Uncovering the Mystery: The Science Behind White Light and the Sun's Spectrum

[Cantstandit]

Okay, it may be a dumb question, I am probably missing something obvious... Let's consider the Sun. There is only, say, hydrogen and helium. Both elements emit light of only certain wavelength, so where would the rest of the spectrum come from?
Or maybe those wavelengths are restricted only to electron changing "orbits", and the fusion of nucleus emits light in the whole spectrum?

 

[russ_watters]

Hot objects emit a continuous spectrum called black body radiation.

 

[Cantstandit]

Does that mean that the black body emits "infinite" number of photons (to fill all the gaps in spectrum) as one photon have only one frequency?

 

[jtbell]

Yes. Most of the light from the sun is emitted via thermal processes, similar to an iron bar glowing when heated.

 

[DrDu]

Hot objects emit a continuous spectrum called black body radiation.

Not all hot objects, only black objects. In the german wikipedia the continuous spectrum of the sun is explained as synchroton radiation from electron plasma moving in the strong magnetic field of the sun.

 

[Sciurus]

Hi, Cantstandit. A black body radiator emits a finite number of photons, but along a probability curve associated with the radiator, which depends on temperate. The sun is a black body radiator (for the most part).

I don't believe the objects have to be black to emit this sort of radiation, since it's due to the temperature -- in the sun's case, the temperature at the surface.

 

[Cantstandit]

Since the number of photons is finite, does that mean we only get white light averaging the frequencies over time? ie. if you could measure Sun's spectrum over very short period of time it would have gaps in it? Is the continuous spectrum continuous to begin with? Or photon can have more than just one exact frequency?
Thanks for the answers, but I still fail to understand how finite number of photons can cover what seems to be infinite number of frequencies in the spectrum...

 

[Sciurus]

Since the number of photons is finite, does that mean we only get white light averaging the frequencies over time?

Yes.

if you could measure Sun's spectrum over very short period of time it would have gaps in it?

Yes, again.

Is the continuous spectrum continuous to begin with? Or photon can have more than just one exact frequency?

Each photon has a unique frequency.

Thanks for the answers, but I still fail to understand how finite number of photons can cover what seems to be infinite number of frequencies in the spectrum...

Although there are a finite number, the number it is a very large number. The curves you see for blackbody radiators are theoretical and something that an actual object only approaches as you measure more and more photons. The photorecepters in the eye (or a camera) are producing results based on the average of a large number of photons. You would need sophisticated equipment to actually weed out individual photons.

I hope this helps. Good luck, Cantstandit.

 

[Smusko]

Thanks for the answers, but I still fail to understand how finite number of photons can cover what seems to be infinite number of frequencies in the spectrum...

I do not officially know what I'm talking about, but here goes.

Maybe theoretically they don't cover every possible infinitesimal change in frequency, but enough to show us that it follows a mathematically smooth probability curve. And when you get the spectrum of a star it comes with a resolution that may be to large to show the small small gaps that may theoretically be there.

Theoretically it may be possible to get a spectrum resolution high enough to show the random gaps in the curve over a small small time interval. Maybe this has been done, I have no idea.

End of speculation.
From now on I know pretty much what I'm talking about.

Seems like you have some flawed notion of how stars shine, so here goes.

Stars are called and ideal black body because they do not transmit any light, and they don't reflect any light. Any light you see from a star is created by the star itself.

They shine due heat generated by nuclear fusion in the core. The energy released goes into exiting the hydrogen and helium particles within. This, and the gravitational pressure asserted on the core is what is causing them to bump into each other emitting photons. This is called thermal radiation. Temperature is in fact a measure of the random motion of particles.
When you have a black body radiating (as a star) you get an idealized light curve (image below) that depends almost entirely on its temperature. The chemical composition plays some part, but it is mostly on the temperature.

When the photon passes through the atmosphere of the sun some light gets scattered by the atmosphere. This causes the continuous spectra of the sun to have dips in its emission lines. These dips are what tells us the chemical composition of the sun, or at least its atmosphere, which we think is representative of the entire star.

I just googled for black body radiation curve and this came up.
White light is the mixture of colours from the whole visible spectra. When the peak in brightness of a black body is smack in the middle of the visible spectrum it comes off as white light.
If the star has higher temperature, the photons escaping will have a higher energy, which translates into shorter wavelength, which result in the star looking more blue.
So the curve of the black body radiation curve depends only on its temperature. Our sun is of the right temperature to radiate white light.

Hope this helps.

 

[Drakkith]

It's kind of odd that our star is called a "Yellow" Dwarf, when it is actually white. But that's just because its most intense output is in the yellow-green region.

