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    q1 : in the spectral classes from O to M says that as the size decreases so does the luminosity and temperature (or so it says in my book) does this mean that white dwarfs and red giants are exceptions?

    q2 : what is a blue super-giant star is it like a main sequence star or what?
    if not then do massive stars ever pass through main sequence? & what do the blue super-giant fuse?

    please help me and thanks so much
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    Yes white dwarfs and red giants are exceptions.

    When the hydrogen is used up and starts fusing helium the star will expand and have a larger surface area, so a red giant is cooler but highly luminous. (As the star expands, the outer surface of the star will cool)

    A white dwarf is extremely hot but has a small core and so is not very luminous because they no longer generate energy by nuclear fusion and because its small.

    I'm not sure what a blue supergiant is but a supergiant is a stage in the life cycle of a star with a core mass >1.4 solar mass (a high-mass star). It's after the main sequence in a high-mass star, just like the red giant stage in a low-mass star. If I were to guess then perhaps since its a "blue" supergiant then maybe it means its a star that belongs to the class either O or B? Don't take my word for that though.

    Hope that helps.
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    (Original post by HappyHatty)
    Yes white dwarfs and red giants are exceptions.

    If I were to guess then perhaps since its a "blue" supergiant then maybe it means its a star that belongs to the class either O or B? Don't take my word for that though.

    Hope that helps.
    It's actually class OB so good guess! Rigel (Orion) is the classic example.

    They inhabit the top left part of the Hertzsprung-Russell diagram but smaller than a Red-Supergiant.

    They have high mass and relatively short life-spans, are larger and brighter than main-sequence stars with and a mass of between 10-100 solar masses.

    Stars above 40M cannot expand into a red-supergiant as they both lose their outer layers and burn too quickly so they go directly to blue-supergiant stage.

    Lower mass blue-supergiants expand until they become small red-supergiants. On-route these stars spend time as a yellow supergiant and yellow-hypergiant.

    However, higher mass red-supergiants can blow off their outer atmosphere and become blue-supergiants for periods during successive pulses of their final stage evolution. Depending on the mass of the red-supergiant, the pulses of blue-supergiants stage occurs during the successive fusing of heavier elements before ending up as either a type II supernovae or finally losing enough mass to collapse to a white dwarf.
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    (Original post by HappyHatty)
    Yes white dwarfs and red giants are exceptions.

    When the hydrogen is used up and starts fusing helium the star will expand and have a larger surface area, so a red giant is cooler but highly luminous. (As the star expands, the outer surface of the star will cool)

    A white dwarf is extremely hot but has a small core and so is not very luminous because they no longer generate energy by nuclear fusion and because its small.

    I'm not sure what a blue supergiant is but a supergiant is a stage in the life cycle of a star with a core mass >1.4 solar mass (a high-mass star). It's after the main sequence in a high-mass star, just like the red giant stage in a low-mass star. If I were to guess then perhaps since its a "blue" supergiant then maybe it means its a star that belongs to the class either O or B? Don't take my word for that though.

    Hope that helps.
    thanks for the help
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    (Original post by uberteknik)
    It's actually class OB so good guess! Rigel (Orion) is the classic example.

    They inhabit the top left part of the Hertzsprung-Russell diagram but smaller than a Red-Supergiant.

    They have high mass and relatively short life-spans, are larger and brighter than main-sequence stars with and a mass of between 10-100 solar masses.

    Stars above 40M cannot expand into a red-supergiant as they both lose their outer layers and burn too quickly so they go directly to blue-supergiant stage.

    Lower mass blue-supergiants expand until they become small red-supergiants. On-route these stars spend time as a yellow supergiant and yellow-hypergiant.

