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If I wanted get the absorption spectra, I suppose I would stand here right on Earth with a diffraction grating collecting the sunlight directly, right? If this is how absorption spectra of the sun is done, then I have one question: The sunlight would have to pass not only its outer layer of gas but also through the Earth's atmosphere in order to get to my diffraction grating. And the atmosphere contains gas. And that gas will absorb some of the frequencies in the incoming light. And so the resulting absorption spectra I see on the grating will not really be the the spectra of the sun but the spectra of sun with the atmosphere interfering. So how did we arrive at this accepted absorption spectra for the Sun?

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The Earth's atmosphere does impose many absorption features on the incident spectrum of sunlight received at the Earth's surface - these are collectively known as telluric features or telluric contamination and can be a problem for ground-based observations of all astronomical sources.

Some of the absorption features that appear in the solar spectrum could not possibly have arisen in the Earth's atmosphere. This would include most of the Fraunhofer lines due to metals like iron, nickel, magnesium, calcium, sodium etc. that are not present in any significant quantity in the Earth's atmosphere. Therefore an observed solar spectrum containing these lines is telling you that these elements must exist in the Sun - there isn't any ambiguity.

A further point to identify these lines as definitely "belonging" to the Sun rather than the Earth's atmosphere or indeed interplanetary space, would be that you can obtain the spectra of other stars (hot stars, see below) where the Fraunhofer lines are mostly absent - this would have been known very early on.

However, the telluric features, caused by absorption due to (e.g.) ozone, molecular oxygen, water, carbon dioxide etc can introduce lines and continuum absorption into a solar spectrum that would not be seen in measurement above the atmosphere and confuse the "astronomical" signal. Contrary to the absorption features of solar origin, these telluric features would vary with time, the state of the atmosphere and the elevation that the Sun was observed at - so they can clearly be distinguished (e.g. Brewster & Gladstone 1860).

There are two basic approaches to correcting the spectrum for these features. The first is to have a good atmospheric model, good knowledge of the atomic and molecular transition probabilities for the relevant species in the atmosphere and then a radiative transfer calculation to work out how much absorption is expected at what wavelengths. The expected absorption can then be retrospectively removed from the solar spectrum. An example of such a model is the HITRAN project.

A second and perhaps more straightforward empirical way of removing relatively narrow telluric features in a spectrum is to observe an astronomical source which is not expected to show any absorption features in that part of the spectrum. These "telluric correction calibrators" are often hot white dwarfs or hot O/B stars that can show very little absorption outside of extremely broadened Balmer lines of hydrogen. By fitting a smooth curve to their spectra and then dividing that smooth curve by the observed spectrum, one obtains a telluric correction spectrum that can be multiplied with any other observed spectrum to correct it for telluric features.

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  • $\begingroup$ These are recent developments in the field, what about the data much older. In 19th century, sun's spectra was known. Does new data are different from older ones in terms of understanding and infirmation. $\endgroup$ Oct 26, 2022 at 11:41
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    $\begingroup$ @NeilLibertine I have added a few edits explaining how you could clearly distinguish between solar and telluric features in the spectrum, even in the 19th century. The hot-star telluric correction technique is very old, not recent. $\endgroup$
    – ProfRob
    Oct 26, 2022 at 12:40
  • $\begingroup$ It was in late 19th century, bolometer and like devices were invented. And you said that they had telluric like features. So they had confident knowledge about star's spectra. It means they knew much more as per their understanding but what ISS helps or changes view drastically. $\endgroup$ Oct 26, 2022 at 14:09
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    $\begingroup$ @NeilLibertine I cannot parse your comment. If you are suggesting we did not understand the spectrum of the Sun prior to the launch of spacecraft, you are incorrect. Brewster identified which of the dark Fraunhofer lines could be attributed to the atmosphere in the 1830s. $\endgroup$
    – ProfRob
    Oct 26, 2022 at 15:59
  • $\begingroup$ But understanding or more study of spectra was possible in late 19th century when theory of radiation from object was attempting to find. $\endgroup$ Oct 26, 2022 at 16:33
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ProfRob’s answer gives good descriptions of ground based approaches to this issue. Also remember that we have a scientific laboratory orbiting above our atmosphere! Here is a recent ISS project monitoring the solar spectrum without atmospheric interference:

https://www.aanda.org/articles/aa/full_html/2018/03/aa31316-17/aa31316-17.html

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