Star

See also: Star (homonymy)

A star is a roughly spherical Celestial object of which the structure is modelled by the revolved. During its formation, a star is primarily made up of Hydrogène and Hélium. During the major part of its existence, its heart is the seat of reactions of nuclear Fusion, of which part of energy is radiated in the form of visible light; its matter is almost completely of it ionized because of high Température which reigns in its center.

The Sun is the star nearest to the Ground, the energy which it radiates allows it the development of the Vie. It appears much more luminous than all other stars because of its proximity: the second star nearest to the Earth, Proxima of the Centaur, is: 250000 times more distant. Except exceptional case, the other stars are visible only the night, in the form of luminous points, when their glare is not drowned by that of the Sun.

The stars are gathered within Galaxie S. a typical galaxy, like ours, the Milky Way, contains several hundreds of billion stars. Within the galaxies, the stars can be dependant in multiple systems (some stars) or cluster (several tens to a few hundreds of thousands of stars). The celestial Sphère also reveals star groupings called Constellation; it is acted in fact of an illusion due to the effect of projection, the stars the component being generally very distant from/to each other.

A star has a mass ranging between 0,07 and approximately 150 times that of the Sun (itself equalizes with: 300000 times that of the Earth, are approximately 2× 10 30   kg). The stars of lower mass do not allow the starting of the reactions fusion nuclear of hydrogen, whereas the stars of higher mass are prone to instabilities involving a loss of mass. The lifespan of a star is primarily determined by speed to which occur the nuclear reactions: the higher the mass of star is, the faster the nuclear reactions are and the lifespan of short star. The most massive stars have one lifespan that a few million the years only, least massive of more than thousand billion years. A star as the Sun has one lifespan of about 10 billion years.

The formation of stars is due to the collapse of a cloud of gas and to its possible fragmentation in several protostars; each one warms up as it contracts. Its temperature can then reach a value such as the heart “ignites”: the Hydrogène amalgamates in Hélium, providing the energy which stops collapse. The star enters the principal Séquence then where it passes the major part of its life. The energy produced by this conversion is gradually evacuated by star at the same time by Convection and by Radiation and escapes finally from the surface of star in the form of Rayonnement, of stellar winds and on Neutrino S. Its later evolution depends primarily on its mass. The higher this one is, the more the star is able to start reactions of fusion with chemical elements increasingly heavy. It can thus synthesize Carbone, then of the Oxygène, Néon, etc the near total of the element S heavier than helium is produced in stars (one speaks about stellar Nucléosynthèse) in the last stages of their evolution. If a star is sufficiently massive to synthesize Fer, then it is dedicated to know a paroxystic end in the form of Supernova: its external heart implose and its layers are dislocated by the process. The residue left by the implosion of the heart is an extremely compact object, which can be either a neutron star, possibly detectable in the form of a Pulsar, or a Black hole. The less massive stars know a less violent end-of-life: they lose little by little the major part of their mass, which forms thereafter a Nébuleuse planet gear, and see their heart contracting slowly to form a white Naine.

Observation

With the naked eye

The night, the stars, because of their distance, appear with the naked eye in the form of brilliant points of gluer white, sometimes so red, orange or blue - generally scintillating because of atmospheric Turbulence , and without immediate apparent movement compared to the other fixed objects of the Vault of heaven. The phenomenon of scintillation is due to the extreme smallness of the angular Taille of the stars (some milli seconds of arc even less), which is lower than that of atmospheric turbulence. Contrary, the Planet S, although seeming points, one actually a sufficient angular size not to be subjected to the phenomenon of scientillation. The absence of movement is due to the fact that if the stars move the ones compared to the others, this own Mouvement is very weak, even for the closest stars, this movement is t-pieces weak, not exceeding a few seconds of arc per annum.

The day, the Sun dominates: the star more the visible brilliance since the Ground is also a star.

The Sun seems much larger than all other stars because those are much more distant: the star nearest to the Earth after the Sun, Proxima of the Centaur, is located at approximately four light-years, that is to say to us close to: 250,000 times the distance which separates us from the Sun, called astronomical Unité.

