Star – Wikipedia

A star is a plasma celestial body that radiates its own light by nuclear fusion reactions, or bodies that have been in this state at a stage of their life cycle, such as white dwarfs or neutron stars ^{[ first ]} . This means that they must have a minimum mass so that the temperature and pressure conditions within the central region – the heart – allow the beginning and the maintenance of these nuclear reactions, threshold below which we speak of objects Substitute. The possible masses of the stars extend from 0.085 solar mass to a hundred solar masses. The mass determines the temperature and brightness of the star.

Most stars are located on the main sequence of the Hertzsprung-Russell diagram, where the stars produce their energy and their radiation by converting hydrogen into helium, by nuclear fusion mechanisms such as the carbon-bezote-oxygen or oxygen or cycle The Proton-Protton chain.

During a large part of its existence, a star is in hydrostatic balance under the action of two forces which oppose: gravitation, which tends to contract and make the star collapse, and kinetic pressure (with pressure radiation for massive stars), regulated and maintained by nuclear fusion reactions, which, on the contrary tends to expand the star. At the end of this phase, marked by the consumption of the totality of hydrogen, the stars of the main sequence are expanded and evolve in giant stars, which obtain their energy from other nuclear reactions, such as the fusion of helium in carbon and oxygen.

A star radiates throughout the electromagnetic spectrum, unlike most planets ^{[ Note 1 ]} (like the earth) which mainly receive the energy of the star or the stars around which they revolve.

The sun is a fairly representative star of those belonging to the same spectral type (G5). Its mass of the order of 2 × 10 ³⁰ kg is common for this kind of stars.

Historically, the stars are the light points of the sky visible only at night and fixed compared to each other, as opposed to the planets which follow trajectories wandering in the night sky during the year. The ancients had an in -depth knowledge of the distribution of the stars in the sky: they used them for navigation and attributed names to the brightest of them as well as to the forms they draw, the constellations. However, they were unaware of their exact nature, often thinking that these were orifices pierced through the celestial sphere ^{[ Note 2 ]} .

It is only with the rise of modern astronomy, that is to say astrophysics, that the stars could be understood as objects of the same nature as the sun but located at considerably larger distances . This hypothesis was stated for the first time by Giordano Bruno at XVI ^{It is} century before being confirmed experimentally in 1838 by the first parallax measure carried out by Friedrich Wilhelm Bessel, as well as by spectrometric observations made thanks to the device invented in 1814 by the optician Joseph von Fraunhofer.

A star is a celestial object in rotation, of shape first spherical ^{[ 2 ]} , consisting mainly of plasma and whose structure is modeled by gravity. During its formation, a star is mainly composed of hydrogen and helium. During most of its existence, its heart is the seat of nuclear fusion reactions, part of the energy of which is radiated in the form of light; The material that composes it is almost completely ionized.

The sun is the star closest to the earth, the energy it radiates allows the development of life. It appears much brighter than all the other stars because of its proximity: the second star closest to the earth, Proxima du Centaure, is 250,000 times more distant. Except in an exceptional case as an eclipse, the other stars are only visible at night when their brilliance is not drowned by that of the diurnal sky, itself resulting from the diffusion of the solar lighting.

The stars are grouped within galaxies. A typical galaxy, like ours, the Milky Way, contains several hundred billions of stars. Within galaxies, the stars can be linked to multiple systems (a few stars) or clusters (several tens to a few hundred thousand stars).

The celestial sphere also reveals groupings of stars, the constellations; It is in fact an optical illusion due to the projection effect. The stars making up a constellation are generally located at very different distances from the earth.

A star has a mass between 0.07 and 300 times about that of the sun (itself equal to 300,000 times that of the earth, about 2 × 10 ³⁰ kg ). The lower mass stars do not allow the priming of nuclear fusion reactions of hydrogen, while higher mass stars are subject to instabilities resulting in loss of mass. The lifespan of a star is essentially determined by the speed at which nuclear reactions occur: the higher the mass of the star, the faster the nuclear reactions and the lifespan of the star is brief. The most massive stars have a lifespan of only a few million years, the least massive, over 50 billion years. A star like the sun has a lifespan of the order of 10 billion years.

The formation of stars is due to the collapse of a gas cloud and its possible fragmentation in several proto-stories, which heat up as they contract. The temperature then reaches a value such as the heart “lights”: hydrogen fuses in helium, providing the energy that stops the collapse. The star then enters the main sequence in which it passes most of its existence. The energy produced by this conversion is gradually evacuated by the star both by convection and by radiation and ultimately escapes from the surface of the star in the form of radiation, stellar winds and neutrinos. Its later evolution depends essentially on its mass. The higher it is, the more the star is able to initiate fusion reactions with increasingly heavy chemicals. It can thus synthesize carbon, then oxygen, neon, etc. Almost all elements heavier than helium are produced in the stars (we speak of stellar nucleosynthesis) in the last stages of their evolution. If a star is massive enough to synthesize iron, then it is doomed to experience a end paroxysmal in the form of a supernova: its heart implodes and its external 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 are experiencing a less violent end of life: they gradually lose most of their mass, which subsequently forms a planetary nebula, and see their hearts slowly contract to form a white dwarf.

