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Introduction :[br][br]A star is an astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. Stars are enormous spheres of ignited gas that light the cosmos and seed it with the materials for rocky worlds and living beings. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. They come in many different types and sizes from smouldering white dwarfs to blazing red giants. Although they emit all colors of light, spectral classification considers only the peak of this emission as an indicator of the star's surface temperature. Using this system, blue stars are the hottest and are called O-type. The coolest stars are red and are called M-type. In order of increasing temperature, the spectral classes are M (red), K (orange), G (yellow), F (yellow-white), A (white), B (blue-white), O (blue). As the coolest stars are invariably the smallest, they are called red dwarfs. Conversely, the hottest stars often called blue giants.[br][br]
[br] Third party image reference[br][br]Yellow Dwarf Stars :[br][br]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. The Sun, Alpha Centauri, and Kepler-22 are Yellow Dwarfs. About 10% of stars in the Milky Way are Dwarf Yellow. Its color ranges from white to a lighter yellow. They have a surface temperature of about 6000° C. However, they do appear yellow when observed through the Earth's atmosphere.[br][br]
[br] Third party image reference[br][br][br]Orange Dwarf Stars :[br][br]Orange Dwarf Stars is a colloquialism for stellar objects that are more properly called "K-type main-sequence stars" and are designated K V. The first is a letter, the second a Roman numeral. Alpha Centauri B and Epsilon Eridani are Orange Dwarf Stars. These are smaller, cooler and live longer than Yellow Dwarfs like our Sun. Like their larger counterparts, they are main sequence stars fusing hydrogen in their cores.[br][br]
[br] Third party image reference[br][br]Red Dwarf Stars :[br][br]A Red Dwarf is a small and cool star on the main sequence of M spectral type. Red Dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Proxima Centauri, Barnard's Star and Gliese 581 are all Red Dwarfs. Red Dwarfs are barely hot enough to maintain the nuclear fusion reactions required to use their hydrogen fuel. However, they are the most common type of star, owing to their remarkably long lifetime that exceeds the current age of the universe.[br][br]
[br] Third party image reference[br][br]Brown Dwarfs :[br][br]Brown Dwarfs are objects which are too large to be called planets and too small to be stars. They have masses that range between twice the mass of Jupiter and the lower mass limit for nuclear reactions. They are too small to generate the heat required for hydrogen fusion. Their cold exteriors emit radiation beyond the red region of the spectrum and to the human observer they appear magenta rather than brown. As Brown Dwarfs gradually cool, they become difficult to identify and it is unclear how many exist.[br][br]
[br] Third party image reference[br][br]Blue Giant Stars :[br][br]In astronomy, a Blue Giant is a hot star with a luminosity class of III (giant) or II (bright giant). Blue Giants are defined here as large stars with at least a slight blueish coloration, although definitions do vary. Not all Blue Giants are main sequence stars. Indeed, the largest and hottest burn through the hydrogen in their cores very quickly, causing their outer layers to expand and their luminosity to increase. These massive stars are more likely to explode in a Supernova before they can cool to reach a later evolutionary stage, such as a Red Super Giant.[br][br]
[br] Third party image reference[br][br]Red Giant Stars :[br][br]A Red Giant Star is a dying star in the last stages of stellar evolution. In only a few billion years, our own Sun will turn into Red Giant Star, expand and engulf the inner planets, possibly even Earth. The outer atmosphere is inflated and tenuous making the radius large and the surface temperature around 5,000 K or lower. In Red Giants, the accumulation of helium causes a contraction of the core that raises the internal temperature. This triggers hydrogen fusion in the outer layers of the star, causing it to grow in size and luminosity. They eventually eject their outer layers to form a planetary nebula, while the core becomes a White Dwarf.[br][br]
[br] Third party image reference[br][br]White Dwarfs :[br][br]A White Dwarf, also called a degenerate Dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A White Dwarf is very dense. Its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. The remnant is called a White Dwarf and examples include Sirius B and Van Maanen's star. More than 97% of stars are theorized to become White Dwarfs. These super hot structures will remain hot for trillions of years before cooling to become Black Dwarfs.[br][br]
[br] Third party image reference[br][br]Neutron Stars :[br][br]A Neutron Star is the collapsed core of a giant star which before collapse had a total of between 10 and 29 solar masses. Neutron Stars are the smallest and densest stars, not counting Black Holes, hypothetical White Holes, quark stars and strange stars. The collapse throws off the outer layers of the star in a Supernova Explosion. Stars larger than 40 solar masses with cores larger than about 2.5 solar masses are likely to become Black Holes instead of Neutron Stars. There are many different types of stars in the universe and it's no surprise that those with the most exotic sounding names receive the greatest levels of attention.[br][br]
[br] Third party image reference[br][br][br][br][br][br]source. : TecH[br] |
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发表于 2019-05-06 14:29:34
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