Nebulae in space are one of the wonders of the Universe, striking in their beauty. They are valuable not only in visual appeal. The study of nebulae helps scientists to clarify the laws of functioning of the cosmos and its objects, to correct theories about the development of the universe and the life cycle of stars. Today we know a lot about these objects, but not all.
A mixture of gas and dust
For quite a long time, right up to the middle of the century before last, nebulae were considered star clusters, remote from us for considerable distances. The use of a spectroscope in 1860 made it possible to establish: many of them consist of gas and dust. The English astronomer W. Haggins found that light from nebulae is different from radiation coming from ordinary stars. The spectrum of the first contains bright colored lines interspersed with dark ones, while in the second case such black bands are not observed.
Further studies have established that the nebulae of the Milky Way and other galaxies are mainly composed of a hot mixture of gas and dust. Often there are similar cold formations. Such interstellar gas clouds also belong to nebulae.
Classification
Several types of elements are distinguished depending on the properties of the elements making up the nebula. All of them are presented in large numbers in the vast expanses of space and are equally interesting for astronomers. Nebulae that emit light for one reason or another are commonly called diffuse or bright. Opposing them in the main parameter, of course, are designated as dark. Diffuse nebulae are of three types:
reflective;
issuing;
supernova remnants.
Emission, in turn, are divided into areas of the formation of new stars (H II) and planetary nebulae. All these types are characterized by certain properties that make them unique and worthy of close study.
Star Formation Areas
All emission nebulae are clouds of luminous gas of various shapes. The main element that makes them up is hydrogen. Under the influence of a star located in the center of the nebula, it ionizes and collides with atoms of the heavier components of the cloud. The result of these processes is a characteristic pinkish glow.
The Eagle Nebula, or M16, is an excellent representative of this type of object. Here is the region of star formation, many young as well as massive hot bodies. The Eagle Nebula is the place where the well-known piece of space, the Pillars of creation, is located. These gas clots, formed under the influence of stellar wind, are a zone of star formation. The formation of luminaries here leads to the compression of gas and dust columns under the action of gravity.
Recently, scientists have learned that we can only admire the Pillars of creation for another thousand years. Then they will disappear. In fact, the destruction of the Pillars occurred about 6,000 years ago due to a supernova explosion. However, light from this area of space has been reaching us for about seven thousand years, so the event calculated by astronomers for us is only a matter of the future.
Planetary nebula
The name of the next type of luminous gas and dust clouds was introduced by W. Herschel. The planetary nebula is the last stage in the life of a star. The shells discharged by the luminary form a characteristic pattern. The nebula resembles a disk, usually surrounding a planet when observed through a small telescope. Today, more than a thousand of such objects are known.
Planetary nebulae are part of the process of transforming red giants into white dwarfs. At the center of the formation is a hot star, similar in its spectrum to the class O luminaries. Its temperature reaches 125,000 K. The planetary nebulae are generally relatively small - 0.05 parsecs. Most of them are located in the center of our galaxy.
The mass of the gas shell discarded by the star is small. It is tenths of a similar parameter of the Sun. A mixture of gas and dust moves away from the center of the nebula at a speed reaching 20 km / s. The shell has existed for about 35 thousand years, and then becomes very thin and indistinguishable.
Features
A planetary nebula can be of various shapes. Basically, one way or another, it is close to the ball. There are round, ring-shaped nebulae, similar to dumbbells, of irregular shape. The spectra of such space objects include the emission lines of a luminous gas and a central star, as well as sometimes absorption lines from the spectrum of the star.
A planetary nebula radiates a huge amount of energy. It is much larger than that for the central star. The core of the formation, due to its high temperature, emits ultraviolet rays. They ionize gas atoms. Particles warm up, instead of ultraviolet light, they begin to emit visible rays. Their spectrum also contains emission lines characterizing the formation as a whole.
Cat's eye nebula
Nature is an expert at creating unexpected and beautiful forms. Noteworthy in this regard is the planetary nebula, due to the similarity called the Cat's Eye (NGC 6543). It was discovered in 1786 and became the first that scientists have identified as a cloud of luminous gas. The Cat's Eye Nebula is located in the constellation Dragon and has a very interesting complex structure.
It was formed about 100 years ago. Then the central star dropped its shells and concentric lines of gas and dust were formed, characteristic of the pattern of the object. To date, the formation mechanism of the most expressive central structure of the nebula remains incomprehensible. The appearance of such a pattern is well explained by the location in the core of the nebula of a double star. However, so far there is no evidence to support this state of things.
The temperature of the halo NGC 6543 is approximately 15,000 K. The core of the nebula is heated to 80,000 K. Moreover, the central star is several thousand times brighter than the sun.
Colossal explosion
Massive stars often end their life cycles with spectacular “special effects.” The explosions, huge in their power, lead to the loss by the luminary of all outer shells. They move away from the center at a speed exceeding 10,000 km / s. Collision of a moving substance with a static one causes a strong increase in gas temperature. As a result, its particles begin to glow. Often supernova remnants are not globular formations, which seems logical, but nebulae of a very different shape. This happens because the substance ejected at great speed unevenly forms clumps and clusters.
Trace of a thousand years ago
Perhaps the most famous supernova remnant is the crab nebula. The star that spawned it exploded almost a thousand years ago, in 1054. The exact date was established by the Chinese annals, where its outbreak in the sky is well described.
The characteristic pattern of the crab-shaped nebula is a gas ejected by a supernova and not yet fully mixed with interstellar matter. The object is located at a distance of 3300 light years from us and is continuously expanding at a speed of 120 km / s.
In the center, the crab-shaped nebula contains a supernova remnant - a neutron star, which emits streams of electrons that are sources of continuous polarized radiation.
Reflecting nebula
Another type of these space objects consists of a cold mixture of gas and dust, unable to independently emit light. Reflecting nebulae glow due to nearby objects. It can be stars or similar diffuse formations. The spectrum of scattered light remains the same as that of its sources, but blue light prevails in it for the observer.
A very interesting nebula of this type is associated with the star of Merop. The luminary from the Pleiades cluster has been destroying a passing molecular cloud for several million years. As a result of the action of the star, the particles of the nebula line up in a certain sequence and stretch towards it. After some time (the exact date is unknown), Merope can completely destroy the cloud.
A dark horse
Diffuse formations are often opposed by an absorbing nebula. The Milky Way Galaxy has a lot of them. These are very dense clouds of dust and gas, absorbing the light of emission and reflection nebulae located behind them, as well as stars. These cold cosmic formations are mainly composed of hydrogen atoms, although heavier elements are also found in them.
A magnificent representative of this type is the Horsehead Nebula. It is located in the constellation Orion. The shape characteristic of the nebula, so similar to the horse’s head, was formed as a result of stellar wind and radiation. The object is clearly visible due to the fact that the background is a bright emission formation. At the same time, the Horsehead Nebula is only a small part of an extended absorbing cloud of dust and gas, which is practically invisible.
Thanks to the Hubble telescope, nebulae, including planetary ones, are familiar to a wide circle of people today. The photographs of the cosmos where they are located are impressive to the core and leave no one indifferent.