Fraunhofer lines and the composition of the Sun

1 Summary

The purpose of this lab is for you to examine the spectrum of the Sun, to learn about the composition of the Sun (it’s not just hydrogen and helium!), and to understand some basic concepts of spectroscopy.

2 Papers and datasets

These documents are all available on the course web page.

• Low resolution spectrum of the Sun (from the National Solar Observatory)

• Absorption lines of various elements (from laserstars.org and NASA’s Goddard Space Flight Center)

• Table of abundances in the Sun

3 Scientific background

3.1 Spectroscopy At its most basic level, spectroscopy is simply splitting light up into different wavelengths. The classic example is a prism: white light goes in, and a rainbow comes out. Spectroscopy for physics and astronomy usually drops the output onto some kind of detector that records the flux at different wavelengths as a “stripe” across the detector. In other words, the output is a data table that lists the measured flux as a function of pixel number. An critical aspect of spectroscopy is wavelength calibration: what wavelength corresponds to which pixel number? Note that that relationship does not have to be — and is often not — linear. That is, the mapping of wavelength to pixel is not a linear relationship, but something more complicated.

3.2 The Sun The Sun is mostly hydrogen and after that mostly helium. You have learned about the fusion processes in the Sun and in other stars that gradually convert hydrogen to helium and then, for some stars, onward to carbon, nitrogen, and oxygen (CNO cycle), and further upward through the periodic table to iron. Our Sun is currently converting hydrogen to helium, but is not doing any of the other more exotic nucleosynthesis processes that produce elements heavier than helium. Neverthe- less, there are many other elements in the Sun besides hydrogen and helium. These traces species are basically inert in the nucleosynthesis process — just along for the ride. These 1 contaminents were simply part of the pre-solar nebula at the time of formation of our Solar System, and got incorporated into the Sun. Interestingly enough, these trace contaminants were present in the protoplanetary disk, and some of them coalesced to become planets .. and people.

3.3 History In 1814, Joseph von Fraunhofer pointed a spectrograph at the Sun. He noticed (as had one or two others before him) that the Sun’s optical spectrum was not continuous and smooth, but rather was interrupted by a number of dark lines. Fraunhofer carefully catalogued these absorption features and recorded their relative positions in the spectrum. Years later, these dark lines were recognized to be absorption features in the Solar System, implying that a number of elemental species were present in the atmosphere (upper layers) of the Sun. Our Sun is the star we know best, of course. The study of absorption features in our Sun has led to understanding of our Sun as a star, and to knowledge of other stars. Almost two hundred years after Fraunhofer first identified these lines, the study of absorption lines in our Sun and in other stars remains an important topic in Solar and stellar astrophysics.

4 Your assignment

Your assignments in this lab is to create a wavelength-calibrated spectrum of the Sun and to identify the major absorption features in the Solar spectrum. The course web site has a low resolution spectrum of the Sun, from the archives of the National Solar Observatory. This file has 948 lines. Please note that this is a “binned down” — that is, converted from high resolution to low resolution — version of the original spectrum, which has 1.1 million lines! This file has two columns: pixel number and flux (arbitrary units). The other table that you’ll need for this lab is also on the course web site: a table listing major absorption features (in the optical) for six of the main elemental species found in the Sun. Here again you have two columns: wavelength (in A)˚ and the relative strengths of those lines. Note that the relative strengths of those absorption features are in arbitrary units, but the same scaling applies across all the elements. Your assignment therefore is to reproduce what Fraunhofer and later 19th century scien- tists did: (1) Produce a wavelength-calibrated spectrum of the Sun and (2) identify the major absorption features present in the Sun. “Identify the major absorption features” means that you should label which element(s) are responsible for which dips that you see in the spectrum. I have not given you explicit step-by-step instructions here, so please show all your work and discuss all the steps that you carry out to answer these two questions. In addition, there are two more discussion-y questions: (3) What are the relative abun- dances of the six elements for which I have given you absorption lists? Compare these relative abundances to the tabulated composition of the Sun given on the web page. Finally, (4) Are 2 there any features in the Solar spectrum that can not be attributed to these six major elemen- tal species? If so, where do you think these mystery features come from? To answer this last question, you may need to do a little research.

5 Final thoughts

You’ll never look at the Sun the same way again.

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