Naked [ modifier | Modifier and code ]

At night, the stars appear with the naked eye in the form of points (because of their removal) shiny white, sometimes also red, orange or blue – generally sparkling and without immediate apparent movement compared to other fixed objects of the sky. The scintillation phenomenon is due to the extreme smallness of the angular size of the stars (a few mills of arc or less), which is lower than that of atmospheric turbulence. Conversely, the planets, although appearing as points, actually have sufficient angular size not to be subjected to the phenomenon of scintillation. If the stars move to each other, this clean movement is very weak, even for the nearest stars, not exceeding a few seconds of arc per year, which explains their apparent immobility compared to the others.

The day, the sun dominates and its light, diffused by the atmospheric layer, obscures that of the stars. But the brightest star visible from Earth is itself a star.

The sun seems much larger than all the other stars because these are much more distant: the star closest to the earth after the sun, Proxima du Centaure, is located about four light years from us, or near From 270,000 times the distance that separates us from the sun (astronomical unit).

Depending on the conditions of observation, the number of stars visible to the naked eye varies strongly and can reach several thousand in the most favorable cases. Apart from the Sun and Sirius – and again, only in excellent observation conditions – the stars are too little shiny to be observable in broad daylight (except during total eclipses of sun and during temporary phenomena such as Novae or Supernovae ). The radiance of the stars is quantified by a quantity called apparent magnitude. For historical reasons, the magnitude is all the smallest since the star is brilliant: the astronomer of ancient Hippark Greece had classified the stars in first -size stars for the brightest, second grandeur for the following, and thus immediately up to fifth grandeur. The precise mathematical definition of the apparent magnitude essentially takes up this classification, with the brightest stars with a magnitude close to 0 (with the exception of Sirius, magnitude -1.5 and canopus, magnitude -0, 7) and the weakest of a magnitude greater than 6. A difference of 1 in magnitude corresponds to a brightness ratio of around 2.5, a difference of 5 to a ratio of 100. The sun has an apparent magnitude of – 26.7, that is to say that seen from the earth, it is about 10 billion times more brilliant than Sirius.

The stars seem associated with more or less simple geometric figures, the constellations; This is a simple optical effect. Real stellar structures are clusters (bringing together a few thousand stars) or galaxies (bringing together the billion stars).

Observation with the naked eye was the first form of astronomy.

With instruments [ modifier | Modifier and code ]

The stars have long remained points in the sky, and this same views through the most powerful magnification instruments, such as the astronomical telescope or the telescope. It is only from the end of the twentieth century and the beginning of the twenty-first that the angular resolution of the best instruments became less than the second of arc and was therefore sufficient to see structures around Certain stars as well as to distinguish these stars as a disc and not as a point. However today the overwhelming majority of stars remain inaccessible to such a direct observation.

Most of the stellar observations therefore focus on data relating to their electromagnetic spectrum, their brightness or their polarization, measured respectively using the spectrograph, the photometer and the polarimeter.

After the eye, the detectors used were photographic plates and then digital detectors like the CCD.

The study of the stars also includes that of the sun, which can be observed in detail, but with appropriate equipment, in particular powerful filters. Sun observation is an activity potentially dangerous for the eye and material: it should only be practiced by an informed and competent audience.

Star catalogs [ modifier | Modifier and code ]

To identify the stars and facilitate the work of astronomers, many catalogs have been created. Among the most famous are the Henry Draper (HD) catalog and the Bonn’s pattern (Comics). The stars are stored there by their contact details, Alpha (right ascent) and Delta (declination) and a number is assigned to them: for example, HD 122653 (famous giant of Population II , very deficient in metals).

A star is characterized by different quantities:

Lot [ modifier | Modifier and code ]

Mass is one of the most important characteristics of a star. Indeed, this grandeur determines his lifespan as well as his behavior during his evolution and the end of his life: a massive star will be very bright but his lifespan will be reduced.

The stars have a mass between approximately 0.08 and 300 times the mass of the sun, that is (very) almost 2 × 10 ³⁰ kilograms (two billion billion billion tonnes). Below the minimum mass, the warm -up generated by the gravitational contraction is insufficient to start the nuclear reactions cycle: the star thus formed is a brown dwarf. Beyond the maximum mass, the gravity force is insufficient to retain all the matter of the star once the nuclear reactions have started. Until recently, we thought that the mass of a star could not exceed 120 to 150 times the solar mass but the recent discovery of a star having a mass 320 times higher than that of the sun made this hypothesis obsolete ^{[ 3 ]} .

Mass limits of stars [ modifier | Modifier and code ]

Low limit [ modifier | Modifier and code ]

The stars having the smallest mass observed (1/ 20 ^{It is} solar mass) are the red dwarfs, which very slowly merge hydrogen with helium.

Below, there are the brown dwarfs that just start the fusion of the deuterium to their formation.

