictures endlessly, especially since they are amazingly beautiful.
When observing the sky through a telescope, you can sometimes stumble upon curious nebulae with rounded outlines. These are planetary nebulae - objects corresponding to the final phase of the existence of stars like the Sun. In fact, each of them is a spherical shell of gas, the outer layer of the star, ejected by it after losing its own stability. These shells then enlarge, expand and gradually become weaker. Observing such nebulae is not easy: most of them have low surface brightness and small angular size. As with other nebulae, dark, moonless nights are required for observation. Very rarely, the identification of a planetary nebula can be helped by a small star located in its center and which gave it its origin.
Ring Nebula
Of all the planetary nebulae visible in the sky, the most famous among astronomy enthusiasts is certainly the M57 nebula, which also has the name Ring Nebula. It is located in the summer constellation Lyra at a distance of about 2300 light years from Earth.
This nebula was discovered in 1779 by the French astronomer Antoine Darquier de Pellepoix. He described it as a perfect disk approximately the size of Jupiter, but with a faint glow and similar to a disappearing planet. Subsequently, in 1785, the English astronomer William Herschel defined it as a “celestial landmark.” He thought that this nebula was a ring of stars.
With a hole
In your telescope, M57 will appear as a small, round, nebulous speck. It makes sense to view it at medium magnification, for example through a 12.5 mm Plössl eyepiece, which provides 80x magnification. At first glance you will notice rounded outlines. After a few minutes of adaptation, if the air is clear and still and there is no interference from the Moon, you will be able to make out some details. By increasing the magnification, you will even be able to discern the central “hole,” especially if you look with “diffused vision,” that is, concentrating your gaze not on the “hole” itself, but on its periphery.
Central star
This nebula was born from the star at its center, which today has become a white dwarf. The surface temperature of this star exceeds 100,000 degrees. Its magnitude is 14.7, making it inaccessible to your telescope. It was discovered in 1800 by the German philosopher and astronomer Friedrich von Hahn.
The nebula is expanding at a speed of approximately 20-30 km/s, and therefore its apparent size is increasing by approximately 1 arcsecond per century.
Nebula formation
After the first planetary nebulae were discovered, their rounded outlines led astronomers to believe that these celestial objects were associated with something similar to planets, most likely gas giants or an emerging planetary system. For this reason, the English astronomer William Herschel (who had recently discovered the planet Uranus) proposed the term “planetary nebula” for such objects. Their true nature was established only in the mid-19th century thanks to spectroscopy (a technique that allows the light coming from a celestial body to be “split” into its primary colors). Then it became clear that before us was a special type of nebula.
Dying Star
All planetary nebulae originate from stars in the final stages of their existence. As we have already noted, a star with a mass comparable to the mass of the Sun, after its birth, goes through a long stage of stability, during which it melts hydrogen nuclei, giving rise to helium nuclei. When the hydrogen contained in the central part of the star runs out, this part heats up and reaches a temperature of 100 million degrees. As a result, the outer layers expand and then cool: the star turns into a red giant. At this point, it loses stability and its outer layers can be thrown out. It is they who form a spherical shell around what remains of the star - around the white dwarf.
Extension
The shell surrounding the star expands at a speed of several tens of kilometers per second and forms a planetary nebula with a characteristic spherical shape. Planetary nebulae, however, face a fairly quick end: as they expand in space, they become rarer and, as a result, become indistinguishable in the firmament. This takes about 25,000 years - a very short period in the life of any star.
Planetary nebulae through a telescope
When observing planetary nebulae, difficulties arise somewhat different than when observing diffuse nebulae, such as the Orion Nebula. Planetary nebulae do not have large angular sizes. With the exception of the Helix Nebula, they appear small and concentrated in the sky. Therefore, they can be difficult to distinguish from stars.
Helix Nebula
In addition to M57, you can observe about a dozen other planetary nebulae with your telescope. The first among them will be the Helix Nebula from the constellation Aquarius. It reaches an impressive size - approximately 13 minutes of arc (which corresponds to a real size of about 3 light years).
