Solar EM spectrum, why only upto UV?

According to this, Sunlight is composed only of the range of the
electromagnetic spectrum from IR to UV, with the majority occurring in
the visible spectrum.

Sunlight - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunlight#Composition

Question, if the Sun generates its photons inside its nuclear core, then
all of that light should be in the form of gamma rays. But by the time
it reaches the surface of the Sun, it doesn't go much over UV in energy.
So what mechanism is in place on the Sun that steps its photon energy
down from the gamma ray range to the lower frequencies?

I assume that there is also a multiplication effect, to conserve energy
balance, where they trade fewer gamma photons for a greater number of
lower energy photons?

Yousuf Khan

Yousuf Khan wrote:
According to this, Sunlight is composed only of the range of the
electromagnetic spectrum from IR to UV, with the majority occurring in
the visible spectrum.

Sunlight - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunlight#Composition

Question, if the Sun generates its photons inside its nuclear core, then
all of that light should be in the form of gamma rays. But by the time
it reaches the surface of the Sun, it doesn't go much over UV in energy.
So what mechanism is in place on the Sun that steps its photon energy
down from the gamma ray range to the lower frequencies?

I assume that there is also a multiplication effect, to conserve energy
balance, where they trade fewer gamma photons for a greater number of
lower energy photons?

The original photons from the core are not the ones that
comprise the spectrum that we see. Those photons are
absorbed and re-emitted over and over again by the material
of the Sun, and in so doing, transfer energy to the
material that shows up in the form of heat. What we see
as the Sun's spectrum is pretty close to that of a black
body radiator at 5,200 or so degrees.

On Oct 27, 4:12*pm, "Greg Neill" wrote:
Yousuf Khan wrote:
According to this, Sunlight is composed only of the range of the
electromagnetic spectrum from IR to UV, with the majority occurring in
the visible spectrum.

Sunlight - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunlight#Composition

Question, if the Sun generates its photons inside its nuclear core, then
all of that light should be in the form of gamma rays. But by the time
it reaches the surface of the Sun, it doesn't go much over UV in energy..
So what mechanism is in place on the Sun that steps its photon energy
down from the gamma ray range to the lower frequencies?

I assume that there is also a multiplication effect, to conserve energy
balance, where they trade fewer gamma photons for a greater number of
lower energy photons?

The original photons from the core are not the ones that
comprise the spectrum that we see. *Those photons are
absorbed and re-emitted over and over again by the material
of the Sun, and in so doing, transfer energy to the
material that shows up in the form of heat. *What we see
as the Sun's spectrum is pretty close to that of a black
body radiator at 5,200 or so degrees.- Hide quoted text -

- Show quoted text -

Photons take 200,000 years from core to Sun's surface. White light is
a mixture of photon wave lengths. We have discussed this over and
over TreBert

On 27/10/2010 4:12 PM, Greg Neill wrote:
The original photons from the core are not the ones that
comprise the spectrum that we see. Those photons are
absorbed and re-emitted over and over again by the material
of the Sun, and in so doing, transfer energy to the
material that shows up in the form of heat. What we see
as the Sun's spectrum is pretty close to that of a black
body radiator at 5,200 or so degrees.

Yes, I understood that they aren't the original photons of the fusion
process. However, in my mind, a gamma ray that is absorbed and
re-emitted should be re-emitted in the form of a gamma ray again, i.e.
it should stay the same frequency. Something must be absorbing it, and
then only partially re-emitting it at a lower energy level.

For example, if an electron absorbs a gamma ray, it gets boosted to a
higher orbital. If that electron then drops back down partially, it'll
emit a photon of a lower energy and frequency, and then perhaps later
it'll drop down some more, and emit another lower energy photon. Is that
what's happening?

Yousuf Khan

Dear Yousuf Khan:

On Oct 27, 3:14*pm, Yousuf Khan wrote:
On 27/10/2010 4:12 PM, Greg Neill wrote:

The original photons from the core are not the ones that
comprise the spectrum that we see. *Those photons are
absorbed and re-emitted over and over again by the material
of the Sun, and in so doing, transfer energy to the
material that shows up in the form of heat. *What we see
as the Sun's spectrum is pretty close to that of a black
body radiator at 5,200 or so degrees.

Yes, I understood that they aren't the original photons of
the fusion process. However, in my mind, a gamma ray
that is absorbed and re-emitted should be re-emitted in the
form of a gamma ray again, i.e. it should stay the same
frequency.

No way. If for no other reason, conservation of momentum for the
"photon plus charge" before has to be equal to the momentum of the
"photon plus charge" after. The electron or nucleon will be
*booking*, and the resulting gamma photon will be net less energetic
and scattered most likely in another direction.

Something must be absorbing it, and then only partially
re-emitting it at a lower energy level.

Correct for *all* cases.

For example, if an electron absorbs a gamma ray, it gets
boosted to a higher orbital.

Gammas completely remove an electron, or if energetic enough, a
nucleon from the system.

If that electron then drops back down partially, it'll emit a
photon of a lower energy and frequency, and then perhaps
later it'll drop down some more, and emit another lower
energy photon. Is that what's happening?

For x-rays or less energetic, yes. But in all cases involving
"hosts", entropy must be increased.

Now to your original question, "sunlight" is filtered by the
atmosphere, and UV-C and more energetic radiation is completely
blocked by nitrogen, oxygen, and ozone pitches in as well (but most
important for absorbing in UV-B).

Lest ye think the Sun does not emit X-rays and such...
http://www.asr.ucar.edu/2004/HAO/tiso.html
.... it does. Just not much.