According to the conditions of observation, the number of observable stars the night with the naked eye strongly varies and can reach several thousands under the most favorable conditions. However, the estimate of the number of stars in the observable Univers oscillates between 10 22 and 10 23 . Except the Sun and Sirius - and still only under excellent conditions of observation - the stars are too not very brilliant to be observable in full day (except at the time of the total eclipses S of Sun and at the time of temporary phenomena like the Nova E or the Supernova E). The glare of stars is quantified by a size called Magnitude connect. For historical reasons, the magnitude is all the more small as the star is brilliant: the astronomer of the ancient Greece Hipparque had classified stars in stars of first size for most brilliant, second size for the following, and so on until fifth size. The precise mathematical definition apparent magnitude takes again primarily this classification, with the most brilliant stars equipped with a magnitude close to 0 (except for Sirius, magnitude -1,5 and of Canopus, magnitude -0,7) and weakest a magnitude higher than 6. A variation of 1 in magnitude corresponds to a report/ratio of luminosity of approximately 2,5, a variation of 5 with a ratio of 100. The Sun has a magnitude connect of -26,7, i.e. seen Earth, it is approximately 10 billion times more brilliance that Sirius.

The stars seem associated in more or less simple geometrical figures, the Constellation S; it is about a simple visual effect. The real stellar structures are cluster (gathering a few thousands of stars) or of the Galaxie S (gathering about the billion stars).

The observation with the naked eye was the first form of Astronomie.

With instruments

Since Galileo, multiple instruments made it possible to reveal varied characteristics of the stars, which are hereafter detailed.

To study stars, the principal instruments are: the Telescope - replaced today by the Telescope (as well on the ground as in space) - the Spectrograph, the Photometer and the polarimeter. For a few years, the techniques of Spectroscopy and Interférométrie have made it possible to increase the angular Résolution limited on the ground by atmospheric turbulence, that is to say approximately a half-second of arc on the best sites of observations. These techniques revealed structures around stars but also give access the angular diameter of a few hundreds of stars. After the eye, the detectors used were the photographic plates then the numerical detectors like CCC.

Star catalogs

To locate stars and to facilitate the work of the astronomers, of many S catalogs were created. Among most famous, let us quote the Henry Draper catalogs (HD) and the Bonner Durchmusterung (data base). The stars are arranged there by their coordinates, alpha (Right ascension) and delta (Déclinaison) and a number their is allotted: for example, HD 122653 (giant Pop II, the very defective one out of metals celebrates).

Principal characteristics

A star is characterized by various sizes:

Mass

The mass is one of the most important characteristics of a star. Indeed, this size determines its existence as well in duration as in its advanced and final phases: a massive star will be very luminous but its lifespan will be reduced.

The stars have a mass ranging between approximately 0,08 and 120 times the mass of the Sun, are some 10 30 Kilogram S (ten billion billion billion billion tons). In on this side minimal mass, the heating generated by the gravitational contraction is insufficient to start the cycle of nuclear reactions: the star thus formed is a brown dwarf . Beyond the maximum mass, the force of gravity is insufficient to retain all the matter of star once the started nuclear reactions.

How to estimate the mass of a star?

The determination of the mass of a star can be made in a precise way only when it belongs to a Binary system by the observation of its orbit. The third law of Kepler then makes it possible to calculate the sum of the masses of two stars of binary as from its period and of the equatorial radius of the described orbit and the distance from the Earth to double star observed. The report/ratio of the masses is obtained by the measurement of the radial Speed of two stars of the binary one. The knowledge of the sum and the report/ratio of the masses makes it possible to calculate the mass of each star. It is the most precise technique.

Other estimates are possible for nonbinary stars (simple) by using the spectroscopic determination of the gravity of surface and the measurement of the ray of star by interferometry. Lastly, if the star is observed in a precise way in photometry and if its distance, its chemical composition and its effective temperature are known, it is possible to position it in a Diagramme of Hertzsprung-Russell (noted HR) which gives immediately the Masse and the age of the star (Theorem of Vogt-Russell).

