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Why is Sirius B hotter than Sirius A?



 
 
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  #1  
Old April 14th 05, 01:17 PM
me
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Default Why is Sirius B hotter than Sirius A?

Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000 degrees
C. Sirius A, a blue/white main sequence star has a surface temperature of
around 12,000 degrees C. Usually the bluer a star is, the hotter it is. Just
wondered why it's not the case here.


  #2  
Old April 14th 05, 10:39 PM
Llanzlan Klazmon
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"me" wrote in
. uk:

Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000
degrees C. Sirius A, a blue/white main sequence star has a surface
temperature of around 12,000 degrees C. Usually the bluer a star is,
the hotter it is. Just wondered why it's not the case here.




The term white dwarf is just a name to distinguish from red dwarf. The
spectra of Sirius B versus A is just as expected by the surface
temperature difference. Note that white dwarf's are slowly cooling down
as they have no internal source of energy production.

K.
  #3  
Old April 15th 05, 03:59 AM
J. Scott Miller
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me wrote:
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000 degrees
C. Sirius A, a blue/white main sequence star has a surface temperature of
around 12,000 degrees C. Usually the bluer a star is, the hotter it is. Just
wondered why it's not the case here.



Essentially, you have answered your own question, assuming you know what a white
dwarf is supposed to be. A white dwarf is supposed to be the core of a now
dead star, a low mass type like our Sun. As the core of a star, they simply
retain the heat from those nuclear fusion processes that were proceeding prior
to the end of the star's life. Now, heat stored within them, they simply
radiate that heat, lowering their temperature eventually to absolute zero over
10s and possibly 100s of billions of years.

And, since they are not generating their own energy but simply radiating stored
energy, comparisons of their color to that of stars is comparing apples to oranges.
  #4  
Old April 15th 05, 02:58 PM
Peter Webb
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"J. Scott Miller" wrote in message
...
me wrote:
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000
degrees C. Sirius A, a blue/white main sequence star has a surface
temperature of around 12,000 degrees C. Usually the bluer a star is, the
hotter it is. Just wondered why it's not the case here.


Essentially, you have answered your own question, assuming you know what a
white dwarf is supposed to be. A white dwarf is supposed to be the core
of a now dead star, a low mass type like our Sun. As the core of a star,
they simply retain the heat from those nuclear fusion processes that were
proceeding prior to the end of the star's life. Now, heat stored within
them, they simply radiate that heat, lowering their temperature eventually
to absolute zero over 10s and possibly 100s of billions of years.

And, since they are not generating their own energy but simply radiating
stored energy, comparisons of their color to that of stars is comparing
apples to oranges.


So what, they don't emit standard black-body radiation?



  #5  
Old April 15th 05, 06:33 PM
Alfred A. Aburto Jr.
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me wrote:
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000 degrees
C. Sirius A, a blue/white main sequence star has a surface temperature of
around 12,000 degrees C. Usually the bluer a star is, the hotter it is. Just
wondered why it's not the case here.


A and B type stars (in the Hertzprung-Russell diagram), and white dwarf
stars, have a white to a blue-white color appearance. I think this is
because the Planck radiation curve peak is broad at those temperatures
and it causes a mixing of colors that we see as looking like white or
blue-white ... this is my opinion ... I've looked for a definitive
reference but have not found it yet ...
Al


  #6  
Old April 16th 05, 12:54 AM
OG
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"me" wrote in message
. uk...
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000

degrees
C. Sirius A, a blue/white main sequence star has a surface temperature

of
around 12,000 degrees C. Usually the bluer a star is, the hotter it

is. Just
wondered why it's not the case here.


Why do you say Sirius B is less blue than Sirius A?


  #7  
Old April 16th 05, 03:46 PM
Alfred A. Aburto Jr.
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Default

me wrote:
Hi

Sirius B, a white dwarf, has a surface temperature of around 25,000 degrees
C. Sirius A, a blue/white main sequence star has a surface temperature of
around 12,000 degrees C. Usually the bluer a star is, the hotter it is. Just
wondered why it's not the case here.


A very nice web site where you can study the color of stars is the Sloan
Digital Sky Survey site: http://cas.sdss.org/dr3/en/proj/advanced/color/
  #8  
Old April 18th 05, 02:46 AM
J. Scott Miller
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Peter Webb wrote:


So what, they don't emit standard black-body radiation?


More likely a better representation of a blackbody curve than that of a star, as
a star's spectrum is littered with spectral lines.

But to see it appear bluish white it must also release enough light from its
surface to trigger the color receptors in our eyes, and they can't do that
because of their small surface area.
 




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