    However, higher mass red-supergiants can blow off their outer atmosphere and become blue-supergiants for periods during successive pulses of their final stage evolution. Depending on the mass of the red-supergiant, the pulses of blue-supergiants stage occurs during the successive fusing of heavier elements before ending up as either a type II supernovae or finally losing enough mass to collapse to a white dwarf.
    so to get this straight blue super-giants are not main sequence stars ?
    if yes ? so the star becomes first a main sequence then blue super-giant then red super-giant? & what do they fuse? sorry for the many questions but this part is poorly explained in my textbooks and thanks
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    (Original post by >>MMM<<)
    so to get this straight blue super-giants are not main sequence stars ?
    if yes ? so the star becomes first a main sequence then blue super-giant then red super-giant? & what do they fuse? sorry for the many questions but this part is poorly explained in my textbooks and thanks
    The term 'main-sequence' is not an absolute 'temporal' interpretation of a typical star. i.e. they do not all start in the top left and work their way down the main-sequence line to the bottom right of the H-R diagram as an evolution.

    The H-R diagram is a distribution plot - a snapshot of observation of all stars taken in an almost infinitely small scale of time compared with the life time of a star. It simply plots the distribution of stars based on what is observed of their brightness and surface temperature.

    (Think of it like taking a snapshot of all the aircraft in the world. Most would be either on the ground clustered at airports or in the air.)

    However, there is a strong correlation between the mass of the star and it's position on the H-R diagram. It's also the mass of a star which is the greatest factor in determining the actual temporal evolution of that star. There is also a correlation between the life-span of a star and it's position on the H-R diagram. It's the mixing/transposing of these facts where many people confuse the interpretation of the H-R diagram.

    Blue stars (progenitors of Blue-Supergiants) are only considered very rare main sequence. They are hot, bright and short lived. i.e. the population density of these stars does not occupy the dense population defining main-sequence other than at the limits of the distribution pattern. Although the dividing line on the Hertspung -Russell diagram looks small, they are actually well differentiated because the luminosity scale is logarithmic.

    This is what wiki has to say:

    " O type main sequence stars and the most massive of the B type blue-white stars become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years.[3] They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars.Supergiants develop when massive main sequence stars run out of hydrogen in their cores. They then start to expand, just like lower mass stars, but unlike lower mass stars, they begin to fuse helium in the core almost immediately. This means that they do not increase their luminosity as dramatically as lower mass stars and they progress nearly horizontally across the HR diagram to become red supergiants. Also unlike lower mass stars, red supergiants are massive enough to fuse elements heavier than helium so they do not puff off their atmospheres as planetary nebulae when their helium becomes depleted. Furthermore, they cannot lose enough mass to form a white dwarf, so will leave behind a neutron stars or black hole remnant, usually after a core collapse supernova explosion.
    Stars more massive than about 40M cannot expand into a red supergiant. They burn too quickly and lose their outer layers too quickly, so they reach the blue supergiant stage, or perhaps yellow hypergiant, and then return to become hotter stars. The most massive stars, above about 100M, hardly move at all from their position as O main sequence stars. These stars convect so efficiently that they mix hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is almost entirely depleted throughout the star, then very rapidly evolve through a series of stages of very similar hot and luminous stars, If supergiants, slash stars, WNh stars, WN stars, and possibly WC or WO stars. They are expected to explode as supernovae but it is not clear how far they evolve before this happens. The existence of these supergiants still burning hydrogen in their cores may necessitate a slightly more complex definition of supergiant: a massive star with increased size and luminosity due to fusion products building up, but still with some hydrogen remaining."
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    (Original post by >>MMM<<)
    so to get this straight blue super-giants are not main sequence stars ?
    if yes ? so the star becomes first a main sequence then blue super-giant then red super-giant? & what do they fuse? sorry for the many questions but this part is poorly explained in my textbooks and thanks
    anything with giant in it's name is off the main sequence by definition. stars of different masses have different fates

    btw I was just in the discount book chainstore 'the works' looking for a pressie and I saw they had some 'collins dictionary of astronomy' in for £1.99 :yep:
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    (Original post by uberteknik)
    The term 'main-sequence' is not an absolute 'temporal' interpretation of a typical star. i.e. they do not all start in the top left and work their way down the main-sequence line to the bottom right of the H-R diagram as an evolution.

    The H-R diagram is a distribution plot - a snapshot of observation of all stars taken in an almost infinitely small scale of time compared with the life time of a star. It simply plots the distribution of stars based on what is observed of their brightness and surface temperature.