Diameter

Compared to our planet (12756 km diameter), the stars are gigantic: the Sun has a diameter of approximately a million and half of kilometers and certain stars (like Antarès or Bételgeuse) have a diameter of the hundreds of times superior to this last.

The diameter of a star is not constant in time: it varies according to its stage of evolution. It can also regularly vary for the variable stars periodic (RR Lyrae, Céphéide S, Miras, etc)

Interferometers like that of VLT of the ESO in Chile or CHARA in California allow the direct measurement of the diameter of the closest stars.

Chemical composition

The chemical composition of the matter of a star or a gas in the Universe is generally described by three quantities of mass number: X (hydrogen), Y (helium) and Z metallicity. They are sizes proportional satisfying the relation: X + Y + Z = 1.

Metallicity

The Métallicité is the quantity (measured of number, or generally by mass) of the elements heavier than the Hélium present in star (or rather its surface). The Sun has a metallicity (noted Z) of 0,02: 2% of the mass of the Sun are made up of elements which are neither of the Hydrogène, nor of the Hélium. For the Sun, they are mainly Carbone, Oxygène, Azote and Fer. Although that seems weak, these two pourcents are very important to however evaluate the Opacité matter of star, intern or atmosphere. This opacity contributes to the color, the luminosity and the age of star (see Diagramme of Hertzsprung-Russell and theorem of Vogt-Russell).

Opacity is directly related to the capacity of star to produce a stellar Vent (extreme case of the stars Wolf-Rayet).

Magnitude

The Magnitude measurement the luminosity of a star; it is a scale logarithmic curve of its radiative flow. The Magnitude connects in a filter given (ex: the visible one noted mv), which depends on the distance between star and the observer, is distinguished from the absolute Magnitude, which is the magnitude of star if this one were arbitrarily placed at 10 Parsec S of the observer. The absolute magnitude is directly related to the Luminosité of star on the condition of taking account of a correction known as bolometric (it is noted BC). The introduction of the logarithmic scale magnitudes comes owing to the fact that the eye also has a sensitivity logarithmic curve, at first approximation (law of Pogson).

Temperature and color

The majority of stars appear white with the naked eye, because the sensitivity of the eye is maximum around the yellow. But if we look at attentively, we can note that many Couleur S is represented: blue, green, yellow, red. The origin of these colors remained a long time a mystery until two centuries ago, when the physicists had sufficient comprehension on the nature of the Lumière and the properties of the Matière at the very high temperatures.

The color makes it possible to classify stars according to their spectral Type (which is in connection with the temperature of star). The spectral types go from most purple to reddest, i.e. hotter towards coldest. They are classified by the letters O B HAVE F G K Mr. the Sun, for example, are of spectral type G.

But it is not enough to characterize a star by its color (its spectral standard), it is also necessary to measure its Luminosité. In fact, for a given spectral type, the size of star is correlated with its luminosity, the luminosity being function of surface - and thus of the size of star. The stars O and B are blue with the eye like β Orionis; the stars has are white like α Canis Majoris (Sirius) or α Lyrae (Vega); the stars F and G are yellow, like the Sun; the stars K are oranges like α Bootis (Arcturus); and finally the stars M are red like α Orionis (Bételgeuse).

One can define an index of color, corresponding to the difference in flow photometric in two spectral bands known as bands photometric (filters). For example, blue (B) and the visible one (V) will form together the index of color B-V whose variation is connected to the temperature of surface of star and thus to its spectral type. The most used indices of temperature are the B-V, the IH and the VI because they are most sensitive to the variation in the temperature.

Number of revolutions

The rotation of the Sun was highlighted thanks to the displacement of the sunspots. For other stars, the measurement this number of revolutions (more precisely, measured speed is the projection number of equatorial revolutions on the line of sight), is obtained by spectroscopy. It results in a widening of the spectral lines. This rotation movement is a remainder of their formation starting from the collapse of the gas cloud. The number of revolutions depends on their age: it decreases during time, under the combined effects of the stellar Vent and the Magnetic field which carry part of the kinetic Moment of the star. This speed also depends on their mass and their statute of simple star, binary or multiple. A star not being a solid body (i.e. rigid), it is animated of a differential Rotation: the number of revolutions depends on the Latitude.