Height limit [ modifier | Modifier and code ]

The mass of a star is limited by the circumstances of the training process and its stability on the main sequence, essentially by the ejection rate of the stellar wind.

The most massive stars generally have a mass of around 50 to 80 masses Solar.
The even more massive stars are unstable because the gigantic radiation pressure which reigns in their center causes the “rapid” expulsion of the matter which constitutes them, thus significantly reducing their mass during their “brief” main sequence.

It is believed that the first generation of stars in the universe, those of the population III , were mainly giant stars, typically more than 100 solar masses, up to 1,000 solar masses. They were able to exist (and maintain themselves during their “short” main sequence), because their metallicity was, so to speak, and the “metallic” ions are much more sensitive to the radiation pressure than hydrogen and ionized helium. A good part of them end up in hypernovas.

In January 2004 , Stephen Eikenberry of the University of California, announced that it had found the most massive star ever observed: LBV 1806-20. It is a very young star who would at least 150 masses Solar. In July 2010 , an international team of astronomers announces the discovery with the VLT In Chile of the Star R136A1 in the Tarentule nebula which would be 265 times more massive than the sun. According to Professor Paul Crowther at the University of Sheffield, his mass at birth would be 320 times the mass of the sun ^{[ 4 ]} .

Estimation [ modifier | Modifier and code ]

The determination of the mass of a star can only be done precisely when it belongs to a binary system by observing its orbit. The third law of Kepler then makes it possible to calculate the sum of the masses of the two stars of the binary from its period and the half-axis of the orbit described and the distance of the earth to the double-observed star. The ratio of the masses is obtained by measuring the radial speed of the two stars of the binary. Knowledge of the sum and the ratio of the masses makes it possible to calculate the mass of each star. This is the most precise technique.

Other estimates are possible for non -binary (simple) stars using spectroscopic determination of surface gravity and the measurement of the star of the star by interferometry. Finally, if the star is observed precisely in photometry and if its distance, its chemical composition and its effective temperature are known, it is possible to position it in a diagram of Hertzsprung-Russell (noted HR) which immediately gives the mass and the age of the star (Vogt-Russell theorem).

Diameter [ modifier | Modifier and code ]

Compared to our planet (12,756 km In diameter), the stars are gigantic: the sun has a diameter of about a million and a half kilometers and certain stars (such as Antarès or Bételgeuse) have a diameter of hundreds of times higher than the latter.

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

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

Chemical composition [ modifier | Modifier and code ]

The chemical composition of the material of a star or a gas in the universe is generally described by three quantities in mass number: X hydrogen, AND L’Hélium and WITH Metalization. These are proportional quantities satisfying the relationship: X + AND + WITH = 1 .

Metacicity [ modifier | Modifier and code ]

Metalization is the quantity (measured in number, or generally by mass) of heavier elements than the helium present in the star (or rather its surface). The sun has a metallicity ( noted WITH ) 0.02: 2% of the mass of the sun is made up of elements that are neither hydrogen nor helium. For the sun, these are mainly carbon, oxygen, nitrogen and iron. Although it seems weak, these two percent are very important to assess the opacity of the star matter, whether internal or in its atmosphere. This opacity contributes to the color, the brightness and the age of the star (see Hertzsprung-Russell diagram and Vogt-Russell theorem).

Opacity is directly linked to the capacity of the star to produce a stellar wind (extreme case of Wolf-rayet stars).

Magnitude [ modifier | Modifier and code ]

The magnitude measures the brightness of a star; It is a logarithmic scale of its radiative flow. Apparent magnitude in a given filter ( ex. : The visible noted MV), which depends on the distance between the star and the observer, is distinguished from absolute magnitude, which is the magnitude of the star if it was arbitrarily placed at 10 parsecs of the observer. Absolute magnitude is directly linked to the brightness of the star provided you take into account a so -called bolometric correction (noted BC). The introduction of the logarithmic scale of magnitudes comes from the fact that the eye has a sensitivity also logarithmic, in the first approximation (Pogson law).

Temperature and color [ modifier | Modifier and code ]

Most stars seem white with the naked eye, because the sensitivity of the eye is maximum around yellow. But if we look carefully, we can note that many colors are represented: blue, yellow, red (the green stars do not exist). The origin of these colors remained a mystery for a long time until there is two centuries ^{[When ?]} , when physicists had enough understanding of the nature of light and the properties of matter at very high temperatures.

The color makes it possible to classify the stars according to their spectral type (which is related to the temperature of the star). The spectral types range from the most purple to the redest, that is to say from the hottest to the coldest. They are classified by letters o b a f g k m ^{[ Note 3 ]} . The sun, for example, is spectral G.

But it is not enough to characterize a star by its color (its spectral type), you also have to measure its brightness. In fact, for a given spectral type, the size of the star is correlated with its brightness, the brightness being a function of the surface – and therefore the size of the star.
The stars O and B are blue like β Orionis (Rigel); Stars A 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 orange like α bootis (Arcturus); And finally the stars M are red like α Orionis (Bételgeuse).

