Speculating About A Star's Spectrum

[hazelm]

It is only a speculative question but I'd like to hear a scientific (speculative) answer to the possibility.  In using the spectra of stars and/or galaxies, we learn that certain colors will be  missing from certain stars.   The missing colors represent missing elements from those stars.  Am I right there?  Now, my speculative question:  Assuming there is intelligent life on the planets of these stars, would these missing elements which can cause missing spectra colors also cause living beings there to see different colors than what we see on Earth? 

 

Just wondering.  Thanks.

[JMJones0424]

No, I don't think your premise is accurate.  We can indeed determine the general constituents of the outer layer of a star by observing its spectrum, but the spectral lines aren't an indication of the absence of an element, instead, they are the fingerprints of the elements that exist. https://en.wikipedia.org/wiki/Emission_spectrum

 

I think we are just beginning to be able to observe the characteristics of light that passes through the atmosphere of nearby extraterrestrial planets.  However, I don't think that there is any reason to believe that our observation of the spectrum of an alien atmosphere should have any particular bearing on how a hypothetical alien species observes color.

 

Color is a fabrication of the mind.  The appearance of color isn't even equivalent among all humans, and it is certainly different between other species that exist on Earth.  The spectral lines that we could conceivably observe from an alien atmosphere determine the constituents of that atmosphere, not what color beings on that world would see.  The eyes of those organisms determine what colors they see.

Edited May 19, 2018 by JMJones0424

[hazelm]

No, I don't think your premise is accurate.  We can indeed determine the general constituents of the outer layer of a star by observing its spectrum, but the spectral lines aren't an indication of the absence of an element, instead, they are the fingerprints of the elements that exist. https://en.wikipedia.org/wiki/Emission_spectrum

 

I think we are just beginning to be able to observe the characteristics of light that passes through the atmosphere of nearby extraterrestrial planets.  However, I don't think that there is any reason to believe that our observation of the spectrum of an alien atmosphere should have any particular bearing on how a hypothetical alien species observes color.

 

Color is a fabrication of the mind.  The appearance of color isn't even equivalent among all humans, and it is certainly different between other species that exist on Earth.  The spectral lines that we could conceivably observe from an alien atmosphere determine the constituents of that atmosphere, not what color beings on that world would see.  The eyes of those organisms determine what colors they see.

All right.  I'll buy your bit about color but, about missing elements, Stephen Hawking says:

 

"We find that certain very specific colors are missing from stars' spectra, and these missing colors may vary from star to star.  Since we know that each chemical element absorbs a characteristic set of very specific colors, by matching these to those that are missing from a star's spectrum, we can determine exactly which elements are present in the star's atmosphere."

 

If I am mis-reading you, my apologies. But, if we can determine "exactly" which elements are present, why can we not tell which  elements are missing.  And that, in fact, was more of my imagination.  If we have some elements they do not have and vice versa, that makes for some very different worlds out there, doesn't it?   Thanks.   I'll check out the link. 

[exchemist]

All right.  I'll buy your bit about color but, about missing elements, Stephen Hawking says:

 

"We find that certain very specific colors are missing from stars' spectra, and these missing colors may vary from star to star.  Since we know that each chemical element absorbs a characteristic set of very specific colors, by matching these to those that are missing from a star's spectrum, we can determine exactly which elements are present in the star's atmosphere."

 

If I am mis-reading you, my apologies. But, if we can determine "exactly" which elements are present, why can we not tell which  elements are missing.  And that, in fact, was more of my imagination.  If we have some elements they do not have and vice versa, that makes for some very different worlds out there, doesn't it?   Thanks.   I'll check out the link. 

Hazel, I suspect the passage of Hawking's you are quoting is not saying certain elements are missing, but that the elements that are present remove light from certain colours of the spectrum. So you have missing colours if the element is there. 

 

What happens is that the interior of the star generates a continuous "white light" light spectrum, from ultra-violet through all the colours of the rainbow and on to infra-red. The atmosphere of the star, which is much cooler, is too cool to radiate, but it will contain gaseous forms of the elements that are present in the star. These gases absorb some of the light on its way out from the stellar interior. It is the electrons in the atoms that do this, but they do this only at certain wavelengths, i.e. certain colours, that are characteristic of each element.

 

The reason is because of a number of things:

 

First, the electrons in an atom can only occupy certain allowed energy levels. They cannot just have any energy they like.

 

Second, the energy gaps between these allowed levels are different for different elements. 

 

Third, absorbing a photon is an all-or-nothing process: you cannot absorb part of a photon. This means that only photons carrying the exact energy difference between two levels in the atom will be absorbed by it.

 

And finally, the energy carried by a photon depends on its frequency or wavelength, i.e. on its "colour". So if the energy levels are quite close together they may absorb a photon with a frequency at the red (less energetic) end of the spectrum. If the energy levels are further apart they may need a photon with a "blue" frequency (more energetic) to have the right energy to kick the electron up to the higher level.  

 

For example sodium absorbs yellow light in two very narrow bands, at specific frequencies, called the "sodium D lines". If you see this pair of lines in a spectrum, you know sodium is responsible. (And also if you heat up sodium enough to excite its electrons, they will give off the same colour light as they jump back down to the ground state. After all, it is the same energy gap: they are just jumping down now instead of jumping up. This is used in sodium street lamps, which are.......yellow. :)

 

If you look at the spectrum of a star you see a bright continuum background, shading from violet through to red, but if you look closely you see lots of dark lines, i.e. gaps in the brightness,  each of which is at a frequency that some element in the atmosphere of the star is absorbing. It looks like this: https://en.wikipedia.org/wiki/Fraunhofer_lines#/media/File:Fraunhofer_lines.svg

 

By assigning these dark lines to the various elements that cause them, astronomers can tell what elements are present in each star.    

