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  #1  
Old December 3rd 16, 09:46 AM
David Levy David Levy is offline
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Posts: 23
Default Black Body

Black Body
https://en.wikipedia.org/wiki/Black_body
"An ideal body is now defined, called a blackbody. A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body). This is true for radiation of all wavelengths and for all angles of incidence. Hence the blackbody is a perfect absorber for all incident radiation.[10]"

A star or planet often is modeled as a black body, and electromagnetic radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a black body, even if it is a simple gas star? In other words, is there any limit in its size or its radiation/energy density in order to be considered as black body?
  #2  
Old December 3rd 16, 04:40 PM posted to sci.astro
Poutnik[_5_]
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Default Black Body

Dne 03/12/2016 v 09:46 David Levy napsal(a):
[....]

A star or planet often is modeled as a black body, and electromagnetic
radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a black
body, even if it is a simple gas star? In other words, is there any
limit in its size or its radiation/energy density in order to be
considered as black body?

An object can be considered as a black body (BB),
if it has for the wavelength a/o frequency range of interest
the negligible reflectance.

E.g. the Earth can be considered as approximately BB
for the thermal IN range, but not for the visible range.

The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
  #3  
Old December 3rd 16, 05:17 PM posted to sci.astro
Poutnik[_5_]
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Posts: 68
Default Black Body

Dne 03/12/2016 v 09:46 David Levy napsal(a):


My question is as follow:
Can we assume that any star, planet, moon would be considered as a black
body, even if it is a simple gas star?


What do you mean by a simple gas star ?
It is either gas, either star.
Plasma is not gas, and has high rate of absoption.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
  #4  
Old December 3rd 16, 05:48 PM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default Black Body

David Levy wrote:
Black Body
https://en.wikipedia.org/wiki/Black_body
"An ideal body is now defined, called a blackbody. A blackbody allows
all incident radiation to pass into it (no reflected energy) and
internally absorbs all the incident radiation (no energy transmitted
through the body). This is true for radiation of all wavelengths and
for all angles of incidence. Hence the blackbody is a perfect
absorber for all incident radiation.[10]"

A star or planet often is modeled as a black body, and electromagnetic
radiation emitted from these bodies as black-body radiation.

My question is as follow:
Can we assume that any star, planet, moon would be considered as a
black body, even if it is a simple gas star? In other words, is there
any limit in its size or its radiation/energy density in order to be
considered as black body?


A star should not be considered a black body because the light emerging is
passing through an atmosphere with lots of absorption lines and variations
of opacity with wavelength. The nearest thing to a BB for a star would be
one that has a "grey atmosphere" with constant fractional absorption
coefficient independent of wavelength. That would not be quite like a BB as
the spectrum would be less peaked at maximum wavelength. The closest thing
to this is probably a very hot star where electron scattering dominates the
opacity.

A planet or moon or asteroid is a reasonable approximation to a BB if it
does not have an atmosphere, and if it does, the overall emission of energy
will balance the absorption of energy if you take into account all the
sources of opacity and reflection (albedo) at different wavelengths.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

  #5  
Old December 3rd 16, 06:15 PM posted to sci.astro
Poutnik[_5_]
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Posts: 68
Default Black Body

Dne 03/12/2016 v 17:48 Mike Dworetsky napsal(a):


A planet or moon or asteroid is a reasonable approximation to a BB if it
does not have an atmosphere, and if it does, the overall emission of
energy will balance the absorption of energy if you take into account
all the sources of opacity and reflection (albedo) at different
wavelengths.


Hm, this is refuted by huan eyes look.
Earth in visible region is far from black body behavior.

The very most surfaces are far from be anywhere near the black.

In the thermal IR region near 10 um
Earth is much closer to BB, especially oceans.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
  #6  
Old December 4th 16, 12:18 PM posted to sci.astro
Mike Dworetsky
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Posts: 715
Default Black Body

Poutnik wrote:
Dne 03/12/2016 v 17:48 Mike Dworetsky napsal(a):


A planet or moon or asteroid is a reasonable approximation to a BB
if it does not have an atmosphere, and if it does, the overall
emission of energy will balance the absorption of energy if you take
into account all the sources of opacity and reflection (albedo) at
different wavelengths.


Hm, this is refuted by huan eyes look.
Earth in visible region is far from black body behavior.


True. I said "reasonable approximation". Human eyes are unable to detect
IR radiation.


The very most surfaces are far from be anywhere near the black.