Radiative spectrum

The spectrum of a source of light and thus of a star is obtained by spectrographs which break up the light into its various components and record them by the means of detectors (historically, of the photographic plates and today of the detecting of type CCC). This decomposition of the light reveals the distribution of luminous energy coming from star according to the Wavelength. It makes it possible to highlight spectral lines in emission and/or absorption revealing the conditions of temperature, pressure and chemical abundances of the external layers of star.

Magnetic field

Like the Sun, the stars are often equipped with magnetic fields. Their magnetic field can have a relatively simple and well organized geometry, resembling the field of a magnet like the Terrestrial magnetic field; this geometry can be also definitely more complex and present arches to more small scales. The magnetic field of the Sun, for example, has these two aspects; its component with large scales structure the solar Crown and is visible at the time of the eclipse S, while its component with more small scales is dependant on the dark tasks which mackle its surface and in which the magnetic arches are anchored.

It is possible to measure the magnetic field of stars through the disturbances that this field induced on the spectral lines formed in the Atmosphère of the star (the Effet Zeeman). The tomographic technique of Imagerie Zeeman-Doppler makes it possible in particular to deduce the geometry from the giant arches that the magnetic field draws up on the surface of stars.

Among magnetic stars, one distinguishes initially cold” or not very massive stars known as the “, whose temperature of surface is lower than 6500 K and whose mass does not exceed: 1.5 solar masses - the Sun thus forms part of this class. These stars are “active”, i.e. that they are the seat of certain numbers of energy phenomena related to the magnetic field, such as for example the production of a crown, of a wind (known as Solar wind in the case of the Sun) or of eruptions. The tasks on the surface of the Sun and stars also testify to their activity; like the magnetic fields, the spots of stars can be charted by tomographic methods . The size and the number of these spots depend on the activity on star, itself function number of revolutions the star. The Sun, which carries out a full rotation on itself in approximately 25 days, is a star having a weak cylic activity. The magnetic field of these stars is produced by $dynamo effect.

There exist also magnetic hot stars. But contrary to the cold stars, which all are magnetic (with various degrees), only a small fraction (between 5 and 10%) of hot stars (massive) has a magnetic field, whose geometry is in general rather simple. This field is not produced by $dynamo effect; it would rather constitute a fossil print of magnetic interstellar paramount, captured by the cloud which will give birth to star and amplified at the time of the contraction of this star cloud. Such magnetic fields were baptized “fossil magnetic fields”.

Structure of a star

Starting from the various measured sizes and simulations resulting from various models, it is possible to build an image of the interior of a star, although it is to us almost inaccessible - the Astérosismologie allowing literally to probe the stars.

In the actual position of our knowledge, a star is structured in various concentric areas, described hereafter starting from the center.

Core

The core (or heart) is the central part of star, concentrating most of the mass of the star, in which the thermonuclear reactions are held which release energy necessary to its stability. The core is the densest zone and hottest, and, in the case of the Sun, reached the temperature of 15,7 million Kelvin S. Under these extreme conditions, the matter is in the form of plasma; by Tunnel effect, the cores of hydrogen (Proton S) or other elements chemical reach speeds enabling them to overcome their electric repulsion and to amalgamate: for example, in nuclear the chains known as proton-proton (or PP1, PP2…), the protons amalgamate by group of four to give a core of Hélium, composed of two protons and of two Neutron S. It then occurs a release of energy according to the following reactions:

2 ({{exp|1}} H + 1 H → {{exp|2}} D + E {{exp|-}} + ν {{ind|E}}) (4,0 MeV + 1,0 MeV)

2 ( 1 H + 2 D → 3 He + γ) (5,5 MeV)

3 He + 3 He → 4 He + 1 H + 1 H (12,86 MeV)

Other thermonuclear reactions exist in the center of stars and contribute more or less to the energy production.

Part of the energy released in the form of Photon S then begins a long voyage towards outside, because a plasma is opaque and the light travels there very with difficulty. It is estimated that a photon puts several million years before reaching the surface of star by transfer of radiation then by convection towards surface.

