We Can See What Stars Are Made of

Super fact 101: We can see what stars are made of and calculate their speed and distance compared to us just by looking at their light.

The picture shows the spectrum visible to humans as a horizontal bar at the top. This spectrum is superimposed on a wider spectrum below as a thin rainbow colored strip. The wider spectrum is also placed horizontally and goes from gamma rays to radio waves.
The visible color spectrum. Sunlight wavelength and increasing frequency vector infographic illustration. Visible spectrum color range. Rainbow electromagnetic waves. Educational physics line. Shutterstock Asset id: 1933622132 by Shutterstock Asset id: 1933622132 WinWin artlab.

Electromagnetic radiation, including visible light, features a lot of different frequencies, which for light correspond to different colors. Red has a lower frequency, and blue has a higher frequency. For more information about the electromagnetic spectrum and light see Human Vision Only Detects a Sliver of the EM Spectrum.

If you place a light bulb (white light includes a spectrum of colors) in front of a container filled with hydrogen and then you let the light pass through a prism, it will split into the different colors (red, yellow, blue, etc.) forming a rainbow, as prisms do. You see this every time you see a rainbow in nature. However, if you measure carefully, as in the experimental setup below, you will  notice that some frequencies are missing. That’s because the hydrogen gas will absorb certain frequencies.

This is true, not just for hydrogen, but for all elements/atoms and molecules. Atoms and molecules have light absorption patterns that are unique to the atom/element in question. The dark lines in the spectra are referred to as Frauenhofer lines. In a sense, all elements have a thumb print resulting from their so called absorption spectra. This makes it possible to identify the elements in a star and their proportions.

On the left a light bulb generating white light, which is split into a rainbow by a prism. On the wall behind the prism you’ll see all the colors, except a few frequencies indicated by black lines, will be missing.
Absorption spectrum / Frauenhofer lines of the hydrogen atom. Shutterstock asset id: 1305568666 by Emir Kaan

Before the discovery of absorption spectra, it was sometimes believed that humanity would never know the chemical elements of stars. Even today many people are surprised to hear that we can indeed know what stars are composed of just by looking at their light. In addition to identifying elements in the upper layers / atmosphere of stars you can use the same absorption spectra to determine the relative velocity of stars compared to us and sometimes the distance to the stars.  This is an old discovery that is surprising and important to our understanding of the world and therefore a super fact in my opinion.

Examples of Absorption Spectra

An interesting, related fact is that Helium was discovered in the Sun before it was found on Earth, with the help of emission spectra (August 18, 1868). Emission spectra are the opposite of absorption spectra. It should be noted that while emission spectra are commonly used for identifying the composition of interstellar gas, absorption spectra are commonly used for identifying the composition of stars.

Absorption spectra showing the colors of visible light with black lines characteristic for Helium.
Absorption and Emission Spectrum of Helium Shutterstock asset id: 1724296909 by MoFarouk
Absorption spectra showing the colors of visible light with black lines characteristic for Carbon.
Absorption and Emission Spectrum of Carbon Shutterstock asset id: 1725934867 by MoFarouk
Absorption spectra showing the colors of visible light with black lines characteristic for the sun.
Solar spectrum with Fraunhofer lines as it appears visually. The solar spectrum is a combination of spectra from multiple elements: nl:Gebruiker:MaureenVSpectrum-sRGB.svg: Phrood~commonswikiFraunhofer_lines_DE.svg: *Fraunhofer_lines.jpg: Saperaud 19:26, 5. Jul. 2005derivative work: Cepheiden (talk)derivative work: Cepheiden, Public domain, via Wikimedia Commons.

Using Redshift to Determine how fast Stars are Moving Compared to us

A star or galaxy that is moving towards us will have a blue shifted spectrum. A star or galaxy that is moving away from us will have a red shifted spectrum. Red shifted means that the absorption lines have moved towards red because the frequency of the light has been shifted due to the motion.

This is called the doppler effect. You can notice this phenomenon for the case of sound when an ambulance is coming towards you and then speeding by you. The sound changes. The astronomer Hubble was using redshift to the determine that the further away a galaxy was from us the faster it was moving away from us. Measuring the redshift of a faraway galaxy or star, not only tells us its speed compared to us but can indirectly help us determine the distance to the galaxy or star.

The top shows a colorful spectrum from blue to red with absorption lines in black. The bottom portion of the picture shows the same thing expect the black absorption lines have moved a bit to the right.
Visualization of redshifted absorption lines are redshifted due to velocity away from observer. Top lines are for an object at rest and in the bottom picture the object is moving away. Maxmath12, CC0, via Wikimedia Commons. This file is made available under the Creative Commons CC0 1.0 Universal Public Domain Dedication.