Surfaces are close to BB in terms of absorption and radiation. But the
atmosphere is not.


In the thermal IR region near 10 um
Earth is much closer to BB, especially oceans.


I think that is what I said, more or less. But the atmosphere tends to
block and reflect some of that IR radiation.

Contrast Venus, which has a very thick and opaque atmosphere. The IR
radiation comes from the upper atmosphere, but the surface is much hotter.
When Venus is studied at radio wavelengths, it looks hotter because the
waves penetrate the clouds and we observe the hot surface directly.

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

  #7  
Old December 4th 16, 01:30 PM
David Levy David Levy is offline
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First recorded activity by SpaceBanter: May 2012
Posts: 23
Default

Quote:
Originally Posted by Poutnik[_5_] View Post
The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.
Please look at the following diagram:
https://en.wikipedia.org/wiki/Black_...ealization.png
It is stated:
"An approximate realization of a black body as a tiny hole in an insulated enclosure"

Why do we need this tiny hole/cavity?
Is it needed just to place the first radiation at that insulated enclosure?
Let's assume that the insulated enclosure is isolated by 100%.
Let's also assume that we have the ability to close that tiny hole at the moment that the first radiation gets in.
So, theoretically, the energy of the radiation will bump the walls forever and at the same amplitude.
Now, if after some time we will measure that radiation.
Would it have a black body signature?
  #8  
Old December 4th 16, 10:14 PM posted to sci.astro
Poutnik[_5_]
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Posts: 68
Default Black Body

Dne 04/12/2016 v 13:30 David Levy napsal(a):
'Poutnik[_5_ Wrote:
;1328995']
The closest approximation to the BB is a hole into a cavity.
Reflection or transmission of radiation coming into the cavity
through the smal enough hole is not measurable.


Please look at the following diagram:
http://tinyurl.com/lfnwltj
It is stated:
"An approximate realization of a black body as a tiny hole in an
insulated enclosure"

Why do we need this tiny hole/cavity?
Is it needed just to place the first radiation at that insulated
enclosure?
Let's assume that the insulated enclosure is isolated by 100%.
Let's also assume that we have the ability to close that tiny hole at
the moment that the first radiation gets in.
So, theoretically, the energy of the radiation will bump the walls
forever and at the same amplitude.
Now, if after some time we will measure that radiation.
Would it have a black body signature?

The idea is easy, incoming radiation gets absorbed
by repeated non ideal absorption. That object behaving like this
actsas BB, followingn the thermodynamic laws.

As if not, there would not be possible the radiative equilibrium.

The is no sense ion closing it.
Neither the incoming rafiation would bounce forever,
but would be absorbed quickly.

The BB thermal radiation within the cavity is another topic.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
  #9  
Old December 4th 16, 10:20 PM posted to sci.astro
Poutnik[_5_]
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Posts: 68
Default Black Body

Dne 04/12/2016 v 12:18 Mike Dworetsky napsal(a):
Poutnik wrote:

Hm, this is refuted by huan eyes look.
Earth in visible region is far from black body behavior.


True. I said "reasonable approximation". Human eyes are unable to
detect IR radiation.


But that has nothing to do with if wtwasfetect it or not.
For visible range, BB is not reasonable approximation.


The very most surfaces are far from be anywhere near the black.


Surfaces are close to BB in terms of absorption and radiation. But the
atmosphere is not.


Surfaces are close to BB in terms of absorption and radiation
for thermal IR.
Athmosphere is close to BB in its IR absoption window.


.......... But the atmosphere tends to
block and reflect some of that IR radiation.


And emit like BB with T lower than surface.


Contrast Venus, which has a very thick and opaque atmosphere. The IR
radiation comes from the upper atmosphere, but the surface is much
hotter. When Venus is studied at radio wavelengths, it looks hotter
because the waves penetrate the clouds and we observe the hot surface
directly.

As above, for absorbing WL the atmosphere acts like BB.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
  #10  
Old December 4th 16, 10:32 PM posted to sci.astro
Poutnik[_5_]
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Posts: 68
Default Black Body

Dne 04/12/2016 v 22:20 Poutnik napsal(a):
Dne 04/12/2016 v 12:18 Mike Dworetsky napsal(a):
True. I said "reasonable approximation". Human eyes are unable to
detect IR radiation.


But that has nothing to do with if wtwasfetect it or not.
For visible range, BB is not reasonable approximation.


Errata .. if it was detected by eyes or not.

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.
 




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