Radiative zone

The energy released by the nuclear reactions of fusions in the core of star is transmitted to the external layers by radiation. In stars not very massive and evolving/moving on the principal Sequence, this zone radiative is surmounted by a convective zone. In the Sun, the radiation produced in the central part puts nearly a million years to cross the radiative zone.

Convective zone

Contrary to the preceding zone, energy is transmitted by macroscopic matter movements: heated at the base of the convective layer, the matter rises under the effect of the Poussée of Archimedes, reheating the matter around (towards surface), cools and plunges towards the base of the convective zone for a new cycle. It is the principle of the Convection. This convective zone is more or less large: for a star on the principal sequence, it depends on the mass and the chemical composition; for a giantess, it is very developed and occupies an important percentage of the volume of star; for a supergéante, this zone can reach the three quarters of the volume of star, as for Bételgeuse.

Photosphere

The Photosphère is the external part of the star which produces the visible Lumière. It is more or less wide: of less than 1 pourcent ray for dwarf stars (a few hundred kilometers) with a few tens of pourcents of the ray of star for giant. The light which is produced there contains all information on the temperature, the gravity of surface and the chemical composition of star. For the Sun, photosphere has a thickness of approximately 400 kilometers.

Crown

The crown is the external, thin zone and extremely heat of the Sun. One can observe it at the time of the eclipse S of Sun. It is thanks to the study of the crown at the 19th century that the astronomer Jules Jansen discovered the existence of the rare gas whose name refers to the Sun (Hélios): the Helium. The fact that the temperature of the crown reaches several million degrees is a difficult theoretical problem and not yet completely solved. It is probable that the majority of stars have crowns.

Theorem of Vogt and Russell

The theorem of Vogt-Russell can be stated as follows: so in all points of a star the knowledge of the values of the temperature, density and chemical composition of plasma interns are sufficient to calculate the pressure, the opacity of plasma and the rate of energy produced, then the mass and the chemical composition of star are sufficient to describe the structure of this one. It results from them the relations mass-ray or mass-luminosity from stars.

Evolution

See also: evolution of the stars

The history of a star is entirely determined by its Masse M and its Chemical composition X, Y, Z (theorem of Vogt and Russell). M determines its duration of existence, and conditions its end. The evolution of a star passes by several phases, the first is the dwarf phase or principal Séquence, the second is the giant phase then supergéante to finish by the final stage a such Supernova or a Nébuleuse planet gear.

Formation

See also: stellar Formation

A star is born from the contraction of a cloud rich in Hydrogène. Under the influence of a wave of density (arm of Galaxy), of a Shock wave (Supernova or Nova near), or of a fluctuation of density within this one, an area starts to contract. By an effect snowball , this area, increasingly dense with it gas attracts more and more. The contraction of gas involves its heating: the Protostar rayon (in the Infra-red ). This radiation slows down by Pression of radiation, but does not stop, the inexorable action of the Gravitation. If the heating is sufficient, it can initiate nuclear reactions in the middle of the cloud. The energy released by these reactions stops the contraction because of pressure of radiation thus generated.

The principal sequence

Under the effect of the contraction, the core of the star (its central part) reached values of pressure and temperature extremes, which go until the lighting of the thermonuclear reactions (see higher). The star enters then during what is called the principal sequence, time during which its helium and hydrogen core, initially and primarily made up, gradually will be transformed into helium.

During this period, antagonism produced energy/gravitation contributes to the stability of the star:

If the flow of energy coming from the core has suddenly decreased, the contraction which follows accelerates the rate/rhythm of energy production which stops the contraction; conversely, a racing of the energy production involves a dilation of star, therefore its cooling, and the racing stops. Thus, it results from it a great stability of the star which is described in the theory of the stellar internal structure under name “peak of Gamow”: it is a kind of thermostat stellar.

