A star is a star like the sun, which shines through nuclear reactions that occur in its center. With the exception of the sun, the stars appear to the naked eye as a bright, sparkling because of atmospheric turbulence, without immediate apparent motion relative to other fixed objects in the sky. All the stars are considerably more distant from Earth than the Sun. The nearest star, Proxima Centauri, is located about 4 light years from the Solar System, about 250 000 times farther than the Sun. The mass of a star is of the order of 1030 kg and the radius of the order of several million kilometers. The power radiated by a star like the Sun is about 1026 watts. The stars are formed following the contraction of a nebula of gas and dust under the influence of gravity. If the heating of the material is adequate, it triggers the cycle of nuclear reactions in the heart of the nebula to form a star. The energy released by these reactions is then sufficient to stop its contraction due to radiation pressure and processed.
The number of stars in the universe is estimated between 1022 and 1023. The Sun aside, the stars are too faint to be observable in daylight. The number of observable stars at night, to the naked eye in clear weather, varies between a hundred and several thousand depending on the conditions of observation.
Image : Birth of a star image made from data of X-ray telescope Chandra (blue) and data from the Spitzer infrared telescope (red and orange). At about 4000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active when the presence of clusters of young blue stars on the image. The one that we see arising in yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen. According to data from different telescopes, astronomers have determined that the birth of stars in this region is triggered by the proximity effect of massive young stars.
Structure of a star
The structure of a star contains various zones, the heart, the radiative zone, the zone convective, the photosphere and the crown. The heart is the part of the star in which take place the thermonuclear reactions supplying the energy necessary for its stability. The heart is thus the warmest zone, achieving for the Sun, a temperature of 15,7 millions of Kelvin The temperature of 0 Kelvin (K) is equal in -273,15°C and corresponds to the absolute zero, a temperature variation of 1 K is equivalent to a variation of 1°C.. The energy freed by the nuclear fusions in the heart of the star is passed on in the external layers by radiation. It is the radiative zone which gets back at first this energy. The radiative zone is surmounted by a zone convective. In this convective zone, the heat is passed on by macroscopic movements of matter: warmed on the base of the layer convective, the matter rises under the influence of the push of Archimedes, warms the matter surrounding (towards the surface), cools and plunges towards the base of the zone convective for a new cycle of convection. This convective zone is more or less big: for a star on the main sequence, it depends on the mass and on the chemical composition.
For a giant, it is very developed and occupies an important percentage of the volume of the star. For a super giant, this zone can reach three quarters of the volume of the star. Then comes the photosphere. It is the external part of the star that produces the visible light. It is more or less spread, of some hundreds of kilometers for the dwarf stars (lower than 1 percent of the beam in some tens of percent of the beam of the star for the most huge. The light which is produced contains all the information on the temperature, the gravity of surface and the chemical composition of the star there. For the Sun, the photosphere has a thickness about 400 kilometers. The crown is the external zone, thin and extremely warm of the Sun. We can observe it during the eclipses of Sun. It is thanks to the study of the crown in the 19th century that the astronomer Jules Janssen discovered the existence of the helium, the rare gas the name of which makes reference to the Sun ( Helios).
Image : Sirius is the brightest star of the night sky. Sirius is over 20 times brighter than our sun and more than twice as massive. Sirius is 8.7 light-years away, this is not the star system nearest to you. Sirius is called the Dog Star because of its importance in the constellation of Canis Majoris (Big Dog). In 1862, we discovered a companion to Sirius which is a binary star, Sirius B is 10 000 times brighter than Sirius A. The Sirius system is captured on the left image in the X-ray.
Image : Alpha Centauri is the closest star at 4.22 light years from our Sun, it is part of a triple star system (captured in the center of the image). It is visible only through a telescope. The brightest star in the Alpha Centauri system is quite similar to our Sun.
The man imagined that the most brilliant stars could constitute figures. These groupings differ from time to the other one and from a civilization to the other one. Figures become traditional, often in touch with the Greek mythology, are called constellations. The stars of a constellation have nothing in common, if it is not to occupy, seen by the Earth, a position is placed next in the sky. It can be very remote some of the others. However, the international astronomical Union defined a list standardized by the constellations, attributing to each a region of the sky, to facilitate the localization of the heavenly objects. Stars have an included mass enter approximately 0,08 and 120 times the mass of the Sun. This size determines the life of the star. A very massive star will be very brilliant but its life cycle will be very reduced. Below the minimal mass, the heating generated by the contraction is insufficient to start the cycle of nuclear reactions, beyond the maximal mass, the force of gravity is insufficient to retain all the matter of the star once the begun nuclear reactions.
