Stars | |||||||||||||||||||||||||||||||
Stars | Automatic translation | Updated June 01, 2013 | |||||||||||||||||||||||||||||
A star is a star like the sun, which shines through nuclear reactions that occur in its center. | 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. 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). | ||||||||||||||||||||||||||||||
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. | For a giant, it is very developed and occupies an important percentage of the volume of the star. | Video: Herschel telescope provides images with a resolution unparalleled in the spectral infrared and submillimeter, a range of light privileged to observe the birth of stars. Star formation at the site Du Big Bang au Vivant. © Groupe ECP, Du Big Bang au Vivant. | |||||||||||||||||||||||||||||
Sirius, the most brilliant | Proxima centauri, the closest | ||||||||||||||||||||||||||||||
Image: Sirius is the brightest star of the night sky. | |||||||||||||||||||||||||||||||
Characteristics | |||||||||||||||||||||||||||||||
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. | 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.
| Image: globular cluster Omega Centauri, taken by the Hubble Space Telescope Wide Field Camera 3 (WFC3), in 2009. Credit: NASA, ESA, and the Hubble SM4 ERO Team. The color used to classify stars according to their spectral type (which is related to the temperature of the star). The spectral types range from the violet to red more, that is to say, the hotter to the colder and are classified by the letters OBAFGKM. The O and B stars are blue in the eye, the A stars are white, F and G stars are yellow, K stars are orange, M stars 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. | 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. * © V. Beckmann (NASA's GSFC) et al., ESA | ||||||||||||||||||||||||||||||
Supernova | |||||||||||||||||||||||||||||||
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. | 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. | ||||||||||||||||||||||||||||||
How big of a star? | |||||||||||||||||||||||||||||||
Thanks to the law of Stefan-Boltzmann, that astronomers can easily calculate the radii of stars (see nota opposite). | 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. Image: Size compared to some super giant stars like Antares, Betelgeuse, Rigel, Aldebaran and some white dwarfs as Arcturus, Pollux, Sirius and the Sun. | ||||||||||||||||||||||||||||||
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. | 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. |