It is no coincidence that this nebula is also one of the closest to the Solar System. Despite its magnitude of 7.6, due to its size it spreads its glow over a very wide area of the night sky. Through a telescope, this nebula appears greenish. It is quite faintly visible. Inside it, the Hubble Space Telescope saw thousands of gas balls, apparently formed at the moment when the dying star ejected its outer shell into space.
Saturn Nebula
In the same zodiac constellation Aquarius, the nebula NCG 7009, known as the Saturn Nebula, is of interest for observation. William Herschel discovered it in 1782. The main difficulty in observing this nebula is its size, which is less than 2 arc minutes.
Nevertheless, at 50x magnification you can understand that this is not a star, and at 100-150x you can discern a characteristic elongated shape. It is for this shape that the nebula received its name, coinciding with the name of the planet with rings.
Another nebula easily accessible for observation is M27 from the constellation Vulpecula. It is also called the “Dumbbell Nebula”. Its apparent diameter is approximately 8 arc minutes and its total magnitude is 7.4. According to astronomers, this nebula formed 3000-4000 years ago. At high magnification you can see her elongated
the form for which she got her name.
There is also a smaller version of M27, at least according to Anglo-Saxon astronomers, who call the planetary nebula M76 the Little Dumbbell. It was discovered by Méchain in 1780, but its membership as a planetary nebula was recognized only in 1918. The 16.6 magnitude star at the center of M76 is too faint for your telescope.
Ghost and Owl
Much more difficult to observe is the nebula NGC3242, which also has the curious name Ghost of Jupiter. This is explained by the fact that in a telescope its diameter is comparable to the diameter of Jupiter. With a 25 mm Plössl eyepiece at 40x magnification you can see it without much difficulty, and at a magnification of over 100 you can even discern its round shape.
Nebula M97, the fourth nebula included in the Messier catalog, also has a funny name. It is located in the constellation Ursa Major. Irish astronomer William Warsons named it the Owl in 1848 because the two dark spots inside it resemble an owl's eyes.
At a magnification of just over 100, you will be able to discern not only the round shape of the nebula, but also two dark areas within it. M97 is believed to be approximately 8,000 years old.
Snowball
It is quite difficult to distinguish the nebula NGl 7662, or the Blue Snowball, in the sky in the constellation Andromeda. In fact, despite its name, it has a reddish tint in a telescope.
At a magnification of over 100, you can also see the “hole” in its center. The advantage of viewing this nebula is that it is located in a constellation that rises very high in our sky in late autumn.
White dwarfs
The planetary nebula NGC 1514, discovered by William Herschel in 1790 in the constellation Taurus, is very difficult to observe because it glows faintly and is barely visible against the celestial background. Much easier to spot is the white dwarf at its center, magnitude 9.4 NGC 1514 can be found about 8 degrees northeast of the Pleiades. Another planetary nebula with a white dwarf visible to your telescope is NGC6826, located in the constellation Cygnus. This is a small and faint nebula: in a telescope it will appear as a blurry star, and only by increasing the magnification to maximum will you be able to see its circular shell. However, if the sky is very dark, then you may notice a star of magnitude 10.4 in its center.
The same can be said about the planetary nebula NGC2392, also known as the Eskimo Nebula, in the constellation Gemini. A white dwarf of magnitude 10.5 will be visible inside the small, faint bluish nebula.
Planetary nebulae as seen by Hubble
Many planetary nebulae, unfortunately, remain inaccessible to observation with an amateur telescope. Although we are often talking about magnificent, very spectacular objects, some of the most beautiful in the sky. The Hubble Space Telescope has photographed some of these nebulae, allowing us to appreciate their brilliant colors and curious shapes.
Even though you won't be able to observe them with your telescope, it is worth talking about the most spectacular and interesting planetary nebulae.
