Summer’s Spectacular Solar Series, Part 2: Spectra Explained
published during a waxing gibbous moon.
04/05/2017

Summer’s Spectacular Solar Series is a recurring column on NOW.SPACE that answers all the questions you’ve ever had about our magnificent Sun but were too afraid to ask.


In the first article of this series, I explored the history of how we answered the question: “Why does the Sun shine?” I have a confession, though, I wrote that first piece only so I could write about one of my all-time favorite scientific images – the one right below this paragraph. I love it because: 1) it’s a rainbow and rainbows are awesome (obvs), and 2) it can be used to explain so much of how we know what we know about the universe.

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High-resolution visible spectrum of our Sun. Wavelength increases from left to right along each strip, and from bottom to top. Each of the 50 slices covers .60 nanometers, for a complete spectrum across the visual range from 400 to 700 nanometers. Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF

Just as planetary scientists use Earth as an analog for other planetary bodies, astrophysicists use the Sun as a proxy for other stars. The more we learn about how the Sun behaves, the more we can deduce about other stars in our galaxy and in the universe as a whole. One of the most important ways we do this is through spectroscopy – the study of spectra, the chemical fingerprints embedded in starlight. The image above represents the solar spectrum; the chemical “fingerprint” of our Sun.

A spectrum in astronomy is a collection of the light emitted by a given object over a range of wavelengths. You can have a spectrum for a given type of electromagnetic radiation (X-ray, infrared, radio, etc.), or you can have one that spans them all. For the purpose of this article, I’m only going focus on the visible spectrum, what we commonly call the rainbow. It covers red to blue light, or roughly wavelengths from 700 nanometers to about 400 nanometers.

The Sun gives off light (or radiates) at all wavelengths which is why bright sunshine looks white to us – it’s the sum of all the colors. You can split the light into its constituent colors with a prism or you can let nature do it for you with raindrops or ice crystals. Scientists have taken things one step further with spectrographs. These are devices that can disperse the light even further into individual wavelengths, allowing for precise measurements of how much of each color is (or isn’t) there. Ooh, foreshadowing…

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Isaac Newton was the first to find the rainbow hiding in the light, but it wasn’t until more than a hundred years later that the rainbow was discovered to be incomplete. William Hyde Wollaston used a precursor to today’s spectrograph to stretch the sunlight out along and project it onto a screen. At that scale, it became apparent that the colors were far from continuous. Instead, they were broken up by black bands of varying thickness. He theorized that the bands were actually marking where one color ended and another began.

A decade and a half later, Joseph von Fraunhofer proved this wasn’t the case. He passed sunlight first through a narrow slit and then through a prism, which splits the light even further, and projected the resulting spectrum onto a wall. Fraunhofer realized the black bands were inherent to the sunlight and labeled them A, B, C, etc. We now call these lines for our Sun Fraunhofer lines.

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Fraunhofer lines in the visible solar spectrum.

It turns out these gaps in the light can be traced back to the sun’s atmosphere. When photons are born in the nuclear furnace deep below the suns surface, they are created with all energies (or wavelengths) across the visible spectrum. But not all of these photons make the trip to Earth. First they must fight their way to the sun’s surface – being absorbed and emitted and re-absorbed and re-emitted by atoms the entire way. This journey can take anywhere from 10,000 to 100,000 years. Once they make it to freedom, there’s one last hurdle: the solar atmosphere. The sun can be divided up into several layers: core, radiative zone, convective zone, photosphere, chromosphere, and corona.

What we typically think of as the surface of the sun is really where the photosphere begins. And the photosphere is where photons at visible wavelengths do one of two things: stream through interrupted or get absorbed and re-emitted in random directions. The photons that are allowed to travel freely are what we see as color in the spectrum; the ones that aren’t are what we see as those black lines.

The technical term for a gap like this in a spectrum is absorption line. And there is a pattern to them. Photons can only be absorbed and emitted at specific energies related to the atoms they interact with. For a photon to be absorbed, it must match the energy required for a given electron in an atom to jump to a higher allowable energy level (or orbital). Similarly, when photons are emitted, their energy matches the difference between the energy level an electron was at and the level it fell to.

Since electron orbits vary by element, the energy of the photons absorbed tells us what atom absorbed them. We know which lines correspond to which element because we can observe them in a controlled laboratory environment here on Earth. Deducing the composition of the sun is then akin to scanning a cosmic barcode.

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Fraunhofer lines identified, and then some. Credit: Randall Munroe, aka xkcd

It turns out, the strongest Fraunhofer lines result from the presence of hydrogen, calcium, magnesium, sodium, oxygen, and iron (though the oxygen signal is contamination from Earth’s atmosphere affecting the sunlight on its way to our detectors). So where does this get us? If we know what our star is made out of, we can then compare its spectrum to the spectrum of other nearby stars.

The National Optical Astronomy Observatory has made similar images for both Procyon (a somewhat sun-like star ~11 ly away) and Arcturus (a red giant ~36 ly away). Comparing these two images to the image at the top of this article, you can see how many more absorption lines Arcturus has than both the Sun and Procyon. This is because it is much older and going into the end stages of its life – it has burned though all its hydrogen and is now powered by helium fusion.

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High-resolution spectrum of Procyon.

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High-resolution spectrum of Arcturus. See top image for detailed descriptions of both images. Credit for both images: N.A.Sharp, NOAO/AURA/NSF

Astronomers have learned to tease more and more information out of these spectra. In addition to composition, spectra can be used to approximate a star’s age and reveal any interactions it may have had with other stars (for those in binary systems). We can also observe the spectrum of an entire galaxy (as a combination of the spectra of its stars) to determine its age and star formation history. Lastly, we can take advantage of the Doppler Effect which causes the lines in a given star’s spectrum to collectively shift towards longer (redder) or shorter (bluer) wavelengths due to the star’s motion. This can help us estimate the proper motion of stars as they orbit the center of the galaxy. Stars moving together are more likely to be related in age and composition than more isolated ones, somewhat like a group of friends at the ice-skating rink.

I could go on and on, but I’ll save some of that for the next installment of this series. For now, I hope the next time you see a rainbow from a random reflection or refraction from something nearby, one of your reactions besides “ooooooh!” is now “ooooooh stellar spectra!” Because, science.