All the Stars in the Sky 5/14/21

Science with Richard Bleil

This post may be a bit ambitious. Last night, as I was trying in vain to fall asleep, I started thinking about the stars, and thought it might be interesting to explain a few things to my readers about how some of the things we know about stars is possible.

Basically, I want to talk about what would be called “Atomic Emission Spectroscopy” in an analytical chemistry lab. Believe it or not, you’re probably familiar with the concept, although you may not be aware of it. A spectrometer breaks down incoming light into its component wavelengths (colors) and analyzes each. When I was in high school, I bought my mother a quartz lead glass ball (back when they were just becoming available) to hang in the window. As light hit it, it threw tiny little rainbows all over the kitchen where she had hung it. That’s what I mean when I say it “breaks down incoming light”. Although it uses something other than a prism, it’s the same basic concept.

As it turns out, different elements have very specific and unique light spectra. In other words, they burn in different colors. You’ve seen this. If you’ve ever tossed a salt in a campfire, you’re performing atomic emission spectroscopy. You might not be measuring the exact wavelengths, but you know, for example, sodium chloride burns orange, and copper sulfate burns green. If you’ve never done this yourself, if you’ve seen fireworks, they do the same thing. They add a bit of salt to the explosive, so they are different colors as they burn. They might add copper sulfate, or lithium chloride (red), potassium chloride (purple), or even magnesium sulfate (white).

By analyzing the color of objects hot enough to give off light, astrophysicists can ascertain the chemical composition of celestial bodies. But it’s a bit more. This same technique can be used to determine if the objects are getting closer, or further away, and how fast they are moving relative to us. This part gets really freaky.

See, these spectra are highly precise, and our instruments are capable of measuring tiny variations in the wavelengths we are measuring, only, there should be no variation. At least not if the object is stationary, but they are in motion, as we are. Most people know that the fastest speed possible is the speed of light in a vacuum, approximately (although slightly less than) 3,000,000,000 meters per second. Here’s the freaky part, though, if we are traveling towards a star, or away from it, this speed does not change. Even adding or subtracting our own speed to this, the speed in which the waves reach us is exactly the same. It’s fixed such that even relative speed does not change it (thanks a LOT, Einstein!). What does happen, however, is a shift in the frequency of the light. Frequency is the number of waves that pass a given point in any given second, and frequency is related to wavelength, and wavelength is related to color. Astrophysicists call this “red shift”, or “blue shift”. Acoustic engineers call it the Doppler effect, and again, you’re familiar with it.

See, sound does the same thing. The sound waves strike our ear at a constant velocity, but the frequency varies. The familiar example used is a train blowing its whistle as it passes. Not around here, of course, because the trains all move very slowly, but in a fast-moving train it’s very noticeable. If the whistle is blowing, as it passes, the pitch will have an obvious drop. It’s a high pitch as it approaches, and low pitch as it moves away. This is because the trains velocity is added to the sound waves as it’s moving towards us, so more waves strike our ear per second giving it a higher pitch. As it’s moving away, the velocity is subtracted so fewer waves strike our ear per second resulting in a lower pitch.

In light, it’s not pitch, be frequency (color). If the star is moving towards us, the velocity of the star is added to the light, and more waves strike us per second from the star. This is a higher frequency, shifting the light towards the blue end (a blue shift) of the spectrum. If the star is moving away from us, the velocity of the star is subtracted resulting in fewer waves striking us per second, or a lower frequency shifting the light towards red (a red shift). We cannot detect these differences with the naked eye, but our instruments are sensitive enough to see the shifts.

By analyzing the color of the light, we can both ascertain the elements present in the star (or object), and by looking at the variations from what we expect, we can measure the velocity relative to the earth. I hope you found this post enlightening. Sorry, that pun was only worth one star.


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