We can define a color index, corresponding to the difference in photometric flow in two spectral strips called photometric strips (filters). For example, blue (b) and the visible (V) will form together the B-V color index, the variation of which is connected to the surface temperature of the star and therefore to its spectral type. The most used temperature indices are B-V, R-I and V-I because they are most sensitive to temperature variation.

Rotation speed [ modifier | Modifier and code ]

The sun rotation was highlighted thanks to the movement of sunspots. For the other stars, the measurement of this speed of rotation (more precisely, the measured speed is the projection of the equatorial rotation speed on the aim line), is obtained by spectroscopy. It results in a widening of spectral lines.

This stellar rotation movement is a remainder of their formation from the collapse of the gas cloud. The speed of rotation depends on their age: it decreases over time, under the combined effects of the stellar wind and the magnetic field which take part of the kinetic moment of the star. This speed also depends on their mass and their simple, binary or multiple star status. A star is not a solid body (that is to say rigid), it is animated by a differential rotation: the speed of rotation depends on the latitude.

In 2011, the Very Large Telescope discover VFTS 102 , the star at the highest speed of rotation ever observed (only pulsars can run much faster), more than two million kilometers per hour ^{[ 5 ]} .

Radiative spectrum [ modifier | Modifier and code ]

The spectrum of a light source and therefore of a star is obtained by spectrographers that break down light into its different components and record them through detectors (historically, photographic plates, then CCD type detectors). This decomposition of light reveals the distribution of light energy from the star as a function of the wavelength. It highlights spectral rays in emission and/or absorption revealing the conditions of temperature, pressure and chemical abundance of the external layers of the star.

Magnetic field [ modifier | Modifier and code ]

Like the sun, the stars often have 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 much more complex and present arches on a smaller scale. The magnetic field of the sun, for example, has these two aspects; Its large -scale component structures the solar crown and is visible during eclipses, while its smaller scale component is linked to dark spots which smell 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 induces on the spectral lines formed in the atmosphere of the star (the Zeeman effect). The Zeeman-Doppler imaging technique allows in particular to deduce the geometry of the giant arches that the magnetic field draws up on the surface of the stars.

Among the magnetic stars ^{[ 7 ]} , we first distinguish the stars called “cold” or not very massive, whose surface temperature is lower than 6 500 K and whose mass does not exceed 1.5 mass Solar – The sun is therefore part of this class. These stars are “active”, that is to say that they are the seat of a certain number of energy phenomena linked to the magnetic field, for example the production of a crown, of a wind (called solar wind in the case of the sun) or eruptions. The spots on the surface of the sun and the stars also testify to their activity; Like magnetic fields, stars spots can be mapped by tomographic methods. The size and number of these spots depend on the activity of the star, itself a function of the speed of rotation of the star. The sun, which makes a complete turn on itself in 25 days About, is a star with low cyclic activity. The magnetic field of these stars is produced by dynamo effect.

There are also magnetic hot stars. But unlike cold stars, which are all magnetic (at different degrees), only a small fraction (between 5 and 10%) of hot stars (massive) has a magnetic field, whose geometry is generally quite simple. This field is not produced by dynamo effect; Rather, it would constitute a fossil imprint of primordial interstellar magnetism, captured by the cloud which will give birth to the star and amplified during the contraction of this star cloud. Such magnetic fields have been baptized “fossil magnetic fields”.

From measures and simulations from different models, it is possible to build an image of the interior of a star, although it is almost inaccessible to us – only asterosismology allows to without the stars.

According to current knowledge, a star is structured in different concentric regions, described below from the center.

Core [ modifier | Modifier and code ]

The nucleus (or heart) is the central part of the star, concentrating a large part of the mass of the star, in which the thermonuclear reactions take place which give off the energy necessary for its stability. The nucleus is the densest and hottest area and, in the case of the sun, reaches the temperature of 15,7 millions of Kelvins. Under these extreme conditions, the material is in the form of plasma; By tunnel effect, hydrogen nuclei (protons) or other chemical elements reach speeds allowing them to overcome their electrical repulsion and to merge. For example, in the so-called Proton-Protton nuclear chains (or PP1, PP2, etc.), protons merge by group of four to give a helium nucleus, composed of two protons and two neutrons. There is then an energy clearance according to the following reactions:

2 ( ^first H + ^first H → ² D + e ⁺ + n _{It is} ) ( 4,0 MeV + 1.0 MeV )

2 ( ^first H + ² D → ³ He + γ) ( 5.5 MeV )

³ He + ³ He → ⁴ He + ^first H + ^first H ( 12.86 MeV ).

Other thermonuclear reactions take place in the center of the stars and contribute variously to the production of energy.

Part of the energy released in the form of photons then begins a long journey to the outside, because a plasma is opaque and the light travels there very difficult. It is estimated that a photon takes several million years before reaching the surface of the star by radiation of radiation then by convection to the surface ^{[Ref. necessary]} .

