Wednesday, August 08, 2018

Ghosts of elements, spectres of the universe: Angelo Secchi SJ's stellar spectra

A plate of Secchi's spectra.

This year marks the 200th anniversary of the birth of astronomer Angelo Secchi, SJ, the pioneer behind stellar spectroscopy, which opened the door to our understanding of what makes up a star.  I'm spending a couple of weeks at the Specola Vaticana outside Rome, of which Secchi is arguably one of its founders, though the official founding of the current incarnation of the Specola would come nearly 15 years after his death.

[A version of this post is cross posted at the Vatican Observatory Foundation blog, The Catholic Astronomer]

If you’ve seen the flash of yellow-orange flames when a pot boils over on a gas stove, you’ve gotten a glimpse of the ghost of an atom, specifically sodium.  The color is part of the atom’s spectrum, which shows which types or frequencies of light are absorbed by that particular atom.

In the late 17th century, Isaac Newton used the Latin word for ghost, spectrum, to describe the bands of colors he saw when light shone through a prism. In 1814 Joseph von Fraunhofer noticed he could see bright lines instead of the bands of colors when looking at certain flames through a prism.  He went on to develop an instrument to measure these spectral lines, called a spectroscope.

Fraunhofer noticed a series of missing colors, dark lines, when looking at the sun’s light through the spectroscope, and went on to characterize the light from several stars as well.  Fifty years later  Jesuit polymath Angelo Secchi invented a series of spectroscopic instruments specifically for examining the patterns of colors in the light from stars and the sun and used it to build a catalog of more than 4000 stars.  Secchi classified the stars by recurring patterns in the light, which were a clue to the star’s composition.

Around the same time Secchi was building his catalog of stellar spectra, Gustav Kirchhoff and Robert Bunsen (the inventor of the ubiquitous Bunsen burner) were involved in a more down-to-earth scheme. Kirchhoff and Bunsen teamed up to create a spectroscope that used Bunsen’s new hotter, gas burner to ignite samples.  They noted that that when they combusted a pure element it produced a characteristic set of lines, a spectral fingerprint, that could be used to identify it.

In October of 1860, Kirchhoff and Bunsen announced they had used their spectroscope to discover a new chemical element, which they named cesium, for the blue color of its principal line.  Chemists quickly began to use Bunsen’s spectroscope to find new elements.  A few months later Kirchhoff and Bunsen found two bright ruby red lines in an extract of a silicate mineral lepidolite, the spectral traces of another new element, rubidium.

Thallium’s ghostly green emanations were first observed by William Crookes, indium, ironically named for its violet lines by its color blind discoverer Ferdinand Reich.  Paul-Émile Lecoq de Boisbaudran spectroscopically painstakingly identified element 66 in a sample extracted from his marble hearth, and instead of naming it for the colors of the lines, called it dysprosium, from the Greek for “hard to get” — because it was.

Hunting for new elements spectroscopically meant you didn’t actually need to have any of it in your lab or even on your planet, as long as you could observe the light from a burning sample.  In 1868 several chemists and astronomers independently observed a faint line in the spectrum of the sun, and assigned it to a new element, helium, which as far as they knew did not exist on earth.  It would take nearly 30 years for two Swedish chemists to confirm that it was present on earth — by matching the spectrum with that of a gas found in a uranium ore.  (All the helium found on earth comes from radioactive decay.)

These ghostly lines produced by elements helped fuel yet another critical discovery that would have far reaching consequences for chemists’ understanding of the periodic table:  quantum mechanics.  Niels Bohr’s quantum mechanical model of the atom opened the door to explaining the line spectra of chemical elements. Though more accurate and sophisticated quantum mechanical models of the atom now exist, Bohr’s model showed the relationship between the lines and an atom’s electron by insisting that the electrons’ energies were quantized, that is, they could only have certain energies.

So why do atoms have ghosts?  When an atom is heated to high temperatures, as in a flame or a star, the energy it absorbs excites its electrons.  You can think of the electrons in an atom as being on an energy ladder. (this isn’t quite correct as far as the quantum mechanics goes, but it is a reasonable approximation and easier to visualize.)  They can only have energies that match the rungs of the ladder, and each type of atom has a unique arrangement of the rungs.