Cat's Eye
You can start from the Cat's Eye Nebula (NGC 6543) in the constellation Draco. In 1864, William Hoggins examined its light with a spectroscope (the planetary nebula was then subjected to such analysis for the first time). Although it was discovered back in 1786, only recently the Hubble telescope revealed its complex and delicate structure, consisting of concentric gas shells, streams and nodules. Astronomers have concluded that approximately every 1,500 years, the central star emits a new shell. The images, taken about 10 years apart, showed that the nebula was expanding.
The nebula NGC 6369 is located in the constellation Ophiuchus at a distance of 2000 to 5000 light years. Its blue-green ring, which measures approximately 1 light-year in diameter, marks the edge of the region where the star's ultraviolet light has ionized the gas, that is, stripped electrons from its atoms. The outer part of the nebula has a more pronounced red tint because the ionization process is less intense at a greater distance from the star. The cloud is expanding at a speed of approximately 20 km/s. Due to this, it will disperse into interstellar space and then disappear after about 10,000 years.
Such as carbon, nitrogen, oxygen and calcium).
In recent years, with the help of images obtained by the Hubble Space Telescope, it has been possible to find out that many planetary nebulae have a very complex and unique structure. Although about a fifth of them are circumspherical, the majority do not have any spherical symmetry. The mechanisms that make it possible to form such a variety of forms remain not fully understood to date. It is believed that the interaction of the stellar wind and double stars, the magnetic field and the interstellar medium may play a large role in this.
History of research
Planetary nebulae are mostly faint objects and are usually not visible to the naked eye. The first discovered planetary nebula was the Dumbbell Nebula in the constellation Vulpecula: Charles Messier, who was searching for comets, when compiling his catalog of nebulae (stationary objects similar to comets when observing the sky) in 1764, cataloged it under the number M27. In 1784, William Herschel, the discoverer of Uranus, identified them as a separate class of nebulae when compiling his catalog ( class IV nebulae) and proposed the term "planetary nebula" for them due to their apparent resemblance to the disk of Uranus.
The unusual nature of planetary nebulae was discovered in the middle of the 19th century, with the beginning of the use of spectroscopy in observations. William Huggins became the first astronomer to obtain spectra of planetary nebulae - objects that stood out for their unusualness:
Some of the most mysterious of these remarkable objects are those that appear as round or slightly oval disks when viewed telescopically. ...Their greenish-blue color is also remarkable, extremely rare for single stars. In addition, in these nebulae there are no signs of central condensation. Based on these characteristics, planetary nebulae stand out sharply as objects that have properties completely different from the properties of the Sun and fixed stars. For these reasons, and also because of their brightness, I chose these nebulae as the most suitable for spectroscopic study.
Another problem was the chemical composition of planetary nebulae: Huggins, by comparison with standard spectra, was able to identify lines of nitrogen and hydrogen, but the brightest of the lines with a wavelength of 500.7 nm was not observed in the spectra of the then known chemical elements. It was hypothesized that this line corresponded to an unknown element. It was given the name nebulium in advance - by analogy with the idea that led to the discovery of helium during a spectral analysis of the Sun in 1868.
Assumptions about the discovery of a new element nebulia were not confirmed. At the beginning of the 20th century, Henry Russell hypothesized that the line at 500.7 nm corresponded not to a new element, but to an old element under unknown conditions.
The resumption of thermonuclear reactions prevents further compression of the nucleus. Burning helium soon creates an inert core consisting of carbon and oxygen, surrounded by a shell of burning helium. Thermonuclear reactions involving helium are very sensitive to temperature. The reaction rate is proportional to T40, that is, an increase in temperature of just 2% will lead to a doubling of the reaction rate. This makes the star very unstable: a small increase in temperature causes a rapid increase in the rate of reactions, increasing the release of energy, which, in turn, causes the temperature to increase. The upper layers of burning helium begin to expand rapidly, the temperature drops, and the reaction slows down. All this can cause powerful pulsations, sometimes strong enough to eject a significant part of the star's atmosphere into outer space.
