Osmio

In Astronomy, a planetary Nébuleuse is a astronomical object which resembles a disc of nebulous aspect when it is observed with low resolution. Because of this aspect, similar to the aspect of the Planet S, the “planetary” adjective was attached to him, and since was maintained to preserve the historical uniformity.

Starting from more detailed observations (in particular spectroscopic) one knows now that planetary nebulas do not have in fact any relationship with planets. When small a star (less than eight solar masses) ages and finished consuming any sound Hydrogène, then its Hélium, its heart crumbles to form a white Naine, while the external layers are expelled by the Pression of radiation. These gases form a matter cloud which extends around star at a speed of expansion from 20 to 30 Kilomètre S by second (: 70,000 to 100,000 km/h). This cloud is Ion ized by the photons Ultraviolet S emitted by the star which became very hot (: 50,000 to 100,000 [[Kelvin K]]).

The energy thus acquired by gas is re-emitted in the form of Lumière less energy, in particular in the field of the visible one.

In fact objects evolve/move rather quickly, one knows some approximately: 1,500 in our Galaxy. They play a crucial role in the enrichment of our universe, transforming paramount hydrogen into element S heavier and expelling these new elements in the interstellar environment.

Planetary nebulas often are very coloured and their images are among most spectacular. One of the famous examples of this type of object is the nebulous of the Ring located in the Constellation of the Lyre from where its other name: nebulous of the Quadrant .

History

Planetary nebulas are in general objects of low brightness, invisible with the naked eye. The first planetary nebula discovered was the Nébuleuse of Haltère in the constellation of the Petit Fox, observed by Charles Messier in 1764 and recorded under the M27 number in its catalogs diffuse objects.

The nature of planetary nebulas remained unknown until the first spectroscopic observations , in the middle of the 19th century. William Huggins was one of the first astronomers studied the spectrum of the astronomical objects by dispersing their light using a prism. Its observations of stars showed a continuous spectrum on which appear dark lines (absorption lines).

In addition, when he studied the Nébuleuse Cats eye, he met a completely different spectrum: a low number of emission lines emerged on continuous almost no one. Most intense of these lines was with a Wavelength of 500,7 Nanomètre S, which did not correspond to any emission of element known on Earth. It thus allotted this emission to a new element, the nébulium, just like the Hélium which had been identified for the first time in the solar spectrum.

Nevertheless, whereas helium was isolated on Earth a few years after its discovery by the astronomers, the enigma of nébulium resisted a long time to the physicists who did not manage to isolate it on Earth. It is only at the beginning of the 20th century which Henry Norris Russell proposed that the luminous lines observed in planetary nebulas are not the fruit of a new element, but emitted well by an element very running, the Oxygène, placed under very extreme conditions (very low density).

Formation and evolution

Planetary nebulas are the result of the evolution of stars of intermediate mass (between 0,8 and 8 times the mass of the Sun). After having passed ten billion years to transform Hydrogen into Helium in their heart, these stars arrive at the end of their hydrogen reserve and do not have thus any more what to produce energy necessary to counterbalance the gravitational force which tends to make them crumble on themselves. The core of star crumbles slowly, increasing its temperature (few tens to a hundred of million of Kelvin S), news fusion S take place then, helium being transformed into Carbone. The external layers of star undergo a strong pressure then and are expelled in the form of a rather slow and dense wind.

The star becomes a Géante red, and its temperature of surface decrease. The star is composed then of two parts: the star itself in the center, which little by little evolves to a white Naine, surrounded by a nebula expanding. One supposes a second phase of wind, this fast time and not very dense, which compresses the first ejected envelope and gives him its form and its structure of rather fine shell.

The star in the center continues its contraction as matter is ejected and its temperature of surface increases until passing to the top of 30,000 K. Starting from this temperature, it emits a considerable quantity of Photon S capable of photoioniser the nebula which surrounds it. The photons must have an energy higher than 13,6 eV, or 1Ry, or a Wavelength lower than 91.2 Nm. Indeed, “to see” planetary nebula, one needs that it emits light, which it does as soon as it is photoionisée by central star.

It is as from this moment that one can speak about planetary nebula. The initial star has a mass ranging between 0,8 and 8 times the mass of the Sun, the dwarf white one resulting from the evolution have a mass between 0,5 and 1,4 solar mass: the major part of initial star was thus reinjected in the interstellar environment.

The evolution is then rather fast, in a few centuries the central star cools in lower part of the temperature corresponding to the emission of ionizing photons (becoming in the long term a black Naine), at the same time as nebula dissolves in the interstellar Milieu, not without sowing it products of fusions which took place in the center of star before this fine tragedy.

Characteristics

Lifespan

The planetary phenomenon nebula is rather transitory, it only lasts some: 10,000 years. The end of planetary nebula comes on the one hand cooling from the central object which ends up more not emitting the photons extreme UV able to ionize nebula and on the other hand dilution of gas constituting nebula.

Number and distribution

Among the few 200 billion stars which account our Galaxy, it was detected only approximately 1500 planetary nebulas. This is due to very the short duration of life of the phenomenon compared with the lifespan of stars itself.

One is also able to detect them in other galaxies, by using images obtained with the typical wavelengths of planetary nebulas (for example 500.7 Nm, are 5007 Å) and by comparing them with the images obtained in close wavelengths: planetary nebulas (as well as the areas HII) appear in the first but not in the second images.