David A. Smith

In sci.physics Yousuf Khan wrote:
According to this, Sunlight is composed only of the range of the
electromagnetic spectrum from IR to UV, with the majority occurring in
the visible spectrum.

Sunlight - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunlight#Composition

Question, if the Sun generates its photons inside its nuclear core, then
all of that light should be in the form of gamma rays. But by the time
it reaches the surface of the Sun, it doesn't go much over UV in energy.
So what mechanism is in place on the Sun that steps its photon energy
down from the gamma ray range to the lower frequencies?

I assume that there is also a multiplication effect, to conserve energy
balance, where they trade fewer gamma photons for a greater number of
lower energy photons?

Yousuf Khan

The Sun emits energy from low radio frequencies to gamma rays with most
of the energy centered at about 500 nm, not just between IR and UV.

http://www.eoearth.org/article/Solar_radiation

Is your question why is most of the energy in the visible region?


--
Jim Pennino

Remove .spam.sux to reply.

On Wed, 27 Oct 2010, Yousuf Khan wrote:

Yes, I understood that they aren't the original photons of the fusion
process. However, in my mind, a gamma ray that is absorbed and
re-emitted should be re-emitted in the form of a gamma ray again, i.e.
it should stay the same frequency. Something must be absorbing it, and
then only partially re-emitting it at a lower energy level.

For example, if an electron absorbs a gamma ray, it gets boosted to a
higher orbital. If that electron then drops back down partially, it'll
emit a photon of a lower energy and frequency, and then perhaps later
it'll drop down some more, and emit another lower energy photon. Is that
what's happening?

The atoms and electrons are interacting with each other, too. Collisions.
These collisions redistribute energy. Excited atom emits photon, ends up
in ground state. Re-excited by collision with other atom, no absorption of
photon required. Absorb a photon, leading to being in an excited state,
transfer that energy, or some of it, to another atom, by collision rather
than radiative emission.

Also lots of the absorption/emission involves bound-free, free-bound, or
free-free processes.

At a sufficient depth, the collisions are frequent enough so that the gas
and the radiation are all in thermal equilibrium, so the spectrum is
blackbody, and the energy distribution of atomic states is Boltzmann,
speeds Maxwellian. The opacity is such that radiation can't travel far
before being absorbed, re-emitted, etc., with collisional redistribution
of energy in there too.

So what you see is close to a blackbody spectrum of the highest layer in
the sun that is in thermal equilibrium.

Above that, the density drops quickly, collision rates drops, and the
radiation field is no longer in thermal equilibrium with the gas. If
collisions are still frequent enough, you still have Boltzmann
distribution of atomic energy states etc (which is LTE, Local
Thermodynamic Equilibrium). Higher still, collisions are frequent enough,
and you don't even have LTE, and temperature isn't well-defined (doesn't
stop people from saying things like "the temperature of the corona is
XXXX").

--
Timo

On 10/27/10 5:14 PM, Yousuf Khan wrote:
However, in my mind, a gamma ray that is absorbed and re-emitted should
be re-emitted in the form of a gamma ray again.

Unless it gives up a bit of kinetic energy to the atom. Where did
you think that core temperature came from?

Yousuf Khan wrote:
On 27/10/2010 4:12 PM, Greg Neill wrote:
The original photons from the core are not the ones that
comprise the spectrum that we see. Those photons are
absorbed and re-emitted over and over again by the material
of the Sun, and in so doing, transfer energy to the
material that shows up in the form of heat. What we see
as the Sun's spectrum is pretty close to that of a black
body radiator at 5,200 or so degrees.

Yes, I understood that they aren't the original photons of the fusion
process. However, in my mind, a gamma ray that is absorbed and
re-emitted should be re-emitted in the form of a gamma ray again, i.e.
it should stay the same frequency. Something must be absorbing it, and
then only partially re-emitting it at a lower energy level.

Think of it this way: As the energy of the photons slowly makes its way
out, at each radius the area of the sphere inside the sun is larger, but the
total flux is the same. Hence, to conserve flux, the average energy of the
photons emitted and reabsorbed must be lower as you move outwards. The
gamma rays are absorbed thermally (they boost the speed of the particles,
etc), so the re-emitted photons are unrelated to the ones absorbed (except
they have the same energy, minus epsilon). They are not the same photons.
If they were, you would have scattering, not absorption.


For example, if an electron absorbs a gamma ray, it gets boosted to a
higher orbital. If that electron then drops back down partially, it'll
emit a photon of a lower energy and frequency, and then perhaps later
it'll drop down some more, and emit another lower energy photon. Is
that what's happening?

No, because in the central regions you have full ionization, even for heavy
elements, and certainly for hydrogen, the main source of opacity. Hence
there are no bound quantum states in the hot plasma, only continuum. Near
the surface that is no longer true and it gets interesting.


Yousuf Khan

--
Mike Dworetsky

(Remove pants sp*mbl*ck to reply)

Yousuf Khan ) writes:
According to this, Sunlight is composed only of the range of the
electromagnetic spectrum from IR to UV, with the majority occurring in
the visible spectrum.

Sunlight - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Sunlight#Composition

Question, if the Sun generates its photons inside its nuclear core, then
all of that light should be in the form of gamma rays. But by the time
it reaches the surface of the Sun, it doesn't go much over UV in energy.
So what mechanism is in place on the Sun that steps its photon energy
down from the gamma ray range to the lower frequencies?

You've had a number of good answers. A slightly different way to look at
it is to remember that to a gamma ray, pretty much every electron is a free
electron (since gamma energies are so much higher than light-atom binding
energies). So a gamma making its way though the sun will undergo a lot of
Compton scattering, almost all of it reducing the photon energy.

--John Park