When an atom absorbs energy, its electrons move to higher rungs.  Excited electrons are unstable. They quickly return to their original arrangement, giving off some their excess energy in the form of light as they fall back to their original rung.  The color (the wavelength) of the light emitted depends on the difference in energy between the rungs.  The colors of light emitted are the ghosts of the energy rungs.  Since each element has a unique pattern of rungs, it will have a unique spectrum of emitted light and so revealing their presence to the sharp eyes of spectroscopists.

The spectra that Secchi so carefully observed (and hand drew!) were not just a way to identify a particular star, but clues to its chemical composition and even more critically to its evolution. Chemists and astrophysicists still use the light emitted and absorbed by atoms and molecules to identify their presence.  We hunt for the structure of the universe in its ghosts.

If you want a way to see the ghosts of atoms for yourself, try this inexpensive DIY folding spectroscope you can attach to your phone. Use it to check out the light from a neon sign or from a street light!

For a wonderful description of the elements, including stories of how they were first discovered, read John Emsley’s Nature’s Building Blocks.

Want to read more about Angelo Secchi, SJ? Try Adam Hincks SJ's piece in American Magazine or my colleague at the Specola Bob Macke SJ's piece about Secchi's more terrestrial scientific pursuits.

This post is a version of an essay written for a collection commissioned for the UN’s International Year of Light in 2015.  


  1. I'm 74, well educated in physics, and have never for a moment believed in the enormously popular "Big Bang" theory. My high school was at Hanford, Washington where we knew when something was a strategic ploy and when it was truth. I worked with Doppler radar on a navy ship recovering astronauts during Project Mercury; visited Nagasaki on the same tour, studied astronomy myself and for a while had a nice 12" Meade SCT. I took a BA in physics at Reed College, worked at Xerox, toured Europe and visited the Vatican, and my wife met Dr. James Watson, one of the discoverers of DNA.
    I took a year of computers science at the University of Washington. I visited Palomar, the VA, and the VLA, and more and more things like that. I knew of the quantum scale with an Edmund Scientific spintharascope before I was in high school. My aunt gave me a copy of the Sourcebook on Atomic Energy, by Glasstone, which was classified, while I was in high school, and I was in Special Fast Learners classes then. I studied Latin and Russian languages. My father taught me a few words of Cherokee. All of this time I never saw a convincing argument or theory or proof of the Big Bang theory.

    I always was partial to the idea of a quantum theory of the Red Shift phenomenon. I knew well of Doppler shift, but Doppler shift is NOT appropriate for the spectral red shift observed in telescopes with spectroscopy, which I understand well.

    It took me a long time, and by the way it was only after I became so much more a believer in the Trinity, in God, and Christ and the Holy Ghost,that I began to make progress on the quantum theory. My first attempt on the internet was
    feeble and far too simple-minded. Years passed and now I am in a retirement home...

    I fetched several more books, by Einstein and Planck and other writers of quantum and relativity theories. I found myself reading them easily, very much thanks to a meaningful sense of faith. It seemed red shift theory was sort of an insult to the pristine perfection of God's starlight, and that's a strong argument, of course.

    To begin with, though, the most important factor is that there is something absolutely constant about light, even from distant galaxies. There are two constants, the speed of light, c, and the Planck action quantum, h. I found the dimensions of physical constant in Woan's "Cambridge Handbook of Physics Formulas". I use other books of course, but Woan's is comprehensive and accurate. Taking care to keep the dimensions constant from the first in my speculations, and then theoretical, mathematical progress, I found that while the speed of light and the action quantum remain constant in numerical magnitude, the energy is conjugate to the wave-time in the photon quantum. This is by the well known equation,
    E = h*nu. The wave-time is the time it takes for one undulation of the wave. Second, the momentum is conjugate to the waveLENGTH. Now it may be observed by the mathematician, that while the magnitude of the action quantum of action, and the magnitude of the speed of light are absolutely constant, and therefore have some Godlike character, the momentum, wavelength, energy and time are free to change WITHIN the photon quantum. It would be possible, therefore, for the energy to slowly diffuse into the wave-time, and the momentum to diffuse into the waveLENGTH.

    Working with this, it took months to understand that only something as small as the Planck constants could be among the solutions to the diffusion equation, whatever it is. There are the Planck Constant of course, and the Planck mass, and the Planck length, and the Planck time.

    These contain all the elements necessary to find the diffusion terms for the observed redshift, and that will ensure the existence of a theory that does not imply that the universe is temporal. or that it began with an explosion.

    Yours sincerely,

    Michael Grant Lewis

  2. I may have sent this twice, only slightly modified...
    Thank you for way cool.