How light informs us about stars and galaxies
Because they are invisible, we tend to forget particles are the building blocks of everything. It’s crazy to think such minuscule, secretive objects are so fundamental to all aspects of today’s science. And yet here they are, teaching us about everything in our elegant universe.
Today and for the duration of this article, photons, the particles of light, will help you complete a mission you and your team of astronomy researchers have been assigned. Using your knowledge of light, you will have to decipher a mysterious star’s composition and determine a galaxy’s distance from our rocky world.
What techniques to use? How to interpret the results? Let’s go on a journey to learn about how the infinitely small is used to understand the most massive structures of our reality.
The stuff of stars
The first thing you’ve wanted to know when you detected this new solar system is what elements the main star was made of. Fortunately, there is a technique called spectroscopy that can help you with that. Here is how it works.
Take any object in your surrounding. It has a color (shocker, right?). Besides that obvious statement, though, there is something interesting. Why does it have a color? That’s because everything as you probably know, white light is formed of all the colors of the rainbow. It encloses all of the different colors’ wavelength. When light hits an object, that object will absorb some of those wavelengths corresponding to a color and reflect some others. To know which elements a star is made of, you have to take advantage of that property.
If you capt this light that’s being reflected back using a spectrometer, you will observe a pattern of black stripes on a multicolor band. How exactly is this done? After the light from the star passes through a narrow opening in the spectrometer to make the observation clearer, it hits a diffraction grating. This is a component of the spectrometer that separates visible light into all the colors of the rainbow. Well, almost all of them. If you observe a particular element, let’s say hydrogen, you’ll see those black lines I mentioned appear on that rainbow band. This is because hydrogen will have absorbed the light that had the wavelength corresponding to those colors. Here is what it looks like:
This pattern of black lines, called an absorption spectrum, is different for every element of the periodic table, and for every compound as well. That spectrum can then tell you a lot about a star’s chemical composition. But only so much.
When scientists have an idea of what elements might produce these black lines, they test their hypothesis by recreating that mixture of elements in a controlled environment here on Earth. They then observe that light being reflected by it through a spectrometer, just like for the star. If the patterns of black lines match, they have got their data point, and so do you and your team of researchers.
A galaxy’s location
Next, you wonder how far away from Earth is that galaxy you’ve spotted. The phenomenon called redshift will help you with that here. But what exactly is redshift?
Redshift is what happens to stars the further they move away from us. Conversely, blueshift describes the behavior of stars that move towards us. Truthfully, it’s not exactly about the actual star. Once again, it has a lot more to do with light, as well as waves and the Doppler effect.
You frequently experience the Doppler effect in your everyday life. Have you ever noticed that shift in pitch when a police car’s siren passes you by? That is due to the sound waves being emitted at regular intervals as the car moves, which get them closer together. As a result, the sound you hear while the vehicle is approaching is higher than the one you hear after you end up behind that car. Indeed, the sound waves reaching you then are on the contrary spread out, so lower than they’d be if that car weren’t moving.
The same principle applies to light. Light is both a particle and a wave and acts sometimes like the former, sometimes like the latter. When we talk about redshift, we are interested in the waves light creates. As you may know, red light has a longer wavelength than blue light. As the light from the star is traveling away from our earthly eyes, the same thing happens to it than what happened with the police siren’s sound. The light waves get spread out as the star moves deeper onto the vast universe. This causes its light spectrum to shift towards the red end. What do I mean by that?
Remember how scientists have to recreate the absorption spectrum of a star here on Earth? Well, when they do that, the pattern will indeed be the same, but it might have slightly shifted towards the right compared to the star’s spectrum. That means the light pattern from that star has shifted to the left, towards the red end of the spectrum, meaning the star in this example would be moving away from us. Here is an image to illustrate the shift:
You can’t get a star’s position using only redshift, though. Because the universe is expanding constantly, and the observed star’s light was emitted at a certain point during its course, by the time that light reaches us, that sun is even further away from what the light indicates. To get the actual position of a star, you have to account for that fact using a formula that is beyond the scope of this article. For this reason, I will not explain it, but here it is anyway for those of you who are curious.
A final word
After you have finally gotten the two data points you were looking for, you realize knowledge doesn’t just happen magically. Nothing is as simple as we are frequently led to think, and every step of the way to discovery is paved with principles that dictate absolutely everything. It is precisely this incredible complexity that makes it so impressive, and only when we start being aware of this complexity can we begin to truly appreciate the beauty in our universe.