A star is a object similar to our sun, which shines through nuclear reactions that occur in the center.
With the exception of the Sun, the stars appear to the naked eye as a bright, glittering due to 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 of the solar system, nearly 250 000 times farther than the Sun.
The mass of a star is the order of 1030 kg and its radius of about a few million miles.
The power radiated by a star like the Sun is about 1026 watts. Stars form due to the contraction of a nebula of gas and dust under the influence of gravity.
If the heating of the material is sufficient, it will trigger 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 the radiation pressure generated.
The number of stars in the universe is estimated between 1022 and 1023.
Apart from the Sun, 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). NB: The astronomers classify stars in dwarf or giant.
At about 4000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active where the presence of clusters of young blue stars in the picture.
That we see born, yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen.
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Yellow dwarfs are stars of medium size. A yellow dwarf is a star belonging to the main sequence of spectral type G and weighing between 0.7 and 1 times the solar mass.
About 10% of stars in the Milky Way are dwarf yellow. They have a surface temperature of about 6000 ° C and shine a bright yellow, almost white.
At the end of his life, a yellow dwarf star becomes a red giant and white dwarf.
The Sun is a typical yellow dwarf. The red giant phase signals the end of life of a yellow dwarf.
Stars reach this stage when the heart has exhausted its primary fuel, hydrogen.
Fusion reactions of helium then trip, and while the center of the star contracts, its outer layers swell, redden and cool.
Transformed into carbon and oxygen, helium is exhausted in its turn and the star dies.
The star then gets rid of its outer layers and center contracts into a white dwarf the size of a planet.
Among the yellow dwarf found, Alpha Centauri A, Tau Ceti, and 51 Pegasi.
Image: The Sun is a yellow dwarf star. NB: The astronomers classify stars in dwarf or giant.
It consists of 74% hydrogen, 24% helium and a fraction of heavier elements. The Sun is spectral type G2-V. "G2" means it is warmer (5770 Kelvin surface around) and brighter than average, with a color yellow-white.