    (Think of it like taking a snapshot of all the aircraft in the world. Most would be either on the ground clustered at airports or in the air.)

    However, there is a strong correlation between the mass of the star and it's position on the H-R diagram. It's also the mass of a star which is the greatest factor in determining the actual temporal evolution of that star. There is also a correlation between the life-span of a star and it's position on the H-R diagram. It's the mixing/transposing of these facts where many people confuse the interpretation of the H-R diagram.

    Blue stars (progenitors of Blue-Supergiants) are only considered very rare main sequence. They are hot, bright and short lived. i.e. the population density of these stars does not occupy the dense population defining main-sequence other than at the limits of the distribution pattern. Although the dividing line on the Hertspung -Russell diagram looks small, they are actually well differentiated because the luminosity scale is logarithmic.

    This is what wiki has to say:

    " O type main sequence stars and the most massive of the B type blue-white stars become supergiants. Because of their extreme masses they have short lifespans of 30 million years down to a few hundred thousand years.[3] They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges, and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars.Supergiants develop when massive main sequence stars run out of hydrogen in their cores. They then start to expand, just like lower mass stars, but unlike lower mass stars, they begin to fuse helium in the core almost immediately. This means that they do not increase their luminosity as dramatically as lower mass stars and they progress nearly horizontally across the HR diagram to become red supergiants. Also unlike lower mass stars, red supergiants are massive enough to fuse elements heavier than helium so they do not puff off their atmospheres as planetary nebulae when their helium becomes depleted. Furthermore, they cannot lose enough mass to form a white dwarf, so will leave behind a neutron stars or black hole remnant, usually after a core collapse supernova explosion.
    Stars more massive than about 40M cannot expand into a red supergiant. They burn too quickly and lose their outer layers too quickly, so they reach the blue supergiant stage, or perhaps yellow hypergiant, and then return to become hotter stars. The most massive stars, above about 100M, hardly move at all from their position as O main sequence stars. These stars convect so efficiently that they mix hydrogen from the surface right down to the core. They continue to fuse hydrogen until it is almost entirely depleted throughout the star, then very rapidly evolve through a series of stages of very similar hot and luminous stars, If supergiants, slash stars, WNh stars, WN stars, and possibly WC or WO stars. They are expected to explode as supernovae but it is not clear how far they evolve before this happens. The existence of these supergiants still burning hydrogen in their cores may necessitate a slightly more complex definition of supergiant: a massive star with increased size and luminosity due to fusion products building up, but still with some hydrogen remaining."
    So the blue super-giants are not main sequence stars but they used to be earlier as blue stars and I'm going to assume they fuse helium elements and above? Plus do they become red giants? Sorry if you might have explained the red giant part but it was a bit above my level to understand (I barely know anything about astronomy ) And thank you sooo much for the help
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    (Original post by >>MMM<<)
    So the blue super-giants are not main sequence stars but they used to be earlier as blue stars and I'm going to assume they fuse helium elements and above? Plus do they become red giants? Sorry if you might have explained the red giant part but it was a bit above my level to understand (I barely know anything about astronomy ) And thank you sooo much for the help
    Yes, as Joinedup said, by definition a supergiant cannot be main sequence. The progenitors of supergiants are the more massive blue stars and because of the mass, they are so hot that they burn helium.

    Whether a blue-supergiant evolves to a red-supergiant or ends life as a luminous blue-variable, is dependent on the progenitor main-sequence mass.

    A picture is worth a thousand words, I found this which is a good explanation of evolutionary tracks on the H-R diagram.

    Name:  690px-Stellar_evolutionary_tracks-en.svg.png
Views: 1413
Size:  101.9 KB


    Note the mass of stars on the main-sequence.

    The collective noun describing any given star on the H-R plot is a general case. The definition for the star becomes blurred for those inhabiting the extremes of the H-R plot density distribution. Hence the demarcation for which evolution a star will take is not clearly defined for the top left part of the H-R diagram, but it is mostly correlated to the young stars mass.
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    (Original post by uberteknik)
    Yes, as Joinedup said, by definition a supergiant cannot be main sequence. The progenitors of supergiants are the more massive blue stars and because of the mass, they are so hot that they burn helium.

