Date: March 14, 2012
Title: Astronomy Word of the Week: Fraunhofer
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Links:
http://www.usno.navy.mil/USNO
http://astrowow.wordpress.com/
Description: How did a Bavarian glass-maker stumble upon one of the most powerful tools in astrophysics? The word of the week is: “Fraunhofer”.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Sponsor: This episode of the “365 Days of Astronomy” podcast is sponsored in-kind by the Planetary Society.
Transcript:
Did you know that the simple act of shining sunlight through a prism and making a rainbow is the first step towards understanding the chemical history of the entire Universe?
A prism – a triangular shaped piece of glass – is a tool remarkable in its simplicity. Sunlight is composed of many different colors – including colors our eyes can’t detect like infrared and ultraviolet – and prisms allow us to spread out those colors so we can see them. The full range of colors that the Sun – or any source of light – produces is called its spectrum and the science of studying how light gets funneled into different colors is known as spectroscopy. But to better understand what’s going on, it’s helpful to stop for a minute and remind ourselves what light really is.
Light behaves like a wave – ripples in the sea of electromagnetic fields that surround us. And these waves of electromagnetic energy have much in common with more familiar waves, like those on the ocean. If you were to stand on a pier and watch the waves go by, you might notice that there’s a few fundamental things about them you could measure. One would be the frequency of the wave: how many waves pass by you in a second. Another closely related property is the wavelength: the distance from the peak of one wave to the peak of another. As the name suggests, it’s quite literally a measure of how long a wave is. Ocean waves generally have wavelengths of several meters.
The wavelengths of visible light – the light your eyes can detect – are much smaller: only hundreds of billionths of meters. Colors are just the way our eyes perceive different wavelengths of light. When you look at a rainbow, you’re actually looking at how much of the Sun’s energy goes into making different light wavelengths.
The science of spectroscopy dates back to the early 1800’s. In 1814, Bavarian-born glass maker Joseph Fraunhofer invented the spectroscope – a tool for measuring the spectra of different light sources – and turned his invention towards the Sun. He was actually looking to see if the Sun’s spectrum was similar to that of a flame from a candle. It couldn’t have been more different. He of course saw a rainbow – that much is probably not surprising. But he also saw something else: a pattern of narrow dark lines spread across the spectrum as if hundreds of very specific colors – or wavelengths of light – had been removed from the sunlight. In truth, Fraunhofer was not the first to notice this strange phenomenon. The English chemist William Hyde Wollaston saw these lines as well twelve years earlier using a crude ancestor of Fraunhofer’s more sophisticated instrument. Wollaston was only able to see a handful of lines and assumed that these were gaps which separated the “primary colors” of the Sun.
Fraunhofer didn’t see that this was the case and undertook a systematic cataloging of these dark lines. He labeled them with letters of the alphabet and carefully measured the wavelengths at which the light went missing. In the end, he identified over 570 of the mysterious features which in time would come to be known as the Fraunhofer lines.
While Fraunhofer was too much of an experimentalist to muse on what was causing this strange occurrence, he did go on to turn his spectroscope on to some of the brightest stars in the night sky and found that the distant stars showed missing colors in their spectra as well. But the exact wavelengths of missing light were in many cases different from those in the Sun and from each other. Every star seemed to have its own unique pattern of spectral lines, though there were some similarites between stars as well. Though it would be many decades before the nature of these lines would be all worked out, Fraunhofer had stumbled upon something truly profound.
So what was causing these lines? Imagine we have a lightbulb and a prism and we let the light from the prism fall on a sheet of paper. We would see on the paper, a rainbow. Now, let’s take a large, clear container filled with, say, hydrogen gas, and place it between the bulb and the prism. How would this change the rainbow? You’d see that a series of narrow, dark lines would appear superimposed on the rainbow as if light from very specific colors had suddenly gone missing – exactly what Fraunhofer saw in his spectroscope! Without getting into the quantum mechanics of it all, the hydrogen gas has the ability to absorb very specific wavelengths of light. Astronomers refer to these lines as “absorption lines”.
If you replace the hydrogen with helium, the lines you saw previously would vanish and a new set of lines would appear. Any element you put in the container – sodium, calcium, oxygen, nitrogen – would create its own unique pattern of absorption lines; essentially a spectral fingerprint. The real power of this remarkable phenomenon is that if you had a container filled with some unknown mixture of gasses, you could pass light through it, look at the absorption lines in its spectrum, and figure out exactly what was in the container.
What Frauhnofer was seeing in his lab nearly 200 years ago was the chemical fingerprint of the Sun’s atmosphere. Light generated by thermonuclear reactions deep in the core of the Sun had to pass through a layer of gas surrounding our star before escaping into space. That gas imprinted on the light it’s chemical makeup by absorbing specific wavelengths. That light then traveled the nearly 100 million miles to Fraunhofer’s spectroscope on Earth where the hydrogen, helium, sodium, calcium, iron, and so on of the Sun’s atmosphere revealed itself by the presence of tiny dark lines on a rainbow.
Fraunhofer had inadvertently given birth to the science of stellar spectroscopy. By attaching a spectrscope to the end of a telescope and matching the absorption line patterns to those of elements measured in labs here on Earth, astronomers could now identify every element in any star, galaxy, planet, or cloud of gas in the Universe – we could precisely measure what the Universe is made of! Helium, in fact, was first discovered not here on Earth, but by unidentified absorption lines in the Sun’s spectrum seen during a total solar eclipse in 1868. The name of the element was taken from the Greek word for sun: helios.
By viewing the Universe through the lens of spectroscopy, the distant stars and galaxies were no longer indistinct points of light, but real physical objects each with a unique and complex tale to tell. Detailed observations of the spectra of celestial objects reveals not just chemical composition but also a diverse array of parameters like gas pressures, temperatures, surface gravity – even how fast objects are moving. With this tool, we have discovered the expanding Universe, teased out the subtle signatures of planets orbiting other stars, uncovered the chemical makeup of stars and nebulae, traced the motions of galaxies, revealed the existence of the enigmatic dark matter, reconstructed the chemical evolution of the Universe, and measured the distances of galaxies sitting at the very edge of existence. Fraunhofer brought the ability to unravel the underlying mechanics of the Universe within our reach!
End of podcast:
365 Days of Astronomy
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