Having generally undergone various stages of contraction, the nucleus of a star frequently turns faster than external layers (by conservation of the kinetic moment). Asterosismology shows, however, that it turns slower than predicting current models. In 2023, digital simulations show that this slowdown in the heart can be due to an internal magnetic field, produced by a Tayler-Sprit dynamo (without convection) ^{[ 8 ] , [ 9 ]} .

Zone radiative [ modifier | Modifier and code ]

The energy released by nuclear reactions in the kernel of the star is transmitted to the external layers by radiation. In the poorly massive stars and evolving on the main sequence, this radiative area is surmounted by an external convective zone; In the red dwarfs, the radiative area has completely disappeared in favor of the convective area. In the sun, the radiation produced in the central part takes almost a million years to cross the radiative area after a Brownian movement.

Zone convective [ modifier | Modifier and code ]

Unlike the previous area, energy is transmitted in the convective area by macroscopic material movements: subject to a gradient of decreasing temperature towards the surface, the fluid develops a convection of the Rayleigh-Bénard type. This convective area is more or less: for a star on the main sequence, it depends on the mass and the chemical composition; For a giant, it is very developed and occupies a large part of the volume of the star; For a supergeant, this area can reach three -quarters of the star volume, as in the case of Bételgeuse. In very low -mass stars (red dwarf) or in protoetors in low mass formation (Tauri type T -type stars), the convective area occupies the entire volume of the star; In the stars whose mass twice exceeds that of the sun, the external convective zone disappears (leaving room for the radiative zone) but the convection remains at the heart of the star ^{[Contradictory passage]} .

It is in the external convective area that the dynamo type magnetic fields of cold stars like the sun and the red dwarfs produce.

Photosphere [ modifier | Modifier and code ]

The photosphe is the external part of the star, which produces visible light. It extends from less than 1% of the radius, for dwarf stars (a few hundred kilometers), a few tenths of the star’s department, for the largest giants. The light produced there contains all the information on the temperature, surface gravity and the chemical composition of the star. For the sun, the photosphere has a thickness of about 400 kilometers .

Crown [ modifier | Modifier and code ]

The crown is the external area, tenuous and extremely hot of the sun. It is due to the presence of a magnetic field, produced in the convective area; It can be observed during sun eclipses. It is thanks to the study of the crown at XIX ^{It is} A century that astronomer Jules Janssen discovered the existence of rare gas whose name refers to the sun (Helios): helium. The fact that the crown temperature reaches several million degrees is a difficult theoretical problem and not yet completely resolved. It is likely that most low -mass stars (containing an external convective area) have magnetic fields and therefore crowns.

Vogt and Russell theorem [ modifier | Modifier and code ]

The Vogt-Russell theorem can be exposed as follows: if in any point of a star the knowledge of the values ​​of the temperature, the density and the chemical composition of the internal plasma are sufficient to calculate the pressure, the opacity of the Plasma and the energy rate produced, then the mass and the chemical composition of the star are sufficient to describe its structure. This results in the mass-ray or luminosity of the stars relationships.

The life of a star can be broken down into several main phases:

After the final phase, the residue of the star is a white dwarf, a neutron star or a black hole.

The spectral analysis of the radiation of a star reveals some of its characteristics, and therefore makes it possible to determine the stage of evolution where it has reached. The Hertzsprung-Russell diagram is often used to locate a star during its evolution ^{[ ten ]} . According to their initial masses (often expressed in solar masses), the stars can follow different developments ^{[ 11 ] , [ twelfth ]} .

Training: Birth of stars [ modifier | Modifier and code ]

The stars are born, often in groups, from the gravitational collapse of an interstellar cloud of gas and dust ^{[ 13 ]} , like a molecular cloud or a nebula (such as the Orion nebula or the eagle nebula). They thus form stellar clusters.

Molecular clouds, extending over hundreds of years, can reach several million sunscreen ^{[ 11 ]} . The stability of a cloud is maintained by magnetic fields and turbulent movements, which prevent it from collapsing on itself ^{[ 14 ]} . However, in the densest and coldest (of the order of 10 K ), the stability of the cloud can be broken ^{[ 11 ]} (sometimes when passing a density wave from a galaxy arm or a supernova). This gravitational instability triggers the collapse phase. It is a series of cloud fragments and contractions in several more and smaller and dense blocks, which end up forming protoetors wrapped in opaque gas and dust clouds ^{[ 13 ]} .

Dust and gas around a protoetoile disperse and flatten under the effect of an emerging rotation to form a protostellar disc, in which possible planets are created ^{[ 13 ]} .

