# I How are apparently 'continuous' blackbody spectra formed?

#### argon1120

Summary
Analysing the blackbody spectra of the Sun appears to produce the standard blackbody curve. My understanding off the mechanics behind this is that the electrons in each atom have a discrete number of energy levels and therefore a discrete number of energy level transitions. This would mean that there is a discrete number of wavelengths which could be emitted by the sun, especially since the sun is mainly hydrogen and helium. How is it that the sun's spectra can appear continuous?
My understanding of this is based upon the assumption that energy level transition is the only mechanism responsible for blackbody photons.

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#### PeterDonis

Mentor
My understanding of this is based upon the assumption that energy level transition is the only mechanism responsible for blackbody photons.
It isn't. In fact, for the Sun this mechanism is negligible, since it's a plasma, which is composed of separate electrons and ions, not neutral atoms.

Even for an ordinary solid object, composed of neutral atoms, energy level transition is not the only mechanism by which photons can be emitted. Atoms in solids can vibrate in various ways without changing the energy levels of any of their electrons, and these vibrations can cause photons to be emitted.

When scientists want to examine the specific frequencies of radiation due to energy level transitions in atoms, they use a vapor of the appropriate substance in order to eliminate, as much as possible, other possible sources of radiation.

#### argon1120

It isn't. In fact, for the Sun this mechanism is negligible, since it's a plasma, which is composed of separate electrons and ions, not neutral atoms.

Even for an ordinary solid object, composed of neutral atoms, energy level transition is not the only mechanism by which photons can be emitted. Atoms in solids can vibrate in various ways without changing the energy levels of any of their electrons, and these vibrations can cause photons to be emitted.

When scientists want to examine the specific frequencies of radiation due to energy level transitions in atoms, they use a vapor of the appropriate substance in order to eliminate, as much as possible, other possible sources of radiation.
Thank you, that makes a lot of sense!

In the context of the sun is it the motion of charged particles which generate photons?

#### PeterDonis

Mentor
In the context of the sun is it the motion of charged particles which generate photons?
Basically, yes, at least for the photons emitted from the surface, the ones we see. There are other things going on in the Sun's interior that involve photons; it's actually fairly complicated to analyze how the energy released in fusion reactions in the Sun's core ends up as energy in the radiation emitted from its surface.

#### vanhees71

Science Advisor
Gold Member
Well, a photon created, in whatever way, in the interior of the sun needs about $10^5 \; \text{yr}$ to get out of the sun. The spectrum is basically a black-body spectrum, which is continuous, because it's thermal radiation. The spectrum gets modified, however, due to absorption in the gas around the Sun, leading to the Fraunhofer lines which lets us analyze precisely what the Sun is made of. Among other things also He has been discovered first in this way (that's why it's called Helium).

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