The study of extragalactic planetary nebulas bring information on for example the gradients of abundances. All planetary nebulas of same a Galaxie (external with our) are almost with the same distance from the observer.

Morphologies

highlighted the fact that according to the chemical ions, planetary nebula always does not have same morphology. For example, there are some which is known as " hydrogen-déficientes" , while others do not have a trace of the line 6583Å once ionized nitrogen. In addition to this problem of stratification in abundance, the stage of evolution of central star plays a great part in the structure of ionization of the surrounding envelope nébulaire. It is canto be able to distinctly separate the effective contributions from hydrogen (via the line H alpha with 6563Å) and from the emission of nitrogen (via the line 6583Å, with only 20Å!) that the team of the Laboratory of Space Astronomy CNRS. The duration also intervenes in classification known as morphological: such nebula, e.g. HS 1-89, or SaWe3.

In addition, the study of spectral high-resolution of the emitted lines makes it possible to obtain information on the dynamics of gas, the Effect Doppler being responsible for the shift wavelength of the emitted photons. This effect is directly connected at the relative speed of the transmitter compared to the observer: the gas which comes towards the observer and the gas which moves away from there are not perceived with the same wavelength. It is thus possible “to rebuild” the morphology of the gas envelope starting from spectral observations, if one gives oneself a relation between the distance to star and the speed of distancing of gas.

The theory of planetary nebulas calls upon many sides of physics and astrophysics. It is necessary on the one hand to include/understand the characteristics and the evolution of central star, the dwarf white one resulting from the evolution of an intermediate star of mass. It is necessary to include in the study of this star the presence of winds (hydrodynamic), to take into account the Nuclear physics which governs the reactions which take place within star and helps to include/understand its chemical evolution via the stellar Nucléosynthèse. It is necessary to call upon the whole of the atomic Physique to reproduce the spectrum emitted by this star, by calculating how the light interacts with the matter. This relates to only the central object, it still remains to study expelled gas.

With the increase in the capacities of calculation and memory of the computers, it is today possible to calculate planetary models of nebulas by taking of to account the majority of the physical phenomena which are with work in star as in ionized gas. The theoretical study of planetary nebulas is done indeed starting from models obtained thanks to computer programs which try to reproduce the physical conditions that one finds within the gas which constitutes them. One can separate in two main categories the programs (codes) data-processing:

Codes of photoionization

These codes calculate the transfer of the radiation emitted by the central star which photoionise nebula. They are based on the assumption that the gas is in balance of ionization (constantly the number of photoionization is equal to the number of recombinations) and in thermal balance (constantly the energy gained by gas due to the absorption of the photons of star is equal to the energy lost by the emission of other photons, those which one observes).

The codes of photoionization are with spherical symmetry (unidimensional) or three-dimensional (3D).

The models obtained by these computer programs do not take into account time, they give an image to a given moment of nebula. The predictions of these programs are mainly the intensities of the emission lines which are produced by nebula, that one can compare with the spectroscopic observations. The models resulting from codes 3D can also give monochromatic images comparable with the observations.

The images of this paragraph were obtained thanks to a code 3D of photoionization.

Current problems concerning planetary nebulas

Although studied since nearly one century, planetary nebulas are far from to have delivered all their secrecies. Among the great debates which worry the specialists, one can quote two important as well by their implications who exceed the only framework of planetary nebulas as by energy than each one puts sometimes in its argumentation in favor of an interpretation/explanation or another:

From which comes the nonspherical forms from planetary nebulas?
: Magnetic field of central star, or presence of a companion beside this one (binary system)? These two schools clash since years without one of them not being essential definitively. As long as we will not have included/understood that, we will not be able to claim to include/understand the phenomenon total of planetary nebulas (in particular its energy assessment) nor to claim to make complete models of these objects. A series of international congresses were dedicated to these problems.
Pourquoi don't the determinations of abundances which one obtains with various techniques give coherent results?
: It is a debate which sees to clash several schools there too. Some speak about fluctuations of temperature, others of fluctuations of chemical composition. As long as we will not have included/understood the cause of these dissensions, we will not be able to claim to determine abundances in other more complex objects or observed less well (because moved away more, such as for example the Galaxie S). It is our comprehension of the chemical evolution of the galaxies and our universe overall which is concerned here.

Another difficulty which arises at the time of the study of planetary nebulas is the difficulty of determining a distance to the object. One could (seldom) apply a method of Parallaxe by comparing the expansion “seen” in projection to the sky between two photographs taken with two times distinct (there is in this case an expansion in unit of angular size per annum) and the speed of expansion determined starting from measurements from the Doppler effect on the gas (this time one obtains an expansion in km/s). The distance between the object and the observer makes it possible to connect these two measurements which one supposes equal. This technique functions only on close and relatively spherical objects (because the Doppler expansion is perpendicular to the expansion projected to the plan of the sky, the two values are equal only if the object undergoes the same expansion in all the directions).

Apart from these rather rare cases, the distance is one of the fundamental unknown factors of the problem: a nebula four times more luminous, but twice larger and twice more distant will be seen in the same way, without possibility of making the difference…

Random links:

Oliver James | Dorfbahn Serfaus | Eugene Laermans | Emigration mahonnaise in Algeria | Charles Soong