Learning! — Fraunhofer Lines

Instead of going over writing advice (as has become the norm), I’m going to talk about something I learned very recently, and that I find fascinating.

Have you ever wondered how we could possibly know what stars are made of? Or how hot they are? Or whether or not they are coming towards or away from us? What about their magnetic field or their rates of rotation?

We can know all of that because of a simple little thing called Fraunhofer lines.

When Sir Isaac Newton was doing his thing, inventing gravity and science and all that, he of course observed the visible spectrum by shining a beam of light through a prism that separated it. He observed that red light waves have a lower frequency and a longer wavelength, whereas violet light has high frequency and a short wavelength.

Centuries later, in 1814, Joseph von Fraunhofer saw that with thorough inspection, the visible spectrum had several black lines going through it, as if there were little pieces missing.

Eventually, he found that these lines were missing in accordance with what the light was made of. If the light was coming from an object made of sodium, for example, two lines in the middle of the yellow spectrum would always be missing. If the object was composed of sodium and magnesium, the lines in yellow would be missing in addition to several lines in the green spectrum. Any object composed of specific colors will always express the same lines in the visible spectrum in the same places. With thorough inspecting of the types of light a star emits, that is how we can tell what it is made of.

But there’s more to it than that. Because light experiences the Doppler effect, a process that shortens or lengthens wavelength based on whether something is coming towards or moving away from the observer, we can also use this here. If we observe that a star contains sodium and magnesium, we will observe their respective lines. But if these lines are shifted left or right of where we would expect them to be, we can use the Doppler effect to measure what direction a star is moving relative to us. If the lines are slightly further towards red (“red-shifted”), we know the star is moving away from us. If the lines are closer to the blue side of the spectrum (“blue-shifted”), we know the star is moving towards us. We can even tell which direction a star is rotating by using the Doppler effect on different areas of the same star.

Fraunhofer lines tell us so much about the world around us that they have, in a sense, singlehandedly birthed the science of astrophysics. Since every object will have distinct chemical signatures, we have been able to use them to analyze and learn why the universe is the way it is in an all new perspective. Using them has told us most of what we know about distant stars and galaxies, and without them we wouldn’t even know the universe is expanding at all.

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