Forensics of the Cosmos

Imagine a burglar just broke into your house. You call the cops and they start investigating the crime scene. What do they check for in the first place? They check if the burglar left behind any traces of him. They search out for hair samples and fingerprints. Now, how does that help with catching the culprit? Hair samples are an excellent source of DNA of a person and fingerprints being unique for each individual, help the cops narrow down the search process.

Photo by Immo Wegmann on Unsplash

Now, back to Physics class! There are different types of objects that one can find on a clear night sky — stars (single and multiple systems), galaxies, nebulae, asteroids and a lot more. But how do we distinguish them? Can we use the same forensic techniques here? The answer is yes. Every object in this universe has its own characteristics so it is possible to use forensic techniques here. Our main concern in this blog is going to be illuminating objects. You may wonder,

“Do they leave hair samples or fingerprints just like burglars?”

Well, this may be surprising for a few but they actually leave traces of them! What they leave behind is the light they emit.

What’s more surprising is that each star has its own type of fingerprint! By fingerprint, what I mean is the light that we receive from them. But light barely looks like a fingerprint, doesn’t it? Remember the rainbow that we used to obtain from a prism? How do we receive them? We pass white light onto a prism and Booyah! We get a rainbow! In scientific terms, the rainbow that we obtained is called a spectrum. Now, don’t you think we are going to get the same result while doing the same with the light that we get from these objects? After all it is white light too. A German guy named Joseph von Fraunhofer attempted to do this with the light that we get from the sun, the nearest star. What he observed was bizarre. He did get the spectrum but he noticed some black lines too! In short, the light that was supposed to be present at the black spot is missing!

So, who is the culprit that stole off the light?

The light from the sun, on its journey, has to cross a long way before reaching us. The first thing that the light encounters is the sun’s atmosphere, the photosphere. When the light passes through the photosphere, the electrons inside the atoms present in the photosphere absorb this light to gain energy and jump to the higher energy level. Thus, when the light reaches us, we find black lines. The photospheres of each star have its own atomic composition. So, the light absorbed ends up being different and thus it helps us differentiate the stars from each other. Brilliant, isn’t it? They honoured the scientist by naming those lines as “Fraunhofer Lines”. But there comes another issue. Some spectra showed only emission of light at particular wavelengths. Later they found out it was not a star, but a planetary nebula. Soon they started getting various results as the observations went on.

The spectra obtained helped the researchers with determining characteristic features of the stellar object like composition, temperature, density and relative motion. To make it easier for analysis, the observed sources were classified under a catalogue and a plot was obtained with spectral type, temperature, absolute magnitude and luminosity. This plot was then named as Hertzsprung-Russell Diagram, a graphical tool that helps us check where a star must be placed according to the spectral features observed. The final spectral classifications were carried out by Annie J. Cannon which was formally accepted by the International Astronomical Union in 1922 which is still being used. According to this classification, stars were classified as O, B, A, F, G, K, M (in this order), together with P for planetary nebulae and Q for three peculiar stars with bright lines. This is called the Harvard system. The hottest and blue stars fall under O type and the coolest and red stars fall under M. Cannon also formulated a mnemonic phrase to remember this:

Oh Be A Fine Guy/Girl — Kiss Me

for which she was known in scientific circles. See? Physicists are indeed known to be sarcastic for some reason.

To the left: Joseph von Fraunhofer; to the right: Annie J. Cannon

Spectral analysis can also be used in archaeological surveys of stellar remnants to make ground breaking discoveries about the beginning of the universe. Spectroscopy is an efficient tool in tracing back the history as well as the evolution of a cosmic object. If you wish to work as an archaeo-astronomer, the best way to start is by studying spectroscopy. The beauty of astronomy lies in spectroscopy.

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