Astronomy For Beginners: Spectroscopy

Spectroscopy is one of the core techniques for understanding distant stars. We can not directly see these stars well enough to know much about them. Thanks to the fundamentals of light and how it behaves, we can collect the radiation from objects and infer many details from them. How spectroscopy is used and its definition is the subject of this article.

 

Spectroscopy

The study of spectral lines emitted by objects is spectroscopy. Objects emit radiation and an instrument called a spectrascope can analyze it. Almost objects will give off radiation which is just another way of saying energy.

 

Spectral Lines From Radiation

When you analyze spectral lines you are creating a spectrum. It is the division of the radiation's component wavelengths. The reason we want to analyze spectra is that it will give us a lot of information about the object that originally emitted the radiation.

The tool used to analyze radiation is the spectroscope. It is one of the core instruments of spectroscopy. Its main component is a prism that splits the light into its wavelengths. There are other smaller parts that help refine the process. You usually look through an eyepiece like on a telescope.

 

Emission Line Spectra

Emission lines are what we see when we split the radiation into parts. Keep in mind when you do this, only the more prominent parts of the spectrum will show up. This is what characterizes each object, these different emission lines for every object. When we repeat this process we get the same type of emission lines each time. If we analyze light from another source then the lines would be totally different.

 

spectroscopy
spectroscopy

 

Absorption Line Spectra

When looking at an object through a spectrascope you should see black lines throughout the spectrum. These black lines are actually gaps. They represent wavelengths that should be there but are not. Something happened to them so they did not show up. These lines are officially called Fraunhofer lines or absorption lines and are a key ingredient to the study of spectroscopy.

 

Kirchhoff's Laws

A German physicist named Gustav Kirchhoff noticed there were similarities between types of spectra. He had a few famous laws.

  1. An object under high heat and pressure gives off a continuous spectrum.
  2. A gas with high heat and low pressure gives off an emission line spectrum.
  3. An object that emits a continuous spectrum and is behind a cool gas under pressure exhibits an absorption spectrum.

 

Decomposing Starlight

In the last couple hundred years the field of spectroscopy has developed quickly and so scientists had several ideas on how to read starlight. One of the most important things that was learned was that the lines on a spectrum corresponded to individual elements. So developing a spectrum for an object meant that we could discern its composition.

 

Atoms and Radiation

Before modern times we thought everything behaved as a wave. It made sense most of the time and was assumed to be true for a long time. Eventually, however, our technology improved as did our instruments and theory. There was something missing, something not quite right. The answer we now know is that light does not behave purely as a wave but as a particle.

Understanding Atomic Properties

Atoms are the basic units from which everything is created. Even at their small size they are affected by radiation. Atoms absorb radiation which in turn changes the atom. Historically the electrons in an associated atom were thought to be on precise orbits, kind of like planets. Scientists now think that it is a very loose orbit around the nucleus of an atom. Some may even be scattered about.

 

Radiation As Particles

Electrons are only found in atoms having a certain transfer. This goes whether they are absorbing or transmitting energy. So the radiation involved in these transactions must correlate to the difference in their separate states. These defined transactions are known as photons. They are particles we now know. A photon is electromagnetic radiation. According to Einstein, the energy of a photon was about the same as the frequency of the radiation. This concept was later worked on in depth by Max Planck and is known as Planck's constant.

 

Spectral Lines 

spectroscopy
spectroscopy

Every element has its own signature on its spectrum. They can be very simple and easy to organize or can be plain bewildering.

 

Hydrogen

The spectrum of hydrogen covers most of the electromagnetic spectrum. When electrons absorb radiation they then have more energy. Electrons do not retain this radiation though. They will lose it. The question is how fast. If it is lost fast then it will emit a single photon with the excess energy but if lost slowly it will emit two photons with the absorbed energy in both. Photons during this process can have differing amounts of energy obviously. So each photon will be at a different wavelength. This is why the spectrum of an object has many lines at different frequencies and wavelengths, the photons that were releases after absorbing energy were each given a different amount of radiation or energy.

Carbon And Later Elements

An atom with more protons will have a much more complicated emission spectra. The reason why is that there are many more electrons that can get charged and developed into an active state. Each one could get a differing amount of radiation and so give off emissions at a different frequency. Every different frequency will make the spectra taken from the atom look different. So you can see how different and complicated this can be very quickly. Carbon and later atoms of course have larger and larger amounts of protons and electrons.

Molecular Spectra

What happens when different atoms join together and make a molecule? Well, things just get interesting. Those different atoms do not stop emitting radiation. They are all doing it at the same time or just a few at once. It depends on what happens with their electrons and what particular atoms are involved in the first place. Since a molecule is just a bunch of different atoms held together by a chemical reaction between their respective electrons, we can expect their to be many more rules about how they interact. The spectra that is emitted from various molecules is therefore a combination of those atoms that make up the molecule.

 

Analysis Of Spectra

Now that you have sort of an idea of how all of this works, what is exactly is the point you may be asking? The very interesting and valuable point is that you use the emission spectra to infer critical information from what you are observing. This can be a star, galaxy, black hole, or planet. Almost everything we know from space is from these studies of radiation that reach us here on Earth.

 

Temperature Of Objects

Measuring the temperature is one of the important tasks for an astronomer. Knowing the temperature alone gives us a little information but put it together with known atoms and molecules and we get a much better picture of what is going on.

Speed Of Objects

The red-shifted lines of an object can tell us its velocity. This is done by the well known Doppler effect.  Looking at the source radiation and checking for either red-shifted or blue-shifted lines can tell us a lot. If they are blue-shifted then the source is moving toward us. Red-shifted lines means the source is moving away from the observer.

Width Of Spectral Lines

Even the width of these spectral lines can tell us a lot about what is going on. The width is an indication of the environment of the source. Most often the velocity tells a lot. A higher velocity will usually mean a higher temperature too. Magnetic fields can also broaden spectral lines. The stronger the magnetic field the more broad the line usually is.

Conclusion

Obviously I am only hitting the basics from each of these sub-topics. There is a lot more to each of them. Examples and more detail is certainly possible but not possible in my vision for this article. This article is designed to give you the basics of each of these topics. I hope you will find one of them very interesting and inspire you in some way. Let me know in the comments if anything in particular is especially fascinating to you or if you have any questions and I will do my best to answer them.