Compared with our planet (approximately 12 000 km in diameter), stars are gigantic: the Sun is one diameter one and a half million kilometers and certain stars as Antares or Betelgeuse is one diameter 800 time superior in in our Sun. The stellar research as for her uses rather the size of the beam rather than the diameter which remains a notion in two dimensions. The magnitude is a logarithmic scale of the radiative stream of the star. We distinguish the visible magnitude which depends on the distance between the star and the observer, and the absolute magnitude, which is the magnitude of the star if this one was arbitrarily placed in 10 parsec of the observer. The absolute magnitude is naturally directly connected to the luminosity of the star. This last size is used by the stellar models of evolution, whereas the magnitude is rather used for the observations, because the eye possesses an also logarithmic sensibility. Most of the stars seem white in the bare eye. But if we look attentively, we can note a beach of colors: blue, white, red and even gilded.
The fact that stars show various colors remained for a long time a mystery. The color allows to classify stars following their spectral type (which is in touch with the temperature of the star). The spectral types go of the most purple to the most red, that is of the warmest coldest verse. They are classified by letters O, for example, is of spectral type G. But it is not enough to characterize a star by its color (its spectral type), it is also necessary to measure its luminosity. For a given spectral type, the more the star is big, the more its luminosity is strong. Stars O and B are blue in the eye, stars A are white, stars F and G are yellow, stars K are orange, and finally stars M are red.
Categories of stars
- The brown dwarfs are not stars or rather they are failed stars. Their mass is situated between those of the small stars and that of the big planets. Indeed, are needed 0,08 solar masses so that a primal star begins thermonuclear reactions and becomes a real star. The brown dwarfs are not massive enough but they shine a little heat, this emitted heat is not more than the residue of its formation. It is possible that at the beginning of their formation they started a thermonuclear fusion but they eventually put out. The brown dwarfs have never affected the critical mass (13 times the mass of Jupiter or 0,08 times the mass of the Sun) to ignite and maintain a durable state. We consider a brown dwarf as cold in 1000°C, and of warm from 2000°C. The brown dwarfs are with difficulty observable, because they emit only a weak radiation in the infrared.
- The red dwarfs are small red stars. These celestial bodies among the smallest as the white dwarfs, the stars with neutrons and the brown dwarfs do not consume nuclear fuel. The mass of the red dwarfs is included between 0,08 and 0,8 solar masses. A temperature of surface between 2 500 and 5 000 K confers them a red color. Because of their small mass, the red dwarfs consume very slowly their hydrogen and thus possess a very long life cycle estimated between some tens and 1000 billion years. They contract and warm up slowly until all their hydrogen is consumed. The red dwarfs are probably the most numerous stars of the universe. Proxima of the Centaur, the star the closest to us is a red dwarf, as well as about twenty the others among the most close thirty stars.
- The yellow dwarfs are stars of average size. (The astronomers classify the stars only in dwarfs or in giants) they have a temperature of surface about 6000°C and shine of yellow one lively, almost white. At the end of her life, a yellow dwarf becomes a red giant then a white dwarf. The Sun is a typical yellow dwarf. The red huge phase announces the end of life ' a yellow dwarf. A star reaches this stage when its heart exhausted its main fuel, the hydrogen. Fusions of the helium start then. Whereas the center of the star contracts, its external layers swell, cool and redden. Transformed into carbon and into oxygen, the helium runs out in his turn and the star dies. The celestial body gets rid then of its external layers and its center contracts to become a dwarf white with the size of a planet.
- The blue giants and red super giants are very warm and brilliant. These stars are ten times as big at least as the Sun. The blue giants are extremely brilliant, of absolute magnitude 5, 6 and more. Very massive, they quickly consume their hydrogen and their life cycle is very short of the order of 10 in 100 million years, thus very rare. When the hydrogen in its heart was consumed, the blue giant merges then the helium. Its external layers swell and its temperature of surface falls until become a great red giant. The star makes then more and more heavy elements: iron, nickel, chromium, cobalt, titanium... At this stage, the fusions stop and the star becomes unstable. It explodes in a supernova and dies. The explosion leaves behind her a strange heart of matter which will remain intact. This corpse is, according to its mass, a star with neutrons or a black hole.
- The white dwarfs are residues of faded stars. It is the last but one phase of the evolution of the stars the mass of which is included between 0,3 and 1,4 times that of the Sun. The density of a white dwarf is very high: a dwarf white with a solar mass has a beam of the order of that of the Earth. The strong density of the matter makes that the quantum phenomena become little by little dominating and we say that the matter is in a state of degeneration. The diameter of the white dwarf does not depend any more on its temperature, but depends mainly on its mass: the more its mass is raised, the more its diameter is weak. However, there is a value over which a white dwarf cannot exist, it is the limit of Chandrasekhar. Beyond this mass, the pressure due to electrons is insufficient to compensate for the gravity and the star continues its contraction until become a star with neutrons.
- Stars with neutrons are very small but very dense. They concentrate the mass of a star as the Sun in a beam about 10 km. They are the vestiges of very massive stars of more than ten solar masses. When a massive star arrives at the end of existence, it collapses on itself, by producing an impressive explosion called supernova. This explosion scatters enormous quantities of matter in the space but saves the heart of the star. This heart contracts and is largely transformed into a star with neutrons. These objects possess very intense magnetic fields. Along the magnetic axis propagates particles in charge, electrons for example, which produce a radiation synchrotron.
