A planetary nebula is an astronomical object, and consists of a shell of gas and plasma that is repelled from an old star at the end of its development.
The name is historical and misleading as those mists have nothing to do with planets. The name comes from the fact that they usually appear in the telescope round and greenish like distant gas planet.
Planetary nebulae usually do not exist for more than a few tens of thousand years. Compared to an average "star of life ", which sometimes takes several billion years, this time period is very short.
In our galaxy, the Milky Way, about 1500 planetary nebulae are known.
Planetary nebulae play a crucial role in the chemical evolution of the galaxy, because the sloughed material enriches the interstellar medium with heavy elements such as carbon, nitrogen, oxygen, calcium, and other reaction products of stellar nuclear fusion. In other galaxies, planetary nebulae are sometimes the only observable objects that provide enough information to learn about the chemical composition.
With the Hubble Space Telescope images were taken of many planetary nebulae. One fifth of the fog has a spherical shape. The majority, however, is a complex structure and has different forms. The mechanisms of shaping are not yet known. Possible causes could be companion stars, stellar winds or magnetic fields.
Planetary nebulae are generally faint objects become and therefore not observable with the naked eye. The first planetary nebula discovered was the Dumbbell Nebula in the constellation of fox. It was discovered in 1764 by Charles Messier and is listed in its catalog with the index M 27.
Due to the relatively low optical resolution of that telescopes saw a planetary nebula like a tiny misty glass. Since the 1781 -discovered planet Uranus a similar sight met, led its discoverer William Herschel in 1785 for this fog to this day retained a name.
The composition of planetary nebulae remained unknown until the mid-19th century, the first spectroscopic observations were carried out. William Huggins was one of the first astronomers who studied the light spectrum of astronomical objects by their light he decomposed spectrally using a prism. His observations of stars showed a continuous, ie continuous spectrum with some dark absorption lines. A little later he found out that some nebulous objects such as the Andromeda Galaxy, a very similar spectrum exhibited. This fog later turned out to be galaxies. However, when Huggins was watching the Cat's Eye Nebula, he found a very different spectrum. This was not continuous with a few absorption lines, but merely drew some emission lines. The lightest line had a wavelength of 500.7 nanometers. This was in no way connected with any known chemical element. First, it was therefore assumed that it was an unknown element, which then received the name nebulium.
1868 had been discovered in the investigation of the spectrum of the sun, the hitherto unknown element helium. Although already the helium was able to demonstrate and isolate in the Earth's atmosphere shortly after this discovery, nebulium not found one. Beginning of the 20th century, Henry Norris Russell suggested that it was not a question, a new element that the wavelength 500.7 nm caused, but rather a known element in unknown conditions.
In the 1920s, physicists showed that the gas has an extremely low density in planetary nebulae. Electrons can reach in the atoms and ions metastable energy levels that may exist only briefly by the constant collisions at higher densities else. Electron transitions in oxygen lead to emission at 500.7 nm spectral lines, which can only be observed in gases with very low densities, forbidden lines are called.
Until the early 20th century, it was assumed that planetary nebulae represent the precursors of stars. It was believed that the fog WOULD CHOOSE together under its own gravity and the center formed a star. Later spectroscopic studies showed, however, that extend all planetary nebulae. Thus, it was found that the fog represent the scuffed outer layers of a dying star that remains to be very hot, but light faint object in the center.
Towards the end of the 20th century helped the progressive technology to better understand the evolution of planetary nebulae. Due to space telescopes, astronomers were able to examine also emitted electromagnetic radiation outside the visible spectrum, which can not be observed because of the earth's atmosphere by ground-based observatories. Through observation and the infrared and ultraviolet radiation components of the planetary nebulae can be the temperature, density and chemical composition determine much more accurately. With the help of CCD technologies, their spectral determine more precise and also make extremely weak lines visible. Planetary nebulae, which had simple and regular structures in ground-based telescopes, showed very complex shapes due to the high resolution of the Hubble Space Telescope.
Planetary nebulae represent the terminal stage of an average star like our sun dar.
A typical star has less than twice the mass of the Sun. His energy is generated in the core, in which runs the nuclear fusion of hydrogen into helium. The resulting radiation pressure prevents the star collapses by its own gravity. It provides a stable state that can persist for billions of years.
After several billion years, the hydrogen inventories are consumed in the core. Radiation pressure decreases and the core is compressed by the force of gravity and is heating up. The temperature in the core rises in this phase of 15 million to 100 million K. Helium in the core then fused to carbon and oxygen. In the "shell " around the core hydrogen fuses into helium. As a result, the envelope of the star expands greatly, he steps on the asymptotic giant in the stage of a red giant.
The helium fusion is very sensitive to temperature. The reaction rate is proportional to the power 30, the temperature, and therefore doubles in a temperature increase of only 2.3%. This makes the star very unstable - a small increase in temperature immediately leads to a significant increase in the reaction rate, which liberates considerable energy, causing the temperature continues to rise. The layers in which is taking place the helium fusion, expand with high speed and thereby cool down again, so that the reaction rate is lowered again. The result is a strong pulsation, which is sometimes strong enough to throw the whole stellar atmosphere into space.
