Francis Berthomieu
"EAAE Summerschools" Working Group
CLEA (France)
Abstract
Pupils are always surprised when they learn that we can determine mass, temperature or chemical composition of a very distant star. This workshop shows how the analysis of the light coming from a star allows us to detect some of its chemical elements. It is a good opportunity to get some knowledge about spectroscopy, elements, emission and absorption lines spectra.
Historical overview
In the Antiquity, stars were seen as luminous points, situated on a giant sphere. And our Sun was not considered as a star. Anaxagoras thought it was an incandescent metal mass and Aristote said it was made of "pure fire".
When Galileo discovered sunspots on its surface, it was thought they were ashes, or some mountains emerging out of a fire ocean. Even in 1798, William Herschel described the sun as a hard dark body, hidden by white luminous clouds.
And even in the middle of the 18th century, nobody could say what was our Sun made of. In his "Cours de philosophie positive", Auguste Comte said that we would never know anything about its composition, because it was unbelievable to remotely determine its chemical composition.
He was wrong: two years after his death in 1857, Robert Bunsen and Gustav Kirchhoff found empirical laws of spectroscopic analysis. The determination of the chemical composition and physical properties not only of our Sun but also of stars was now possible.
Their work investigated thoroughly some properties of matter and light, discovered by Newton. White light is a mixture of all the colors of the rainbow. It is easy to show that with a prism and obtain a pure spectrum of the white light. The prism deviates the blue light more that the red light, and colors appear in the following order : purple, indigo, blue, green, yellow, orange and red. When he discovered that all these colors were present in the white light, Newton was introducing a fabulous method: spectroscopy.
Matter is able to emit or absorb light: Studying how it is emitted or absorbed by the atoms of a gas, Kirchhoff and Bunsen deduced informations about composition, temperature and density of this gas.
And they found 3 experimental rules:
Rule 1: A hot and opaque solid, liquid or highly compressed gas emits a continuous spectrum.
Rule 2: A hot, transparent gas produces an emission spectrum with bright lines. The number and colors of these lines depend on which elements are present in the gas, constituting an identity card of these elements.
Rule 3: If a continuous spectrum passes through a gas at a lower temperature, the transparent cooler gas generates dark absorption lines, whose number and colors depend on the element in the cool gas.
However, from 1814 the German physicist Joseph von Fraunhoffer observed black lines in the spectrum of the Sun. He drew what he could observe, assigning a letter to each line or groups of lines.
Using the Kirchhoff and Bunsen third rule, it was quite easy to identify which elements were responsible for these dark lines: D1 and D2 lines were caused by sodium, C an F lines by hydrogen, etc.
Kirchhoff deduced that these elements were present in the atmosphere of the Sun and were absorbing their characteristic wavelengths. He published in 1861 the first atlas of the solar spectrum, obtained with a prism; however, these wavelengths were not very precise: the dispersion of the prism was not linear at all.
In 1869, the Swedish physicist Anders Ångström published a new atlas, obtained with a diffraction grating, whose dispersion is linear, and replaced Kirchhoff's arbitrary scale by the wavelengths, expressed in the metric system, using a small unit (10-10 m) with which his name was to be associated.
A photographic atlas, published by Henry Rowland about 1890, and still currently used, covers a wavelength field ranging from 300 to 700 nm and contains approximately 20 000 lines.
In 1863, a detailed study of the solar spectrum showed new strange lines: they only appeared when the Sun was low above the horizon. Lab experiments, carried on by French astrophysicist Jules Janssen, showed that these lines were caused by the Earth atmosphere.
In 1868, during a very interesting eclipse, Janssen found a new unknown black line in the yellow part of the spectrum.
The British astronomer Norman Lockyer, using the Greek name of the Sun, proposed the name of "helium" for this new unknown element: 27 years later, helium was found down here on our Earth by Ramsey, and we know nowadays that helium is the second more abundant element in the Sun, after hydrogen.
The spectral analysis made possible to know the detailed and precise chemical composition of our Sun, and demonstrated a surprising fact: all elements known on Earth were present there. And you are now ready to use the same method with the light of stars.
Documents
Document 1: You can observe a photograph of the argon emission spectrum.
A table gives the wavelengths of the most intense Argon emission lines.
Emission lines spectrum
Argon
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Document 2: You can find in these black and white photographs the visible part of star spectra. On both side of the star absorption spectrum, you can observe, analyse and use the emission line spectrum of argon gas, as a reference for wavelength measurements.
a: spectrum of Markab (α Pegasus)
b: spectrum of Rigel (β Orionis)
c: spectrum of the Sun.
These photographs were taken by Daniel Bardin & Jean Ripert - CLEA Mission - Observatoire du Pic du Midi
Document 3: This table indicates the wavelengths of some characteristic lines and elements.
Wavelengths of some characteristic lines and elements (nanometers)
| Hydrogen | (Balmer serie) | 656,3(Ha) | 486,1(Hb) | 434,2(Hg) | 410,2(Hd) | 397,0(He) | |||||||
| Hélium | He I | 388,9 | 404,6 | 414,4 | 447,1 | 471,3 | 492,5 | 501,6 | 504,8 | 587,6 | 667,8 | 706,5 | 728,1 |
| Hélium | He II | 468,6 | |||||||||||
| Sodium | Na I | 589,0 | 589,6 | ||||||||||
| Magnesium | Mg I | 470,3 | 516,7 | 517,3 | 518,4 | ||||||||
| Magnésium | Mg II | 279,5 | 280,3 | 448,1 | |||||||||
| Calcium | Ca I | 422,7 | 458,2 | 526,2 | 527,0 | 616,2 | 616,9 | 650,0 | |||||
| Calcium | Ca II | 393,4 | 396,8 | ||||||||||
| Chromium | Cr I | 435,2 | 461,3 | 464,6 | |||||||||
| Titanium | Ti I | 466,8 | 469,1 | 498,2 | |||||||||
| Iron | Fe I | 404,6 | 423,4 | 425,1 | 426,0 | 427,2 | 438,3 | 452,9 | 459,3 | 489,1 | |||
| ... | . | 491,9 | 495,7 | 501,2 | 508,0 | 527,0 | 532,8 | 537,1 | 539,7 | 543,0 | |||
| ... | . | 543,4 | 544,7 | 545,6 | 561,6 | ||||||||
| Nickel | Ni I | 508,0 | 508,5 | ||||||||||
| Oxygen | (Earth atmosphere) | 686.7 |
Important note:
I means neutral atom Fe I = Fe
II means ionized i.e. Ca II = Ca+
Exercises
Use document 1 and verify that the wavelengths scale on the photograph is a linear one.
Determine the wavelengths of the principal absorption lines for each star.
Try to identify the elements responsible for these lines.
Other questions
Do you think that there are no more elements than those you have found in the external envelope of the star?
Did you detect Hydrogen in the 3 stars through lines Ha, Hb and Hg?
What can you say about Helium, which is always present in the stars?
More informations and didactic material
Some links...
Many informations about light and spectra (in French)
Ce que nous apprend la lumière
A wonderful web site about Physics and Astronomy by J.F. Noblet.
Don't miss his Periodic table of elements and spectra and download it!
Find also his software to study RIGEL's spectrum and download it.
Bibliography
Méthodes de l'astrophysique - L.Gouguenheim - Liaisons scientifiques -Hachette CNRS.
"Fraunhoffer" Cahiers Clairaut n° 37 - K. Mizar.
Les Cahiers Clairaut - HS-7 - Etude du spectre du Soleil - GRP CLEA.
La lumière messagère des astres : Fascicule pour la formation des maîtres en astronomie n°3 (publication CLEA).