End of a star

The more massive one star is, the more it consumes its hydrogen quickly. A large star will be thus very brilliant, but will have one short duration of life. When the nuclear fuel is done too rare in the core of star, the reactions of fusion stop. The pressure created by these reactions not compensating for the forces of gravitation more, the star crumbles on itself. The larger one star is, the more the end of its existence will be cataclysmic, being able to go until even taking the form of a gigantic explosion (Supernova) followed formation of a neutron star in the extreme cases (according to mass of star) of a Black hole.

Types of stars

The astronomers classify stars by using the effective temperature and the luminosity. This classification with two parameters makes it possible to define spectral types (luminosity) varying from VI to I, dwarf being classified V. the Sun is of class V. Among these classes one distinguishes various categories related to the temperature of surface. For example them: dwarf brown, dwarf reds, dwarf yellows, giant reds, giant blue, supergéantes red, dwarf white, neutron stars and black holes. If the majority of stars are placed easily in one or the other of these categories, it is necessary to keep at the head which they are only temporary phases. During its existence, a star changes form and color, and can pass from a category to another.

Dwarf brown

See also: Dwarf brown

The dwarf brown ones are not stars, or rather, they are “missed” stars. Their mass is located between those of small stars and large planets. Indeed, at least 0,08 solar Masse is necessary so that a protostar starts thermonuclear reactions and becomes a genuine star. The dwarf brown ones are not sufficiently massive to start these reactions. They can shine however slightly by gravitational contraction.

Dwarf reds

See also: Dwarf red

Dwarf the reds are… small red stars! One regards them as smallest stars as such. The smaller stars like the white dwarf , the dwarf brown neutron stars and do not consume nuclear fuel. The mass of dwarf the reds lies between 0,08 and 0,8 solar mass. Their temperature of surface between: 2,500 and 5,000 K confer a red color to them. These stars burn their fuel slowly, which ensures a very long existence to them. They are most abundant: at least 80% of stars of our Galaxy are dwarf reds. Nearest close to the Sun, Proxima of the Centaur, in is one. It is the same of the second stellar system, the nearest solar system, the star of Barnard is also dwarf a red.

Dwarf yellows

See also: Dwarf yellow

Dwarf the yellows are stars of intermediate size - the astronomers classify stars only into dwarf or giants. Their temperature of surface east of approximately 6,000 K and they shine of a sharp yellow, almost white. At the end of its existence, dwarf a yellow evolves/moves as a red giantess, who by expelling her external layers - deploying then a Nébuleuse planet gear -, reveals dwarf white.

The Sun is dwarf a typical yellow.

Red giantess

See also: Giant red

The red giant phase announces the end of existence of the star, which reaches this stage when its core exhausted its principal fuel, the Hydrogène: reactions of fusion of the Hélium start, while the center of star contracts, and that its external layers inflate, cool and redden. Transformed into Carbon and Oxygen, helium becomes exhausted in its turn and the star dies out. The external layers of the star move away and its center contracts, revealing a white Naine.

Blue giantess and supergéante red

See also: Giant blue, red Supergéante

On the diagram HR, the left higher corner is occupied by very hot and brilliant stars: blue giants. These very massive stars, at least ten times larger than the Sun, consume their hydrogen quickly.

When the core of a blue giantess does not contain any more hydrogen, the fusion of helium takes over. Its external layers swell and its temperature of surface decreases. It becomes a red supergéante then.

The star manufactures then increasingly heavy elements: Iron, Nickel, chromium, Cobalt, Titanium… At this stage, the reactions of fusion stop and the star becomes unstable. It explodes in a supernova and leaves behind it a strange matter core which will remain intact which will become according to its mass, a neutron star or a black hole.

Dwarf white

See also: Dwarf white

The dwarf white ones are the residues of the evolution of stars of low mass (between ~0,8 and ~5 to 8 solar masses). The Sun having (by definition) a mass of a solar mass, it will also finish dwarf white. The dwarf white ones are dead stars since they are not any more the place of thermonuclear reactions producing the light. On the other hand, they are very hot, and have at the beginning, a relatively white color (see Loi of Wien). Gradually, they cool by radiation, like cold stars. Their size is approximately equal to that of the Earth.