Within the protoetoile, the gas contraction continues and leads to its warm -up (by converting gravitational energy into thermal energy). During its warm -up, the Protoetoile emits infrared radiation before becoming visible. It enters the main pre-sequence. In the Hertzsprung-Russell diagram, the protoétoile manifests itself first in the region of red giants and its brightness decreases rapidly, while its temperature increases: it descends the Hayashi line ^{[ 11 ] , [ 15 ]} . In the center of the protoetoile, when the temperature reaches approximately a million degrees (10⁶ K), the merger of the deuterium begins (this is the first nuclear fusion within the protoetoile) ^{[ 16 ]} . Then at a dozen million degrees (10⁷ k) the temperature is sufficient to trigger the Proton-Protton chain (fusion of helium hydrogen) ^{[ 14 ]} . During this phase, the contraction ceases: the kinetic pressure due to the thermal agitation of the particles and the radiative pressure are significant enough to counterbalance the gravitational pressure. The Prototoile then becomes a full-fledged star, located on the main sequence of the Hertzsprung-Russell diagram ^{[ 11 ]} .

Main sequence [ modifier | Modifier and code ]

Under the effect of the contraction, the kernel of the star (its central part) reaches extreme pressure and temperature values, which go as far as the ignition of thermonuclear reactions (see above). The star then enters into what is called the main sequence, a period during which its nucleus, initially and essentially made up of hydrogen and helium, will gradually transform into helium.

During this period, antagonism released energy / gravitation contributes to the stability of the star: if the flow of energy coming from the nucleus comes to decrease, the contraction which ensues accelerates the rate of energy production which stops the contraction; Conversely, an excitement of energy production leads to dilation of the star, therefore its cooling, and the excitement stops. Thus, the result is a great stability of the star which is described in the theory of the stellar internal structure under the name “Pic de Gamow” ^{[Ref. to confirm]} : it is a kind of stellar thermostat .

End of a star [ modifier | Modifier and code ]

The more massive a star, the more it quickly consumes its hydrogen. A big star will therefore be very brilliant, but will have a short lifespan. When nuclear fuel is too rare in the kernel of the star, fusion reactions stop. The radiation pressure maintained by these reactions no longer compensating for the gravitation forces, the star collapses on itself. The larger a star, the more the end of its existence will be cataclysmic, which can go so far as to take the form of a gigantic explosion (supernova, even hypernova) followed by the formation of a neutron star (pulsar, magnetar, etc. ) or even in extreme cases (depending on the mass of the star) of a black hole.

Astronomers classify the stars using effective temperature and brightness.
This classification with two parameters makes it possible to define spectral types (brightness) varying from WE To I . Dwarfs for example (whose sun) are listed IN . Among these classes there are different categories related to surface temperature. We distinguish the black, brown, red, yellow and white dwarfs, red and blue giants, red supergeants, neutron stars and black holes. If most stars are easily placed in one or the other of these categories, it must be kept in mind that these are only temporary phases. During its existence, a star changes shape and color, and goes from one category to another.

Brown [ modifier | Modifier and code ]

The brown dwarfs are not stars, but substantial objects sometimes qualified as “missed stars”. Their mass is located between those of small stars and large planets. Indeed, at least 0.08 solar mass is necessary for a proto-star to start thermonuclear reactions and become a real star. The brown dwarfs are not massive enough to start these reactions. However, they can radiate weakly by gravitational contraction.

Red dwarf [ modifier | Modifier and code ]

The red dwarfs are small red stars. They are considered to be the smallest stars as such, because the smaller stars like white dwarfs, neutron stars and brown dwarfs do not consume nuclear fuel. The mass of red dwarfs is between 0.08 and 0.8 solar mass. Their surface temperature between 2 500 And 5 000 K gives them a red color. The least massive of them (below 0.35 mass solar approximately) are fully convective. These stars slowly burn their fuel, which ensures them a very long existence. They are the most abundant: at least 80% of the stars in our galaxy are red dwarfs.

The closest neighbor of the sun, Proxima of the centaur, is one. The same is true of the second stellar system closest to the solar system, the Barnard star is also a red dwarf.

Yellow [ modifier | Modifier and code ]

The yellow dwarfs are medium -sized stars – astronomers classify the stars only in dwarf or giants. Their surface temperature is about 6 000 K And they shine bright yellow, almost white. At the end of its existence, a yellow dwarf evolves into a red giant, which by expelling its external layers – then deploying a nebula – reveals a white dwarf.

The sun is a typical yellow dwarf.

Red giants [ modifier | Modifier and code ]

The giant red phase announces the end of existence of the star, which reaches this stage when its nucleus has exhausted its main fuel, hydrogen: helium fusion reactions are triggered, and while the center of the ‘Star contracts under the effect of increasing its internal gravitation, its external layers swell under the effect of the energy released by the fusion of helium, cool and reddish. Transformed into carbon and oxygen, the helium is exhausted in turn and the star goes out, its size and therefore its gravitational energy being insufficient to trigger oxygen fusion reactions. The external layers of the star move away and its center contracts, revealing a white dwarf.

Giants, bright, supergeting and hypergenous giants [ modifier | Modifier and code ]

On the Hertzsprung-Russell diagram, beyond a certain luminosity, the stars successively take the names of giant, bright giant, supergeant and hypergenous.
In the case of giant stars, when the nucleus of a blue giant no longer contains hydrogen, the fusion of helium takes over. Its external layers swell and its surface temperature decreases. According to its mass, it then becomes a red giant or a red supergeant.

