I recently purchased a 1886 lithograph from an esty shop in australia and framed it; my initial impulse for the acquisition was that the plate was a beautiful piece of art and history, related to one of the few things I can base my personality on – astronomy. The plate, titled “SPECTRA OF THE SUN AND STARS,” is from a late 19th century book titled The Story of the Heavens.
The book is essentially free on google, and reads like a 19th century Carl Sagan wrote it:
The history of Astronomy is in one respect only too like other histories. The earliest part of it is completely and hopelessly unknown. The stars had been studied, and some great astronomical discoveries had been made, untold ages before those to which earliest historical records extend. For example, the perception the apparent movements, of the sun and of the moon, and the recognition of the planets by their movements, are both to be classed among these discoveries of the pre-historic ages. Nor is it to be said that these achievements were all of a very obvious or elementary character. To us of the present day who have been familiar with such truths from childhood, they may now seem simple and rudimentary; but in the infancy of science the first man who arose to demonstrate one of these great doctrines was indeed a most sagacious philosopher.page 2, The Story of the Heavens
But what am I looking at?
If you’ve hung around earth for a while, you’ve ran into Pink Floyd’s Dark Side of the Moon album cover art (even if you haven’t listened to the album).
The cover art depicts a prism receiving a beam of “white” light from the left, and splitting it into a rainbow that exits the other side of the triangle. This represents a physical phenomenon, as anyone who’s seen the effect in person can attest. The door to the apartment I lived in as a child faced due west, so when the sun would set, its peephole would act like a prism and shine a small rainbow on the opposite wall. As a kid I would always try to catch the rainbow.
This splitting effect occurs because of two properties of traveling light: Ibn Sahl’s law (sometimes referred to as Snell’s law) and the Huygens principle. أبو سعد العلاء ابن سهل (Ibn Sahl) and some first year physics tells us that light will bend when passing from one medium (air → prism glass) because light moves at a different speed in different stuff; if the stuff is easier to move through (less dense, or some other qualifiers) the light will move faster. While “light” particles, photons, travel at one speed, “light” waves which are collections of lots of particles, take longer to move through different media because the individual particles which take up the light wave will bounce around within the medium, cutting a path that is longer than a straight line would be through the medium.
But why does passing from oen medium to another make light bend? When a wave slows down or speeds up, its wavelength changes. This animation from Oleg Alexandrov explains the effect well: when the wavelength of one part of the wave changes, but the other doesn’t, you get a bending effect as one end of the wavefront wants to move faster than the other.
So our light is bent, but how does it split? Different wavelengths (colors) of light bend differently! As Pink Floyd shows us, shorter wavelengths (bluer colors) of light will bend more (shorter wavelength means a more exaggerated pull on its lagging wavefront).
Now imagine letting that light paint a wall opposite the prism+light source; you’d see something like this:
If you point a telescope at a star, put a prism at the bottom of the telescope, and let the star’s light be split into all it’s component pieces, you get a spectrum, like Plate 13. A lot of questions come to mind when examining Plate 13: What are those black bars? Why do different stars have different spectra? Why is this important?
Atoms, molecules, compounds
I’m not a chemist, and I don’t pretend to be. As an astronomer, the only particle I bother myself with is Hydrogen. Let’s talk about hydrogen then. Neutral hydrogen has one proton and one electron. If the electron is too energetic, it will spit out a photon of light to calm down. Sometimes the atom will get hit by a photon of light from somewhere else, and the electron will swallow up the light and get a little more energetic. When the electron has finished digesting that light, it’ll spit it back out again in a random direction. Light that was supposed to go one way goes another because it “bounced” off the hydrogen. Left on its own, a bunch of hot hydrogen gas will spit out photons to calm (cool) itself down.
The funny thing about quantum particles is that they’re picky. These hydrogen atoms don’t swallow or spit up any kind of light that comes their way, they have specific colors of light they eat: everything else they ignore. Hydrogen in particular likes a few “series” of light, most interesting to us is the Balmer series, which is comprised of colors of light humans can see.
Lets put a bunch of neutral hydrogen around a star, in its photosphere (you can imagine this as the star’s atmosphere), which is a bit cooler than the center of the star where all the photons are generated. If light tries to come out of the star, the hydrogen will let it pass unless it is a color of light the hydrogen wants to swallow, in which case the hydrogen will have a snack, blocking that color from traveling from the star to the earth. When an astronomer splits the star’s spectrum, they’ll see a gap in those colors of light the star’s atmosphere ‘ate.’
The red color in the Balmer series is commonly referred to as Hydrogen alpha (Hα). Stars have a lot of hydrogen, and if you look back at Plate 13, you can see that labeled in the red swatch of the spectra is an “absorption line” titled Hα which appears in each star.
The rest of the absorption lines? Other atoms, molecules, and chemical compounds in the star’s atmosphere that all have different tastes; if you can learn who likes to absorb what color, you can now tell what kind of stuff is in each star!
Hot star, cool star, red star, blue star
Wait, sorry, that should be the other way around. Hot stars are blue and cool stars are red. A campfire (~1000°C) glows red/orange, while a welding torch (~3500°) glows blue. Heat tends to melt complex things into simple things; like a smelter melting complex ore down into smooth iron. The same things happen in stars; cooler things (like planets, brown dwarfs, and cool stars) have lots of complex chemicals in their atmospheres, while hot things have relatively simple compounds and usually simple atoms. Complex chemicals means more and more varied absorption lines in a star’s spectrum.
Taking another look at Plate 13, what can we say? If we rank the 4 stars based on the complexity of their spectra, we’d end up with a list something like: Sirius, Sol, Aldebaran, Betelgeuse. We can then say that Sirius is the hottest star, followed by our Sun, and so on (we’d be right). Sirius’ surface temperature is 9,940K, and it glows blue, while Betelgeuse glows a ruddy orange at 3590K.
Taking spectra of stars has evolved a long way since 1886, but it remains an essential tool for astronomers to further understanding the numerous and wonderful objects that inhabit the universe. They look may look beautiful, but the real beauty is hidden in each line, each color, telling its own story.
Author’s note: This series may continue with an in-depth look at the 4 stars who are shown in plate 13. Thank you for reading, I hope you enjoyed this piece on stellar spectra. Happy astronomizing!