The ejected gas forms an expanding shell around the exposed core of the star. As more and more of the atmosphere is stripped away from the star, deeper and deeper layers with higher temperatures are revealed. When the exposed surface (photosphere of the star) reaches a temperature of 30,000 K, the energy of the emitted ultraviolet photons becomes sufficient to ionize the atoms in the ejected material, causing it to glow. Thus, the cloud becomes a planetary nebula.
Lifespan
The matter of the planetary nebula flies away from the central star at a speed of several tens of kilometers per second. At the same time, as matter flows out, the central star cools, emitting remaining energy; Thermonuclear reactions stop because the star no longer has enough mass to maintain the temperature required to fuse carbon and oxygen. Eventually, the star will cool so much that it will no longer emit enough ultraviolet light to ionize the outlying shell of gas. The star becomes a white dwarf, and the gas cloud recombines, becoming invisible. For a typical planetary nebula, the time from formation to recombination is 10,000 years.
Galactic Recyclers
Planetary nebulae play a significant role in the evolution of galaxies. The early Universe consisted primarily of hydrogen and helium, from which type II stars. But over time, as a result of thermonuclear fusion, heavier elements were formed in stars. Thus, the matter of planetary nebulae has a high content of carbon, nitrogen and oxygen, and as it expands and penetrates into interstellar space, it enriches it with these heavy elements, generally called metals by astronomers.
Subsequent generations of stars, formed from interstellar matter, will contain a larger initial amount of heavy elements. Although their share in the composition of stars remains insignificant, their presence significantly changes the life cycle type I stars(See Stellar Population).
Characteristics
physical characteristics
A typical planetary nebula has an average extent of one light year and consists of highly rarefied gas with a density of about 1000 particles per cm³, which is negligible in comparison, for example, with the density of the Earth's atmosphere, but about 10-100 times greater than the density of interplanetary space on distance of the Earth's orbit from the Sun. Young planetary nebulae have the highest density, sometimes reaching 10 6 particles per cm³. As nebulae age, their expansion causes their density to decrease.
Radiation from the central star heats gases to temperatures on the order of 10,000. Paradoxically, the temperature of a gas often increases with increasing distance from the central star. This happens because the more energy a photon has, the less likely it is to be absorbed. Therefore, low-energy photons are absorbed in the inner regions of the nebula, and the remaining high-energy photons are absorbed in the outer regions, causing their temperature to increase.
Nebulae can be divided into poor in matter And radiation poor. According to this terminology, in the first case, the nebula does not have enough matter to absorb all the ultraviolet photons emitted by the star. Therefore, the visible nebula is completely ionized. In the second case, the central star emits not enough ultraviolet photons to ionize all the surrounding gas, and the ionization front passes into neutral interstellar space.
Since most of the gas in a planetary nebula is ionized (that is, plasma), magnetic fields have a significant effect on its structure, causing phenomena such as filamentation and instability of the plasma.
Quantity and distribution
Today, in our galaxy, consisting of 200 billion stars, 1,500 planetary nebulae are known. Their short lifespan compared to stellar ones is the reason for their small number. Basically, they all lie in the plane of the Milky Way, and are mostly concentrated near the center of the galaxy, and are practically not observed in star clusters.
The use of CCD matrices instead of photographic film in astronomical research has significantly expanded the list of known planetary nebulae.
Structure
Most planetary nebulae are symmetrical and almost spherical in appearance, which does not prevent them from having many very complex shapes. Approximately 10% of planetary nebulae are practically bipolar, and only a small number are asymmetric. Even a rectangular planetary nebula is known. The reasons for this diversity of shapes are not fully understood, but it is believed that gravitational interactions between stars in binary systems may play a large role. According to another version, existing planets disrupt the uniform spreading of matter during the formation of a nebula. In January 2005, American astronomers announced the first detection of magnetic fields around the central stars of two planetary nebulae, and then suggested that they were partly or entirely responsible for creating the shape of these nebulae. The significant role of magnetic fields in planetary nebulae was predicted by Grigor Gurzadyan back in the 1960s. There is also an assumption that the bipolar shape may be due to the interaction of shock waves from the propagation of the detonation front in the helium layer on the surface of the forming white dwarf (for example, in the Cat's Eye, Hourglass nebulae, it will be possible to calculate the expansion rate along the line of sight. Comparison of the angular expansion with the obtained expansion rate will make it possible to calculate the distance to the nebula.