There are other things you can tell from the light from stars and from their electromagnetic spectrum such as the type of star, the size of the star, and the age of the star, but that is for another post. This post is long enough.

To see the other Super Facts click here

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Author: thomasstigwikman

My name is Thomas Wikman. I am a software/robotics engineer with a background in physics. I am currently retired. I took early retirement. I am a dog lover, and especially a Leonberger lover, a home brewer, craft beer enthusiast, I’m learning French, and I am an avid reader. I live in Dallas, Texas, but I am originally from Sweden. I am married to Claudia, and we have three children. I have two blogs. The first feature the crazy adventures of our Leonberger Le Bronco von der Löwenhöhle as well as information on Leonbergers. The second blog, superfactful, feature information and facts I think are very interesting. With this blog I would like to create a list of facts that are accepted as true among the experts of the field and yet disputed amongst the public or highly surprising. These facts are special and in lieu of a better word I call them super-facts.

33 thoughts on “We Can See What Stars Are Made of”

  2. So interesting! Thank you Thomas for this excellent presentation. I did not know any of this, I assumed the stars were so far away that specifics on their composition was impossible. Amazing how somethings like color, prisms and the doppler effect, that are known ways to explain everyday occurences of light and sound, can be used to determine information among the stars. Great share as always I learned some new things.

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    1. Thank you so much for your kind words Suzette. I think most people are unaware of the things that we can tell from star light and the rest of the electromagnetic spectrum. It is surprising how much we can know.

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  3. So excited by this post, Thomas! My son was telling me about the electromagnetic spectrum recently, and he will be truly fascinated by this post, especially when you talk about the absorption spectra used for identifying the composition of stars. Thanks for sharing!

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    1. Thank you so much Ada. Your son is amazing. I should add that there are other things you can tell from the light from stars and from their electromagnetic spectrum such as the type of star, the size of the star, and the age of the star, but that is for another post.

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    1. It is interesting and you can determine the speed of the ambulance compared to you based on how much the sound changes. The same is true for stars, but with light. Thank you so much Robbie.

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    1. Yes you are so right Craigavad. Also I just added this “There are other things you can tell from the light from stars and from their electromagnetic spectrum such as the type of star, the size of the star, and the age of the star, but that is for another post. This post is long enough.”

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  7. This is very interesting, Thomas! For one thing, I have always been fascinated by colors, color theory and the physics of color… and it is so nice to find out that this is how we can determine the composition of stars and how they travel. 🙂 And speaking about colors, I remember a while ago I found out a quite funny fact about the color pink, that it somehow “doesn’t exist”, being a non-existent color? Better said it is like a mental construction, they say, obtained from “white minus green” which gives our brains the perception of “pink”. I still can’t seem to wrap my mind about it too well, but I understand “the idea of it” in theory.

    In fact… I feel exactly the same as when I try to find reflections and photograph them, on the surface of a lake or a puddle after rain. More often than not I am left with that sensation that… ok, the mountain or the trees are high up over there, and the surface down here, so… somehow that reflection exists only because I observe it, standing opposite the respective subject across that water surface from it, if you know what I mean. 😅 Funny thing… quite often when I take reflection photos I end up exclaiming something like this: “Oh my… where exactly is that reflection? That reflection actually doesn’t exist unless I am here to “see” it?!” ☺

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    1. Thank you for those very interesting facts and observations. I did not know about pink. That is interesting. What you say about reflections is very interesting. It is the same with a rainbow. There is no specific location of the rainbow. If two people are standing next to each other they see it in the same place. But then if one walks away the rainbow will move from his perspective while the guy still standing in the old spot will see it in the same place as before.

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        1. I just read your other post and left a comment. I am amazed that it took the comment despite the post being 10 years old. Sometimes that does not work. Anyway, it was very interesting to learn that pink is the absence of green.

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  9. Another wonderful share, Thomas. I didn’t know any of this, and now my evening gaze is enriched with understanding. 😊 Thank you!

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    1. Yes you are right Priscilla and it reminds me I haven’t been at the darksite in Atoka/Oklahoma in a while. Jacqui also suggested another Astronomy superfact that stars twinkle but not planets. Thank you so much Priscilla.

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    1. Yes you are right Java Bean. We can figure out a lot from the starlight and I only told half the story. It is also amazing all the things that dogs can figure out just by sniffing the ground and each other and people.

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    1. Yes I agree, it is amazing. In addtion to their chemical composition and motion compared to us, we can also figure out type of star, size and age from the starlight. Thank you so much for your kind words Debbie.

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