    Whether a blue-supergiant evolves to a red-supergiant or ends life as a luminous blue-variable, is dependent on the progenitor main-sequence mass.

    A picture is worth a thousand words, I found this which is a good explanation of evolutionary tracks on the H-R diagram.

    Name:  690px-Stellar_evolutionary_tracks-en.svg.png
Views: 1413
Size:  101.9 KB


    Note the mass of stars on the main-sequence.

    The collective noun describing any given star on the H-R plot is a general case. The definition for the star becomes blurred for those inhabiting the extremes of the H-R plot density distribution. Hence the demarcation for which evolution a star will take is not clearly defined for the top left part of the H-R diagram, but it is mostly correlated to the young stars mass.
    Ok so they fuse helium and above right? Plus another question , if not too much since i appreciate your answers. To say like if the blue stars become blue supergiants then red supergiants. Is this because when hydrogen is fully is burned in blue stars and helium fusion starts and the star expands to become blue supergiant and then finally red
    supergiants?(i.e. when expansion starts due to helium fusion blue stars transform to blue supergiants and then when expansion continue and then is large enough the temperature distributed of large surface area makes it transform from blue supergiant to red supergiant)
    Sorry for the long message and again thanks ALOT
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    (Original post by uberteknik)
    Yes, as Joinedup said, by definition a supergiant cannot be main sequence. The progenitors of supergiants are the more massive blue stars and because of the mass, they are so hot that they burn helium.

    Whether a blue-supergiant evolves to a red-supergiant or ends life as a luminous blue-variable, is dependent on the progenitor main-sequence mass.

    A picture is worth a thousand words, I found this which is a good explanation of evolutionary tracks on the H-R diagram.

    Name:  690px-Stellar_evolutionary_tracks-en.svg.png
Views: 1413
Size:  101.9 KB


    Note the mass of stars on the main-sequence.

    The collective noun describing any given star on the H-R plot is a general case. The definition for the star becomes blurred for those inhabiting the extremes of the H-R plot density distribution. Hence the demarcation for which evolution a star will take is not clearly defined for the top left part of the H-R diagram, but it is mostly correlated to the young stars mass.
    I mean like this kind of life cycle when the blue super-giant becomes a red super-giant (4th and 5th stage) --------> http://linus.highpoint.edu/~mdewitt/...mass-cycle.jpg
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    (Original post by uberteknik)
    Yes, as Joinedup said, by definition a supergiant cannot be main sequence. The progenitors of supergiants are the more massive blue stars and because of the mass, they are so hot that they burn helium.

    Whether a blue-supergiant evolves to a red-supergiant or ends life as a luminous blue-variable, is dependent on the progenitor main-sequence mass.

    A picture is worth a thousand words, I found this which is a good explanation of evolutionary tracks on the H-R diagram.

    Name:  690px-Stellar_evolutionary_tracks-en.svg.png
Views: 1413
Size:  101.9 KB


    Note the mass of stars on the main-sequence.

    The collective noun describing any given star on the H-R plot is a general case. The definition for the star becomes blurred for those inhabiting the extremes of the H-R plot density distribution. Hence the demarcation for which evolution a star will take is not clearly defined for the top left part of the H-R diagram, but it is mostly correlated to the young stars mass.
    I like the diagram - where did you find it?
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    The diagram came from the Harvard University website but I don't have the original address. I've had it on my PC as a useful resource for a while.

    >>MMM<<, you may like to look at the NASA Chandra X-Ray Observatory education pages. Here they give a truly excellent introduction to stellar-evolution along with fairly detailed descriptions. This link contains some 13 pages on stellar evolution and well worth reading from the beginning.
    It also gives an insight as to the state of knowledge on stellar-evolution. For instance, the ultimate fate of Luminous Blue Variables is unknown at this time, that kind of thing.

    The address is: http://chandra.harvard.edu/edu/forma...ry/index3.html

    Hope this helps.
 
 
 
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