The existence of such a variety of nebula shapes is a topic of heated debate. It is widely believed that this may be due to interactions between matter moving away from the star at different speeds. Some astronomers believe that binary star systems are responsible for at least the most complex shapes of planetary nebulae. Recent studies have confirmed the presence of powerful magnetic fields in several planetary nebulae, which has already been suggested several times. Magnetic interactions with ionized gas may also play a role in determining the shape of some of them.
At the moment, there are two different methods for detecting metals in the nebula, based on different types of spectral lines. Sometimes these two methods give completely different results. Some astronomers are inclined to explain this by the presence of weak temperature fluctuations within the planetary nebula. Others believe that the differences in observations are too dramatic to be explained by temperature effects. They hypothesize the existence of cold clumps containing very small amounts of hydrogen. However, clumps, the presence of which, in their opinion, could explain the difference in the estimate of the amount of metals, have never been observed.
Physics of planetary nebulae. - M.: Nauka, 1982.
In the fifth article of the series "Observations of deep space objects" I will introduce you to some recommendations for observing planetary nebulae. In the previous four articles, you learned how to observe globular clusters, open star clusters, galaxies and diffuse nebulae. All recommendations are preferable for telescopes with an aperture of 110 mm or more. For planetary cameras, a lens diameter of 150 mm or more is better.
Almost all planetary nebulae have a very small angular size, which is comparable to the size of Jupiter (40″). The surface brightness of these nebulae is quite high. It is recommended to use telescope magnification: 80x - 200x.
But there are planetary nebulae with low brightness, for which there is no point in using a higher magnification eyepiece or a Barlow diverging lens, which gives higher magnification. For such nebulae, it is difficult to select recommendations and give advice on the use of magnification; everything is very subjective and the reader can choose (select) himself. Faint “planetaries” include: M 27, M 76, M 97, NGC 4361).
Planetary nebula with weak surface brightness
Let me remind you that when you have found the desired object for observation (in our case, a planetary nebula), follow the following instructions. It will help you learn and gain as much information as possible in practice. Don't forget to keep notes, this will speed up your memorization process and will be useful in the future for comparing objects with others of the same type, and will also teach you to distinguish and notice the subtleties of each of the objects.
Observing a planetary nebula
- As always, we start by estimating the angular size of the desired object. For a better and more accurate estimate, compare it with the planet Jupiter, which can be seen at the same magnification.
- What shape does the nebula have? Hollow inside, round, oval, incomprehensible? Is it possible to see and give any information about the edges of the nebula? What are they?
- Is the brightness distributed evenly from the center to the edges? Maybe one area is saturated, another less so, or some color is visible?
- What general color is visible through a telescope? Is the nebula completely gray? Or maybe bluish-gray? Is there a reddish tint visible?
- Take a look around. What can you say about the stars behind the “planetary”, around it? Are there any very bright ones?
- What is the approximate brightness of the object under study?
- Lastly, when the eye and brain have absorbed enough information, determine what the nebula looks like? Are there any similarities with any object?
That's all...Take yourself away from the telescope for a few seconds and give your eyes a rest. Visualize in front of you what you just observed. Take another look at the eyepiece and fix it. Check your notes. If all is well, then you can finish observing this planetary nebula and, after a short pause, switch to a new object.
Here are a few simple, but in my opinion very useful and necessary recommendations worth following when observing planetary nebulae. Until new articles, take care of your eyes and don’t miss a single cloudless starry night.