Undergrad Solar Spectrum: Continuous & Absorption Confusion

[xoxo1001]

Ok, I'm a bit confused with the spectrum of the Sun. Is the spectrum of the Sun continuous or absorption? Better yet, is it both? Or am I totally confusing myself? I understand that the source itself is continuous but it is partially absorbed (wrong phrasing?) as it passes through the outer layers of the Sun, which should give us an absorption spectrum, right? But looking over many sources, even ones here, many people say that it is continuous. Why is that? If it is continuous, then how do we get the Fraunhofer lines? Or if it really is just continuous, then why do other sources say that the Sun's spectrum shows an absorption spectrum?

 

[Orodruin]

An absorption spectrum is a continuous spectrum where particular wavelengths have been removed due to absorption.

 

[xoxo1001]

An absorption spectrum is a continuous spectrum where particular wavelengths have been removed due to absorption.

I get that part, but maybe I'm just not seeing an obvious answer or something. If I look at the spectrum of a beam of sunlight, would I see a continuous one or one with dark lines on it? I thought I would see an absorption one (like this https://www.extremetech.com/g00/3_c-7x78x78x78.fyusfnfufdi.dpn_/c-7NPSFQIFVT34x24iuuqtx3ax2fx2fx78x78x78.fyusfnfufdi.dpnx2fx78q-dpoufoux2fvqmpbetx2f3124x2f21x2ftvo-751y537.kqhx3fj21d.nbslx3djnbhf_$/$/$/$/$/$), but it sounds like it would just be continuous? So then why do some sources say that the spectrum is absorption? #confused

 

[andrew s 1905]

The spectrum would look like the one you linked to. It is a continuous spectrum with lines removed by absorption formed in the photosphere.

You can't have a pure absorption spectrum. There has to be some continuous spectrum for the electron transitions to absorb.

Regards Andrew

 

[xoxo1001]

The spectrum would look like the one you linked to. It is a continuous spectrum with lines removed by absorption formed in the photosphere.

You can't have a pure absorption spectrum. There has to be some continuous spectrum for the electron transitions to absorb.

Regards Andrew

Ok, there lies some of my confusion. You said that it is a continuous spectrum with lines removed by absorption. Is that not an absorption spectrum? I guess I am confused by the usage of these two terms at this point. Are they both correct? As in, the spectrum of the Sun IS continuous, but since there are lines removed by absorption in the photosphere, the spectrum of sunlight is also an absorption spectrum? Or am I on the wrong track? Thanks.

 

[andrew s 1905]

Yes it is an absorption spectrum. However, while there are a significant number of absorption lines there is also a significant amount of the continuous spectrum left so it could reasonably be referred to as a continuous spectrum.

Some cool stars have so many lines due to molecular transition almost none of the continuum remains.

I think you are expecting too clear a distinction when there is a range of common usage.

Regards Andrew

 

[xoxo1001]

Yes it is an absorption spectrum. However, while there are a significant number of absorption lines there is also a significant amount of the continuous spectrum left so it could reasonably be referred to as a continuous spectrum.

Some cool stars have so many lines due to molecular transition almost none of the continuum remains.

I think you are expecting too clear a distinction when there is a range of common usage.

Regards Andrew

Ahhhhh ok! Cleared up a lot of confusion for me. Thanks a bunch!

 

[lomidrevo]

I think the most clear way is just to say "spectrum". Basically spectrum of all stars is mainly formed by continuum originated at photosphere, so called blackbody radiation, characterized by effective temperature. The absorption (or emission) lines are produced in the atmosphere as photons of particular wavelength are absorbed during ionization (or emitted during recombination). These lines are superimposed on the continuum, as we observe them. So the distinction between the continuous and absorption spectrum in this context is not very meaningful. Just my opinion..

 

[andrew s 1905]

Absorption lines in the visible spectrum of the Sun are due to bound-bound transitions not bound free transitions i.e. not ionization and recombination.

Regards Andrew

 

[lomidrevo]

Absorption lines in the visible spectrum of the Sun are due to bound-bound transitions not bound free transitions i.e. not ionization and recombination.

Regards Andrew

You are right! I don't know where my thoughts were when I was writing the post... Of course, the absorption (or emission) lines are caused by bound-bound transitions (not only in the visible part of the spectrum). Bound-free absorption (ionization) or free-bound emission (recombination) are contributing to the "continuous" part of the spectra, as the energy of free electrons is not restricted to energy levels as in case of bound electrons. Thanks for correcting me.