The star then manufactures increasingly heavy elements: titanium, chrome, iron, cobalt, nickel, etc. At this stage, fusion reactions stop and the star becomes unstable. It explodes in a supernova and leaves behind it a strange nucleus of matter which will remain intact and which will become, depending on its mass, a neutron star or a black hole.

Luminous giant stars are class stars brightness II .

The supergearies and the hypergeling are the most massive and luminous stars of the observable universe.

Blue light variable stars [ modifier | Modifier and code ]

A blue variable blue star is a hypergal blue with variable brightness which occasionally expels large quantity of material.
It can evolve in star Wolf-rayet and finally finish in supernova.

Wolf-rayet stars [ modifier | Modifier and code ]

Wolf-rayet stars are very massive stars at the end of life which expel very large amounts of material in the form of high speed solar winds so intense that they form a cloud around it. Thus one cannot directly observe its surface as for the other stars but only the matter which it ejects. They have a very brief lifespan of only a few million years, before exploding in supernovæ.

Population stars III [ modifier | Modifier and code ]

The stars of population III are an extremely massive and bright type of stars, observed for the first time in 2015 in the CR7 galaxy ^{[ 17 ]} , made up exclusively of light elements (hydrogen and helium, with perhaps a little lithium), which would be the first stars formed at the beginning of the universe, approximately 400 millions years after Big Bang .

White [ modifier | Modifier and code ]

White dwarfs are the residues of the evolution of low -mass stars (between ~ 0.8 and ~ 5 to 8 solar masses ). The sun having (by definition) a mass of a solar mass, it will also end in white dwarf. White dwarfs are “dead” stars since they are no longer the place of thermonuclear reactions producing heat. However, they are initially very hot and relatively white in color (see Wien’s law). Little by little, they cool by radiation, to become cold stars. Their size is approximately equal to that of the earth.

White dwarfs, like neutron stars, consist of material degenerate . The density mean of a white dwarf is such that a teaspoon of material of such a star would, on earth, the weight of an elephant, or about 1 t cm ⁻³ . In fact, in this matter, the electrons, being very close to each other, then begin to repel vigorously. The main factor of pressure then comes from the principle of exclusion from Pauli; It is the degeneration pressure that opposes that of gravitation. The white dwarf is therefore in balance despite the absence of nuclear fusion in its nucleus. The pressure of the electrons can withstand a mass of 1.44 times that of the sun: it is the limit of Chandrasekhar.

If a white dwarf becomes more massive (aspiring the material of another star, for example), it explodes in supernova and is largely sprayed in nebula. This is the guy I a Thermonuclear supernovas.

Procyon B and Sirius B are white dwarfs.

Black dwarfs [ modifier | Modifier and code ]

Like a heated plate that is turned off, the white dwarfs cool inexorably. However, this is done very slowly, because of their highly reduced emissive surface (the size of a telluric planet) compared to their mass (of the order of that of the sun). They gradually lose their brilliance and become invisible after ten billions of years. Thus, all white dwarf turns into a black dwarf.

The universe, old of 13.7 billion Years are still too young for producing black dwarfs.

After his death, the sun will become a white dwarf then a black dwarf. This spell awaits him in about 15 billion years.

Neutron stars and black holes [ modifier | Modifier and code ]

The neutron stars are very small but very dense. They concentrate the mass of one and a half that of the sun within a radius of about ten kilometers. These are the vestiges of very massive stars of more than ten solar masses whose heart has contracted to achieve values ​​of extraordinarily high density, comparable to those of the atomic nucleus.

When a massive star arrives at the end of his life, she collapses on herself, producing an impressive explosion called Supernova. This explosion disperses most of the material of the star in space while the nucleus contracts and turns into a neutron star ^{[ Note 4 ]} . These objects have very intense magnetic fields (for the most intense, we speak of magnetar). Along the magnetic axis propagate charged particles, electrons for example, which produce synchrotron radiation.

The kinetic moment of the star being kept during the collapse of the nucleus, the neutron star has an extremely high speed of rotation, which can reach a thousand laps per second. If by chance an observer on earth looks in the direction of a neutron star and that the aiming line is perpendicular to the axis of rotation of the star, he will then see the synchrotron radiation of the charged particles moving on magnetic field lines. This rotating lighthouse phenomenon is called Pulsar’s phenomenon. There are pulsars in remains of supernovas, the most famous being the pulsar of the crab nebula, born from the explosion of a massive star. This supernova was observed by Chinese astronomers since the morning of the July 4, 1054 , in broad daylight for three weeks and overnight for almost two years.

Sometimes the core of the dead star is too massive to become a neutron star. It contracts inexorably until a black hole form.

Variable stars [ modifier | Modifier and code ]

While most stars are of almost constant brightness, like the sun which practically no measurable variation (about 0.01% on a cycle of 11 years old ), the brightness of certain stars varies perceptibly over much shorter periods of time, sometimes in a spectacular way.

