Fraunhofer lines.....

[mrjaffa]

Hey guys. Just want to ask about these Fraunhofer lines which I've been doing a little reading on. Without going in to too much detail in my question, if we look at these lines from the spectrum coming from our sun, are they only showing us what the suns atmosphere is made of and not the sun itself?

Thanks!

[Merlin66]

No, they show both!

The reason the lines appear as dark bands is that the light from the sun goes through a
"gaseous" layer on it's way outwards and this region accounts for the absorption features....

All the elements in the sun are accounted for.....

[mrjaffa]

Hmmmm. Thanks for the reply. Not sure if this helps me though. I understand what you say, but everything I've read say the lines work when the light passes through a gas, i.e the suns atmosphere, a nebulae. So that seems to say the light travels from the sun and only when it passes through its atmosphere these absorption lines become present. Telling us what the atmosphere is made of, not the sun itself. I know I'm wrong but trying to understand it better.

[Merlin66]

Trying to keep it simple...

Kaler in his "Stars and their Spectra", p28/29 does a better job than me....

He simplifies the solar environment to three regions - the core, the envelope and the atmosphere.

The core is hydrogen under going fusion at millions of degrees, the envelope "squeezes' the core to maintain the high core temperatures. Radiation cannot escape from the core and the envelope absorbs the gamma rays which otherwise would destroy life as we know it. The outer atmosphere is what we "see" and where the spectra originate. Transparent gasses at around 6000 deg.

The spectrum seen does in fact show what the sun (and the stars) are made of as all the elements originated in the solar nuclear furnace.

So observing the outer shell is as good as it gets.

[robin_astro]

The Fraunhofer lines we see are from the cool outer atmosphere of the star. For stars sitting on the main sequence like the Sun what we see it what the star was made from, not the elements which are being formed in the suns core (Only later on in the life of a star we may see elements dredged up from lower depths which were formed in the star.)

This means what we see in a main sequence star spectrum are the elements that were made in previous generations of stars and then ejected (eg as stellar winds or in novae/supernovae explosions) 

The metallicity of a star (ie the proportion of other elements relative to Hydrogen)  is a measure of how many recycles the material has gone through before forming the current star.

Robin

[robin_astro]

Note that the photosphere is made from the same material as the rest of the star, except at the core (which is a very small proportion of the total mass) where nuclear fusion is gradually converting some of the H to He, and then later on in the life of the star to C, O, N etc.  The solar photosphere and the spectrum produced there  is therefore indeed representative of the material which makes up  the sun.

Robin 

[robin_astro]

Note also that just because a particular element is present, it does not neccesarily mean it will show up as absorption lines in the visible spectrum. This depends on the exact conditions in the gas (eg temperature) The classic example of this is Helium. This certainy exists in the material which makes up the sun but does not appear anywhere in the solar absorption spectrum. It can only be seen in emission from  the tenuous chromosphere eg during total solar eclipses (the so called "flash spectrum" eg as here on my website

http://www.threehillsobservatory.co.uk/astro/spectra_27.htm

Cheers

Robin

[mrjaffa]

Thanks for your time chaps. I'll have a look at that link robin.