Edited May 19, 2018 by exchemist

[hazelm]

Hazel, I suspect the passage of Hawking's you are quoting is not saying certain elements are missing, but that the elements that are present remove light from certain colours of the spectrum. So you have missing colours if the element is there. 

 

What happens is that the interior of the star generates a continuous "white light" light spectrum, from ultra-violet through all the colours of the rainbow and on to infra-red. The atmosphere of the star, which is much cooler, is too cool to radiate, but it will contain gaseous forms of the elements that are present in the star. These gases absorb some of the light on its way out from the stellar interior. It is the electrons in the atoms that do this, but they do this only at certain wavelengths, i.e. certain colours, that are characteristic of each element.

 

The reason is because of a number of things:

 

First, the electrons in an atom can only occupy certain allowed energy levels. They cannot just have any energy they like.

 

Second, the energy gaps between these allowed levels are different for different elements. 

 

Third, absorbing a photon is an all-or-nothing process: you cannot absorb part of a photon. This means that only photons carrying the exact energy difference between two levels in the atom will be absorbed by it.

 

And finally, the energy carried by a photon depends on its frequency or wavelength, i.e. on its "colour". So if the energy levels are quite close together they may absorb a photon with a frequency at the red (less energetic) end of the spectrum. If the energy levels are further apart they may need a photon with a "blue" frequency (more energetic) to have the right energy to kick the electron up to the higher level.  

 

For example sodium absorbs yellow light in two very narrow bands, at specific frequencies, called the "sodium D lines". If you see this pair of lines in a spectrum, you know sodium is responsible. (And also if you heat up sodium enough to excite its electrons, they will give off the same colour light as they jump back down to the ground state. After all, it is the same energy gap: they are just jumping down now instead of jumping up. This is used in sodium street lamps, which are.......yellow. :)

 

If you look at the spectrum of a star you see a bright continuum background, shading from violet through to red, but if you look closely you see lots of dark lines, i.e. gaps in the brightness,  each of which is at a frequency that some element in the atmosphere of the star is absorbing. It looks like this: https://en.wikipedia.org/wiki/Fraunhofer_lines#/media/File:Fraunhofer_lines.svg

 

By assigning these dark lines to the various elements that cause them, astronomers can tell what elements are present in each star.    

And that is why Stephen Hawking said "May be missing".  But see your last sentence.  If they can tell what is present, can't they also say what is missing?  For example, they could see that "that star" has no helium or no iron, or such. Then there is the question of seeing that a star has an element that we do not have.  Hmmmm? 

 

Actually, my question started out with no consideration of those black lines.  I just thought it was certain colors missing from what the see on the spectral line.  But, maybe that's the same thing.I think I see what you mean.  One day last week we had a rainbow.  Only half the usual colors were there.  That doesn't mean we suddenly lost the other colors.  Different cause, I know.  Just an example.

 

I'll read your explanation again and no doubt get still more out of it.  Wave lengths puzzle me and I have a question about photons.  But those are another topic for another thread.

 

Thank you.

Edited May 20, 2018 by hazelm

[exchemist]

And that is why Stephen Hawking said "May be missing".  But see your last sentence.  If they can tell what is present, can't they also say what is missing?  For example, they could see that "that star" has no helium or no iron, or such. Then there is the question of seeing that a star has an element that we do not have.  Hmmmm? 

 

Actually, my question started out with no consideration of those black lines.  I just thought it was certain colors missing from what the see on the spectral line.  But, maybe that's the same thing.I think I see what you mean.  One day last week we had a rainbow.  Only half the usual colors were there.  That doesn't mean we suddenly lost the other colors.  Different cause, I know.  Just an example.

 

I'll read your explanation again and no doubt get still more out of it.  Wave lengths puzzle me and I have a question about photons.  But those are another topic for another thread.

 

Thank you.

Yes I'm sorry the explanation is a bit involved.

 

Suggest having a good look at the picture I linked to, showing the dark lines subtracted from the continuous spectrum.

 

Also very important to know Planck's relation: E=hν. This shows that the energy E of a photon is proportional to ν, the Greek letter "nu", which is the frequency of the radiation. So the higher the frequency (i.e. the shorter the wavelength) the greater the energy. (h is the fixed constant of proportionality relating the two, called "Planck's constant".

 

This is why UV light (even higher frequency than violet) is so bad for your skin. Too much energy in the photons! 

[hazelm]

Yes I'm sorry the explanation is a bit involved.

 

Suggest having a good look at the picture I linked to, showing the dark lines subtracted from the continuous spectrum.

 

Also very important to know Planck's relation: E=hν. This shows that the energy E of a photon is proportional to ν, the Greek letter "nu", which is the frequency of the radiation. So the higher the frequency (i.e. the shorter the wavelength) the greater the energy. (h is the fixed constant of proportionality relating the two, called "Planck's constant".

 

This is why UV light (even higher frequency than violet) is so bad for your skin. Too much energy in the photons! 

Involved only because it is somewhat beyond my ken.  But I like that solar spectrum.  That is what I was envisioning.  This book that I mentioned elsewhere only has the blue lines and the red lines for oxygen and for nitrogen.  It also has a bar of "the electron order filling" if that means anything.

[exchemist]

Involved only because it is somewhat beyond my ken.  But I like that solar spectrum.  That is what I was envisioning.  This book that I mentioned elsewhere only has the blue lines and the red lines for oxygen and for nitrogen.  It also has a bar of "the electron order filling" if that means anything.

Ah yes, das aufbauprinzip. Very useful for understanding the Periodic Table.