The stars rarely form alone. When a gas cloud (proto-stud) gives birth to a heap of stars, all the stars of this cluster does not seem to be distributed at random, but seems to follow a distribution law called initial mass function (abridged IMF in English), of which we know little currently; It accounts for the proportion of stars according to their mass. We do not know if this MFI function depends on the chemical composition of the proto-stud cloud. The most adopted function currently has been proposed by Edwin Salpeter and seems to give satisfactory results for the study of galaxy clusters.

Binary and multiple systems [ modifier | Modifier and code ]

The binary systems are made up of two stars linked gravitational and orbiting one around the other. The brightest element is said to be primary and the least shiny, secondary. When a system has more than two components it is qualified as a multiple stellar system.

Binary systems can be detected by imaging, when the telescope manages to solve the pair; In this case the binary is said to be visual. In other cases, the two companions cannot be resolved, but the Doppler-Fizeau lag of spectral lines makes it possible to detect the orbital movement of one or two stars. In this case, the binary is said to be spectroscopic. If only one spectrum is visible and varies we speak of binary SB1, if the spectrum of the two stars is clearly visible we speak of 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 the primary star if the secondary is very little bright; In this case, the binary is said to be astrometric. Finally, we speak of interferometric binary when the secondary is detected by interferometry.

Amateur astronomy speaks of apparent binary when two distant stars in space and not linked gravitational are close to the sky by perspective effect.

Loves [ modifier | Modifier and code ]

Stellar clusters are local groups of stars linked gravitational and formed at the same time. As a result, they constitute a reference population to study the lifespan of a star according to its size (see Hertzsprung-Russell diagram). It can be used to determine the age of the oldest stars of our galaxy.

We distinguish open clusters (AO) made up of a few dozen to a few thousand stars and generally in any form and globular cluster (AG) made up of several thousand to several million stars.

AOs are young, from a few tens to a few hundred million years old. Among the oldest, M67 (4.6 billion years like the sun) is also among the biggest. In our galaxy, the AOs are rich in metals (typically like the sun). The GAs are spherical in shape, hence their name. Their stars are poor in metals and they are among the oldest objects in the galaxy. They are distributed in the spheroid of the galaxy called the halo. Their age is between around 10 and 13.5 billion years. Omega du Centaure is among the biggest. Its stellar population is not unique which shows that it has had an origin spread over time allowing the formation of several of them (at least three). It is considered to be the residue of a dwarf galaxy having been captured by the Milky Way. NGC 6397 On the contrary is a unique stellar pile with an abundance of metals of one hundredth from that of the sun. The most poor in known metals is M92 with almost a thousandth of solar abundance.

Associations [ modifier | Modifier and code ]

Stellar associations are similar to clusters, except that they are not a gravitational linked system. So associations disperse after a while. Example of association: O-B associations consisting mainly of very massive and very hot stars. They can be considered as very young open clusters still having a lot of ionized gas in the vicinity of the stars. We meet them in our galaxy mainly in the arms.

Galaxies [ modifier | Modifier and code ]

A galaxy is a large set of stars. Galaxies differ from piles by their size (several hundred billion stars against a few thousand to a few million for stellar clusters), their organization and their history.

By observing the night sky, the human being imagined that the brightest stars could constitute figures. These groupings generally differ from one time to another and from one civilization to another. Figures that have become traditional, often related to Greek mythology, are called constellations.

The stars of a constellation have first Nothing in common, if not to occupy, seen from the earth, a neighboring position in the sky. They can be very distant from each other. However, the international astronomical union has defined a standardized list of constellations, attributing to each a region of the sky, in order to facilitate the location of celestial objects.

The stars can be accompanied by bodies gravitating around them. Thus, the solar system is made up of a central star, the sun, accompanied by planets, comets, asteroids.
Since 1995, several thousand exoplanets have been discovered around other stars than the sun, making the solar system lose its supposedly unique character. All these planetary systems are discovered indirectly. The first star around which planets were revealed by velocimetric measures is 51 Peg (OHP observations with the Élodie spectrograph). Many other planetary systems have since been discovered. Due to current detection limitations, they have similar characteristics, with giant planets on very eccentric orbits: they are called “hot Jupiter”. The majority of these stars are richer in metals than the sun. Statistics on these planetary systems make it possible to conclude that the solar system has no equivalent so far. From space, the hunt for planetary systems by photometry has started with the satellite Corot (CNES). This was relayed in 2009 by the American satellite Kepler .

Notes [ modifier | Modifier and code ]

References [ modifier | Modifier and code ]

Bibliography [ modifier | Modifier and code ]

• Marc Seguine and Benoît Villeneuve, Astronomy & Astrophysics: Five big ideas to explore and understand the universe , Paris, Masson, 1995 , 550 p. (ISBN  978-2-225-84994-7 )
• Joachim Herrmann , Astronomy Atlas , Paris, the pocket book, coll. “Today’s encyclopedias”, 1995 , 287 p. (ISBN  978-2-253-06453-4 )

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