The BLUE SUN by MagikUnicorn

Bryce • Space • posted on Oct 09, 2014

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Description

Blue Giant Star In astronomy, a blue giant is a hot star with a luminosity class of III (giant) or II (bright giant). In the standard Hertzsprung–Russell diagram, these stars lie above and to the right of the main sequence. The term applies to a variety of stars in different phases of development, all evolved stars that have moved from the main sequence but have little else in common, so blue giant simply refers to stars in a particular region of the HR diagram rather than a specific type of star. They are much rarer than red giants, because they only develop from more massive and less common stars, and because they have short lives in the blue giant stage. The name blue giant is often misapplied to other high-mass luminous stars, such as main-sequence stars, simply because they are large and hot. Blue giant is not a strictly defined term and it is applied to a wide variety of different types of stars. What they have in common is: a moderate increase in size and luminosity compared to main sequence stars of the same mass or temperature, and are hot enough to be called blue, meaning spectral class O, B, and sometimes early A. They have temperatures from around 10,000 K upwards, ZAMS masses greater than about twice the Sun (M☉), and absolute magnitudes around 0 or brighter. These stars are only 5–10 times the radius of the Sun (R☉), compared to red giants which are up to 100 R☉. The coolest and least luminous stars referred to as blue giants are on the horizontal branch, intermediate mass stars that have passed through a red giant phase and are now burning helium in their cores. Depending on mass and chemical composition these stars gradually move bluewards until they exhaust the helium in their cores and then they return redwards to the asymptotic giant branch (AGB). The RR Lyrae variable stars, usually with spectral types of A, lie across the middle of the horizontal branch. Horizontal branch stars hotter than the RR Lyrae gap are generally considered to be blue giants, and sometimes the RR Lyrae stars themselves are called blue giants despite some of them being F class. The hottest stars, blue horizontal branch (BHB) stars, are called extreme horizontal branch (EHB) stars and can be hotter than main sequence stars of the same luminosity. In these cases they are called blue subdwarf (sdB) stars rather than blue giants, named for their position to the left of the main sequence on the HR diagram rather than for their increased luminosity and temperature compared to when they were themselves main sequence stars. There are no strict upper limits for giant stars, but early O types become increasingly difficult to classify separately from main sequence and supergiant stars, have almost identical sizes and temperatures to the main sequence stars from which they develop, and very short lifetimes. A good example is Plaskett's star, a close binary consisting of two O type giants both over 50 M☉, temperatures over 30,000 K, and more than 100,000 times the luminosity of the Sun (L☉). Astronomers still differ over whether to classify at least one of the stars as a supergiant, based on subtle differences in the spectral lines. BEAUTIFUL PICTURES OF GIANT BLUE STARS http://becuo.com/blue-giant-star

Comments (26)

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MagikUnicorn

9:08PM | Thu, 09 October 2014

Géante bleue Une géante bleue est une étoile très chaude, très brillante et très massive (plus de 18 masses solaires), de couleur bleue et de type spectral O ou B. Dans le diagramme de Hertzsprung-Russell, les géantes bleues se trouvent dans le coin supérieur gauche à cause de leur luminosité et de leur couleur. Ce sont, généralement des étoiles de la séquence principale qui brûlent leur hydrogène. Cependant leur masse est suffisante pour leur permettre d'entamer la combustion de l'hélium une fois celle de l'hydrogène achevée. Très rapidement, l'étoile passera à la combustion du carbone, puis à celle de l'oxygène. Elle aura alors constitué un noyau de fer, élément trop stable pour que sa combustion soit possible, quelle que soit la masse de l'étoile. L'équilibre maintenu dans l'étoile par les réactions thermonucléaires étant subitement rompu, elle explosera en une gigantesque supernova. Les géantes bleues sont extrêmement lumineuses, de magnitude absolue -5, -6 ou même moins. Leur température de 20 000 K ou plus est suffisamment élevée pour qu'une partie non négligeable de l'énergie qu'elles émettent soit dans le domaine de l'ultraviolet, donc invisible à nos yeux. La majorité des étoiles de ce type se trouvent dans les associations O-B, des grands groupes d'étoiles jeunes. Du fait de leur masse élevée, leur durée de vie est très courte, de l'ordre de 10 à 100 millions d'années. Les théories actuelles prédisent que la plupart d'entre elles finiront en supernova. Certaines des étoiles les plus brillantes visibles dans le ciel sont des géantes bleues ; par exemple Beta Orionis (Rigel) et Alpha Cygni (Deneb), mais dans l'ensemble, à cause de leur courte durée de vie, les géantes bleues sont assez rares.

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mikeerson

9:21PM | Thu, 09 October 2014

I'm watching Football, so I didn't have time to read your book - lol.... but, I saw your title... came and looked and I think this would be a better title: "Blue Moon with a Crack Down the Center" lol

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magnus073

9:35PM | Thu, 09 October 2014

Magik, c'est un presenation merveilleux de la syster solaire. J'ai bien aimé apprendre à propos de Blue Giants, et comment elles se développent. Votre galerie est une expérience de l'enseignement et de l'apprentissage.

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