The gas of the stellar envelope expands initially at a rate of 20 to 40 kilometers per second and has a temperature of about 10,000 K. This relatively slow stellar wind forms the bulk of the nebula. To the extent in which the star gradually loses its outer shells and exposing the increasingly hotter core, its color changes from orange to yellow to white and eventually blue - a visible sign that its surface temperature rises to about 25,000 K. If the exposed surface is approximately 30,000 K hot, enough high-energy ultraviolet photons are emitted to ionize the previously ejected gas. The gaseous envelope becomes visible as a planetary nebula. The star at the center has reached the stage of a white dwarf.
Time in view of the nebula
The ejected gases of planetary nebulae are moving away at a speed of several kilometers per second from the center. The stellar wind subsides over time completely and the gas enters recombination, making it invisible. For most planetary nebulae is the time between the formation and recombination of about 10,000 years.
Galactic producer of elements
Planetary nebulae play an important role in the evolution of a galaxy. The early universe consisted almost entirely of hydrogen and helium. Only by running in the stars nucleosynthesis heavier elements were created that are called in astrophysics and metals. Planetary nebulae consist of significant parts also from elements such as carbon, nitrogen and oxygen, with which they enrich the interstellar matter.
Subsequent generations of stars exist in a small proportion of these heavier elements, which has an influence on the stellar evolution. The planets are to a large portion of heavy elements.
In addition to the planetary nebulae and supernova explosions come in the final stages of massive stars from heavy elements.
Typical planetary nebulae are composed of about 70 % hydrogen and 28 % helium. The residual portion consists primarily of carbon, nitrogen, oxygen and traces of other elements.
They have a diameter of about one light year and consist of extremely rarefied gas with a density of approximately 1000 particles per cubic centimeter. The highest density have " young " planetary nebula with up to one million particles per cubic centimeter. Over time, the extent of the mist leads to a reduction of its density.
The star in the center fueled by its radiation the gases to a temperature of about 10,000 K. Contrary to expectations, the gas temperature is usually higher, the further one moves away from the center. The reason is that high-energy photons are absorbed less frequently than less energy. In the peripheral regions of the nebula, the low -energy photons have been absorbed and the leftover high-energy photons lead to a stronger increase in temperature.
Planetary nebulae can be either described as " matter- bounded " or " radiation limited ". In the former case is so much matter around the star that all ultraviolet photons that have been emitted, absorbed and is thus surrounded the fog of neutral gas. In the other case, the radiation is limited by the material. In this case, enough photons are emitted in order to ionize the entire gas of the mist.
Fog that contain regions of ionized hydrogen, is called emission nebula. They consist mainly of a plasma in the ionized hydrogen ( protons) and free electrons occur. Unlike a "simple" gas mist is replaced by the plasma characteristics such as magnetic field, plasma bilayers synchrotron radiation and plasma instabilities.
Number and incidence
Currently around 1,500 planetary nebulae are in our galaxy, which consists of about 200 billion stars, is known. When you see the very short existence of fog in relation to the entire "Star Life", the small number is understandable. They are usually found around the plane of the Milky Way, with the greatest concentration in the Galactic center. There are only one or two known planetary nebulae in star clusters.
Recently, a systematic photographic survey of the sky has increased the number of known planetary nebulae drastically. Although CCDs have the chemical film in modern astronomy already been replaced, while a Kodak Technical Pan film was used. In combination with a special filter for the isolation of typical hydrogen lines, which occur in all planetary nebulae, even very faint objects were detected.
In general, planetary nebulae have a symmetrical and approximately spherical shape, but also there are very different and complex forms. Approximately 10% are strongly pronounced bipolar, some are asymmetrical; a copy is - seen from us - even rectangular. The causes of the extreme diversity of forms are not yet precisely known, they are currently being debated. Gravitational effects of companion stars could help. Another possibility would be that massive planets disturb the flow of matter when ausformt the fog. In January 2005, the first magnetic field was found around the central stars of two planetary nebulae. It is believed that this field is partially or entirely responsible for the unusual structure.
2011 photographed by the Hubble Space Telescope, a planetary nebula in the constellation arrow which, Necklace Nebula ' (, collar - fog '), which consists of a ring glowing knots of gas that sparkle like diamonds of a necklace.
Current research subject
A major problem in the study of planetary nebulae is that you can their removal is difficult to determine. In relatively nearby planetary nebulae, the distance determination using the parallax is possible. Given the small number near the fog and the variety of forms no standard of comparison is however derivable therefrom. With a known distance can be achieved by many years of observations, the expansion velocity of the nebula determine perpendicular to the direction of observation. By spectroscopic analysis of the Doppler effect is also obtained the expansion velocity in the direction of observation.
The proportion of heavy elements in planetary nebulae can be determined by two methods, with results sometimes differ greatly. Some astronomers think that this could be due to temperature fluctuations within the nebula. Others are of the opinion that the differences are too large to be explained by this temperature effect and lead to deviations cold regions back with very little hydrogen. Such areas have, however, been observed.