The dwarf white ones, as the neutron stars consist of degenerated matter . The density on the surface of dwarf white is such as a spoon with matter the of such a star would have, on Earth, the mass of a elephant is approximately 1 kg·mm^-3. In fact, in this matter, the electron S, being very close from/to each other, then start to be pushed back vigorously. The principal factor of the pressure comes then from the Principe of exclusion of Pauli; it is the Pression of degeneration which is opposed to that of the gravitation. The dwarf white one is thus balances some in spite of the nuclear absence of fusion in its core. The pressure of the electrons can support a mass of 1,44 times that of the Sun: it is the Limite of Chandrasekhar. If dwarf white becomes more massive (by aspiring the matter of another star, for example), it explodes in supernova (of Ia type) and can sometimes be completely destroyed.

Procyon B and Sirius B is the dwarf white ones.

Dwarf black

See also: Dwarf black

Like a hotplate that one extinguishes, the dwarf white ones cool unrelentingly. However, that is done very slowly, because of their mass. They fade little by little their and become invisible at the end of ten billion years. Thus, very dwarf white transforms itself into dwarf black.

The Universe, old man of 13,7 billion years, is still too young to have produced the dwarf black ones.

After its death, the Sun will become dwarf white then dwarf black. This fate awaits it in approximately 15 billion years.

Neutron star and black hole

See also: Neutron star, Black hole

The neutron stars are very small but very dense. They concentrate the mass of a star like the Sun in a ray of approximately ten kilometers. They are the very massive star vestiges of more than ten solar masses.

When a massive star arrives at the end of the lifetime, it crumbles on itself, by producing an impressive explosion called Supernova. This explosion disperses enormous quantities of matter in space but saving the core of star. This core contracts and is transformed mainly into a neutron star. These objects have very intense magnetic fields (for most intense, one speaks about Magnétar). Along the magnetic axis is propagated charged particles, electrons for example, which produce a Synchrotron radiation.

The kinetic Moment of star being preserved during the collapse of the core, the neutron star has a number of extremely high revolutions, being able to reach the thousand of turns a second. So by chance an observer on Ground looks in the direction of a neutron star and that the line of sight is perpendicular to the axis of rotation of star, this one will then see the synchrotron radiation of the particles charged moving on the lines with magnetic field. This phenomenon of revolving headlight is called the phenomenon of Pulsar. One finds pulsars in remainders of supernovas, most famous being the pulsar of the Nébuleuse Crab, born from the explosion of a massive star. This supernova was observed by the Chinese astronomers since the morning of July 4th, 1054, in full day during three weeks and during the night on nearly two years. Its (Re) discovered was made by the radioastronomer S.

Sometimes, the core of dead star is too massive to become a neutron star. It contracts unrelentingly until forming a Black hole.

Variable star

See also: variable Star, eruptive Star

Whereas the majority of stars are of almost constant luminosity, as our Sun which practically does not have measurable variation (approximately 0,1% on a 11 years cycle), the luminosity of certain stars varies in a perceptible way over periods of time much shorter, sometimes in a spectacular way.

Stellar systems

The stars are seldom only formed. When a cloud of gas (proto-stellar) gives rise to a cluster of stars, the whole of stars of this cluster does not seem to be distributed randomly, but seems to follow a law of distribution known as function of initial mass (IMF), which one currently knows little thing; it gives an account of the proportion of stars according to their mass. It is not known if this function IMF depends on the chemical composition of the proto-stellar cloud. The function currently most adopted was proposed by Edwin Salpeter and seems to give satisfactory results for the study of the clusters of the Galaxie.

Binary systems and multiples

The binary systems consist of two stars bound gravitationally and orbiting one around the other. The element more the brilliance is known as primary education and less shining, secondary. When a system comprises more than two components it is described as multiple stellar system.

