Stellar Fingerprints: How Do We Know What Stars Are Made Of? by Lanz Lagman



Stars have fascinated us since we first walked on our legs and discovered how to make flames. These little jewels floating in the sky, so far away that they seem to be beyond our grasp, and forever out of our reach. As the nursery rhyme goes, 

Twinkle, twinkle, little star
How I wonder what you are
Up above the world so high
Like a diamond in the sky
Twinkle, twinkle, little star
How I wonder what you are

For the curious, the most striking line might be, "How I wonder what you are". What are stars anyway, and what are they made of? More importantly, how do we know what they are made of?

Scientists - chemists, especially - determine the composition of particular substances through spectroscopy. It basically involves projecting light towards a sample, then analyzing their interaction by interpreting the resulting spectra.

Wait, what? 

Concepts of quantum mechanics can indeed be hard to understand, and even harder to explain in simple terms. Nature can be tricky to comprehend, as we gaze into it at the smallest of scales. To make things easier, let's break this article up into smaller bits!

The Nature of Light as a Wave

Stars emit light, but we can't scoop star stuff directly. Not only do we lack the technology to go near one, it's also impractical since any instruments would be destroyed once they get close to the surface of a star. This is why we're limited to analyzing the light that comes from them. Fortunately for us, that's quite enough.

First, let's discuss the nature of electromagnetic waves. Light, as it's commonly known, exists in two natures: as a wave, and as a particle (called a photon), at the same time! Weird, right? For most of our discussions, however, let's focus more on light as a wave. Light as a wave, is a combination of electric and magnetic waves - why they're called electromagnetic waves.

This is how an electric wave (red) and a magnetic wave (blue) form the EM wave.
Image credit: NASA

Throughout our discussion, we must keep in mind this fundamental relationship:

The wavelength. 
Image credit: NASA

What can we learn from this relationship? The longer the wavelength, the less energetic the EM wave is. That's simply because the larger the divisor, the smaller the quotient. The opposite happens when the wavelength is shorter. In scientific jargon, the energy of light is inversely proportional to its wavelength. 

What we've learned from this section would prove to be crucial soon. Now, let's discuss light at different wavelengths.

The Electromagnetic Spectrum: Light at Different Wavelengths

The EM wave's energy depends on its wavelength, and as it travels, it loses energy. Thus, its wavelength increases over time and distance from its source. The range of how long a wavelength can be is only constrained by our universe; a wavelength could theoretically be as long as the entire universe, or even shorter than atoms. 

In order to make it easier for us to understand its entirety, we must divide it into parts by various ranges. Now, we'll introduce the electromagnetic spectrum: 

The electromagnetic spectrum.
Image credit: NASA


Our eyes only allow us to see within that very thin section of the spectrum, and hence, wavelengths that are too short or too long won't be detected by our eyes. This range will be called the visible range. Notice how short its range is; it's like having a strip of bacon, but only being allowed a slice thinner than an atom (which really sucks if you're not vegetarian).

Yeah, it's analogous to this situation.
Image credit: Explosm.net
Our discussion however, will proceed and focus mostly on the visible range, since astronomers observe and apply spectroscopy in this specific region when determining the composition of stars.

Hopefully these make things about the EM wave clear. We'll now discuss how they interact with atoms in Part 2 of "Stellar Fingerprints".

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