 

[ibysaiyan]

Since the number of photons is finite, does that mean we only get white light averaging the frequencies over time? ie. if you could measure Sun's spectrum over very short period of time it would have gaps in it? Is the continuous spectrum continuous to begin with? Or photon can have more than just one exact frequency?
Thanks for the answers, but I still fail to understand how finite number of photons can cover what seems to be infinite number of frequencies in the spectrum...

In reply to the bold bit: Each photon has one frequency.Energy of a Photon: hf
And the way this works in the atmosphere of most stars is by Absorption radiation.An atom absorbs a photon of a certain frequency (aggregating to the energy 'gap') which then re-releases the very photon in a random direction 'Fraunhofer radiation'/lines).

As mentioned by other posters, our perception of star is judged by the wavelength ( max output)which transmitted successfully through the atmosphere(earth).The curve of intensity against wavelength explains clearly of this shift.

 

[Cantstandit]

Thanks a lot for all the answers, I think I understand it now. Just to clarify some things:

This thermal radiation is due to the temperature i.e. movement of particles, so this is how I understand it works:

In the core there is enough pressure to start nuclear reaction, higher elements are formed and gamma rays are emmited. Those gamma rays are "absorbed" by higher layers of gas(plasma?), increasing its temperature (velocity). This new layer emits again radiation and so on. With "height" the temperature and frequency of the radiation are decreasing (because the area is increasing), and the uppermost part of the atmosphere have particles that are the source of the visible light.

Now, those ions can't move with constant velocity, if they "give away" some of the energy in form of photons, so are they colliding with each other? I think they must be, because if this thermal radiation was because of e.g. angular acceleration due to magnetic field people wouldn't emit infrared radiation ;)

What puzzles me is how exactly are those photons produced. Aren't those collisions elastic? I don't want to bother you guys too much, can someone point me to some source, because I don't even know what keywords should I google ;)


Thanks again it is very helpful.

 

[bcrowell]

The sun's core is dense -- about a 10^2 times as dense as water, at the center. Under these conditions, individual hydrogen and helium atoms don't have the same spectrum of electronic energy levels they have in a low-pressure gas. Since all the electrons are coupled together, they act like a quantum-mechanical n-body system, where n is huge. A quantum-mechanical n-body system is basically a classical system, so we won't see any quantum-mechanical effects such as a discrete line spectrum. We get a continuous spectrum (which is approximately a blackbody spectrum with a temperature characteristic of the least deep layer at which the atoms don't behave as individual atoms).

The outer layers of the sun are low-density gas, where each atom exists by itself and acts like a quantum-mechanical n-body system with a small n. This causes the absorption lines.

All of this is completely independent of, and more fundamental than, any ideas about blackbody spectra. For example, if there are temperature differences between different parts of the surface at which the final continuous spectrum is emitted (which I'm sure there are), then the spectrum will be an average of blackbody spectra corresponding to different temperatures, and its shape will differ from a blackbody curve to some extent.

Now, those ions can't move with constant velocity, if they "give away" some of the energy in form of photons, so are they colliding with each other? I think they must be, because if this thermal radiation was because of e.g. angular acceleration due to magnetic field people wouldn't emit infrared radiation ;)

What puzzles me is how exactly are those photons produced. Aren't those collisions elastic?

For example, you could have a collision between two neutral atoms in which one of them is ionized: atom+atom -> atom + ion + electron + photon. Since the energy of the liberated electron can take on a continuous spectrum of values, so can the photon. Depending on how deep this is in the sun, it may not even be valid to think of the atoms as having their own, independent quantized energy levels.

 

[DrDu]

I read a little bit about that. Apparently the mechanism was clarified in 1938 by Rupert Wildt. The continuous emission of the sun is due to bound to continuous (ionization) transitions of hydrogen anions. The electrons necessary for the formation of these anions stem from metals (mostly Na) which are completely ionized under the prevailing conditions.
The articles by Wildt are available here:
http://adsabs.harvard.edu/full/1939ApJ...90..611W
http://adsabs.harvard.edu/full/1941ApJ...93...47W

 

[twofish-quant]

Aren't those collisions elastic?

Nope. When you have a photon hit an electron or a nucleus, the photon can give up some of its energy to thing it is colliding with.

 

[twofish-quant]

The other thing is that the sun is not a black body. The interior of the sun is pretty close to a black body, but then when you get into the upper layers, you it goes through gasses that block out some of the light from the interior.

The sun is a third generation star which means that it has a lot of iron lines that makes the spectrum quite "dirty".