The binary systems can be detected by imagery, when the telescope manages to to solve the pair; in this case the binary one is known as visual. In other cases, the two companions cannot be solved, but the shift Doppler-Fizeau of the spectral lines makes it possible to detect the orbital movement of one or two stars. In this case the binary one is known as spectroscopic. If only one spectrum is visible and varies one speaks about binary SB1, if the spectrum of two stars is quite visible one speaks about binary SB2. It is also possible to detect the apparent movement in the sky of the binary star, which corresponds to the orbital movement of primary star if the secondary is far from luminous; in this case the binary one is known as astrometrical. One finally speaks about binary interferometric when the secondary is detected by interferometry.

The Astronomie amateur speaks about binary apparent when two distant in space and nongravitationally dependant stars are close in the sky by effect of prospect.

Clusters

See also: stellar Cluster

The stellar clusters are local star regroupings bound gravitationally and formed at the same time. So they constitute a population of reference to study the lifespan of a star according to its size (see Diagramme of Hertzsprung-Russell). One can make use of it to determine the age of the oldest star populations of our Galaxy.

One distinguishes the clusters open (AO) made up from a few tens to a few thousands of stars and generally of an unspecified form and the globular clusters (AG) consisted of several thousands to several million stars. The AO are young, of a few tens to a few hundreds of million of years. Among oldest M67 (4,6 billion years as the Sun) is also among largest. In our galaxy, the AO are rich in metals (typically like the Sun). AG are of form spherical from where them name. Their stars are low in metals and they count among the oldest objects of the Galaxy. They are distributed in the spheroid of the Galaxy which one calls the halation. Their age lies between approximately 10 and 13,5 billion years. Omega of the centaur is among largest. Its stellar population is not single what shows that it had an origin spread out in time allowing the formation of several of them (at least three). He is regarded as being able to be the residue of a dwarf galaxy having been captured by the Milky Way. NGC6397 is on the contrary a cluster has stellar population single with a metal abundance of a hundredth of that of the Sun. The AG lowest in metals known is M92 with almost thousandths of solar abundance.

Associations

Stellar associations are similar to the clusters, with this close which they do not constitute a system bound gravitationally. Also associations disperse at the end of a certain time. Example of association: the associations O-B made up mainly of very massive stars and very heats. One can regard them as small very young opened clusters still presenting much gas ionized in the vicinity of stars. One meets them in our Galaxy mainly in the arms.

Galaxies

See also: Galaxy

A galaxy is a vast whole of stars. They differ from the clusters by their size (several hundreds of billion stars compared with a few thousands to a few million for the stellar clusters), their organization and their history.

Constellations

By observing the night sky, the man imagined that the most brilliant stars could constitute figures. These regroupings generally differ from a time with another and a civilization to another. The figures become traditional, often in connection with the Greek Mythology, are called Constellation S.

The stars of a constellation have a priori nothing jointly, if is not to occupy, seen Ground, a close position in the sky. They can be very distant from/to each other. However, the international astronomical Union defined a standardized list of the constellations, allotting to each one an area sky, in order to facilitate the localization of the celestial objects.

Planetary systems

The stars can be accompanied by body revolving around them. Thus, the Solar system is composed of a central star, the Sun, is accompanied by Planet S, Comet S, Astéroïde S. Recently, of the planet S were discovered around other stars that the Sun, making lose with the solar system its presumedly single character. All these planetary systems are discovered in an indirect way. The first star around which planet S were revealed by velocimetric measurements is 51 Peg (observations carried out with OHP with the Spectrographe Elodie). Different numbers planetary systems have had for summer discovered. Because of the current limitations of detection, they show similar characteristics, with giant planets on very eccentric orbits: one names them of “Jupiter heats”. The majority of these stars are richer in metals than the Sun. The statistics on these planetary systems make it possible to conclude that the solar system does not have for the moment not an equivalent. Since space, the tracking of the planetary systems by photometry started with the satellite CoRoT (CNES). This one will be relayed in 2009 by the American satellite Kepler.

Star catalogs

To locate stars and to facilitate the work of the astronomers, of many S catalogs were created. Among most famous, let us quote the Henri Draper catalogs (HD) and the Bonner Durchmusterung (data base). The stars are arranged there by their coordinates, alpha (Right ascension) and delta (Déclinaison) and a number their is allotted: for example, HD 122653 (giant Pop II, the very defective one out of metals celebrates).

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