- The black holes, sometimes, the heart of the dead star is too massive to become a star with neutrons. It contracts inexorably until form this astronomical object that is the black hole. Envisaged from the 18th century, the theory supporting the existence of the black holes stipulate that it is about so dense objects that their escape speed is superior at the speed of light - that is even the light cannot overcome their gravitational strength of surface, and stays prisoner. Of this disturbing characteristic result the "black " and " dark" qualifiers, but the most exact term would "be certainly "invisible", because it is very there about a total absence of luminosity. The theory defines also exactly the intensity of the gravitational field of a black hole. It is such as no particle crossing its horizon, theoretical border, can escape from it.
If most of the stars take place easily in the one or other one of these categories, it is only about temporary phases. During its existence, a star changes shape and color, and can pass from a Category to the other one.
A giant that explodes as a supernova, is what can be seen on this image that combines data obtained in different wavelengths through the space telescopes Chandra and Hubble. This supernova is known as the reference e0102-72, it is about 190 000 light-years away in the Small Magellanic Cloud. E0102 has been observed by the Chandra Observatory X-ray in 1999. An analysis of these data indicates that the general shape of e0102 is probably not that of a sphere but that of a cylinder given by one of its ends. The intriguing result implies that the explosion of a massive star produces a form similar to that observed in some planetary nebulae associated with stars of lower mass. A strong source of X-rays, Chandra has allowed the identification.
Image : Supernovae E0102-72 seen in this Chandra image of the cloud of matter expansion that followed the explosion of a giant star in a supernova. Crédit: Rayons-X - NASA / CXC / MIT / D.Dewey et al., NASA / CXC / SAO / J.DePasquale;
How big of a star?
Thanks to the law of Stefan-Boltzmann, that astronomers can easily calculate the radii of stars (see note below cons). In 1879 the Austrian physicist Josef Stefan, who is interested in the radiation of hot bodies, discovered that the total energy emitted by an object is proportional to the 4th power of its absolute temperature. The biggest star discoveries are Sagitarii kilowatts, V354 Cephei and KY Cygni, are about 1 500 times larger than our Sun. Our Sun has a diameter of 1 392 000 km. Betelgeuse is a red super giant, one of the largest stars known. Its radius is estimated at about 900 times solar, if Betelgeuse was the center of our solar system would extend between the orbit of Mars and that of Jupiter. Antares super giant red closest to us has a diameter of about 700 times that of the Sun, or nearly 1 billion miles. Aldebaran is a red giant of magnitude 0.86 and spectral type K5 III, which means it is orange, large and has left the main sequence after using all its hydrogen. It basically burns helium and reached a diameter 45 times solar.
Rigel is a blue super giant, 55 000 times brighter than the Sun. With a diameter of nearly 116 000 000 km, about 35 times solar, Rigel extend to the orbit of Venus in our solar system. Arcturus is 20 times bigger than the sun, its magnitude is -0.04 and its distance from the sun is ≈ 37 light years. Pollux is about 8 times larger than the sun, its magnitude is 1.09 and its distance from the sun is ≈ 33.7 years light.
nota : Thanks to the law of Stefan-Boltzmann, astronomers can calculate the radii of the stars. The brightness of a star is written L = 4πσR2T4 L is the luminosity, σ is the constant of Stefan-Boltzmann constant, R is the radius of the star and T its temperature.
The spots of Betelgeuse or Alpha Orionis
Betelgeuse (α Orionis) is a super cool red giant, one of the largest stars known, located 640 light years away in the constellation of Orion. Its radius is estimated at about 900 times solar, if Betelgeuse was the center of our solar system would extend between the orbit of Mars and Jupiter. Betelgeuse is 600 al of the solar system and despite its huge diameter, it shines in our sky as a simple bright point, even in the most powerful telescopes. However, using a observational technique called interferometry (virtual reconstruction of a giant telescope from several large telescopes networked) in the infrared wavelength, astronomers from the Observatoire de Paris have managed to solve the surface of Betelgeuse and produce this image of the red supergiant. This amazing image reveals the presence of two huge bright spots like two giant convective bubbles rising from the depths of the supergiant. Unlike the spots that we can observe our sun, they are brilliant because they are warmer than the rest of the surface but colder than the surface of our Sun.
Also known as Alpha Orionis, Betelgeuse is about 600 light years from us.
Image : Betelgeuse is a star at the end of life that has an absolute magnitude -5.3 to -5.0. Its temperature is about 3 600 K. It forms one corner of the Winter Triangle with Sirius (α Canis Majoris) in the constellation Canis Major and Procyon (α Canis Minoris) in the constellation Canis Minor. Betelgeuse is doomed to explode as a supernova, it will be easily visible from Earth. Credit: Xavier Haubois (Observatoire de Paris) et al.