A note on the Hydrogen spectrum

The emission spectrum of atomic hydrogen is given by this amazing spectral series diagram:

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Let’s take a closer look at only the visible portion of the spectrum i.e the Balmer series.

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If a hydrogen lamp and a diffraction grating just happen to be with you, you can take a look at the hydrogen lamp through the diffraction grating, these lines are what you would see:

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Source

These are known emission lines and they occur when the hydrogen atoms in the lamp return to a state of lower energy from an excited energy state.

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           Representation of emission and absorption using the Bohr’s model

Here’s another scenario that could also happen:

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You have a bright source of light with a continuous spectrum and in between the source and the screen, you introduce a gas (here, sodium)

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Source: Harvard Natural sciences

The gas absorbs light at particular frequencies and therefore we get dark lines in the spectrum.

This is known as absorption line. The following diagram summarizes what was told thus-far in a single image:

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The absorption and emission spectrum for hydrogen look like so :

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Stars and Hydrogen

One of the comments from the previous post was to show raw spectrum data of what was being presented to get a better visual aid.

Therefore,the following spectrum is a spectrum of a star taken from the Sloan Digital Sky Survey (SDSS)

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                                 Plot of wavelength vs median-flux

Here’s the spectrum with all the absorption lines labelled:

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Source: SDSS

You can clearly see the Balmer series of hydrogen beautifully encoded in this spectrum that was taken from a star that is light-years away.

And astronomers learn to grow and love these lines and identify them immediately in any spectrum, for they give you crucial information about the nature of the star, its age, its composition and so much more.

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Source: xkcd

Have a great day!

*If you squint your eyes a bit more you can find other absorption lines from other atoms embedded in the spectrum as well.

On the strong 5577Å spectrum line


The above is a plot of the Wavelength(in Å) in the x-axis vs the flux of some objects from the Sloan digital survey ( consists of galaxies, young stars, Quasars, etc)

But  there is one strong peak in all of those plots that seems to stand out: 5577 Å

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And if you like, the color that it represents is the above  (Made with Stanford’s color matcher app)

A nightmare for the astronomer

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This line at 5577.338 is what astronomers refer to as a ‘skyline emission’ or a ‘mesospheric night-glow’ and arises from the recombination of atomic oxygen in the mesosphere.[2]

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                                               Source

This line is of no significance to an astronomer who is looking to find out properties about a far away astronomical object. Yet, this line pops up in every spectrum of any object that you look at in outerspace!

In addition since the line is so strong, it contaminates the nearby pixels making the nearby data unusable and also messes up the scaling of the plot.

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Example of contaminated pixel columns in an image because of bright object

Wavelength Calibration

What do you do with something that is always there but has no use for you? – Re-purpose it!

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Having noticed that this peak was consistent at 5577.338, Astronomers decided that they would use this peak line in the data as a reference to calibrate their actual data. (known as ‘zero-point correction’).

This ensures that all the spectrum lines in the data are aligned and any errors that might have occurred during observation are corrected for.

Other lines ?

There are other lines at 6300,6363, etc which are sometimes as bright or brighter than the 5577 line that are also used for calibration.

If you are interested in learning more, the following are three papers that this post was inspired from and they dive deeper into more technical details that underlie this fascinating topic:

[1] Night-Sky High-Resolution Spectral Atlas of OH and O2 Emission Lines for Echelle Spectrograph Wavelength Calibration

[2] Mesospheric nightglow spectral survey taken by the ISO Spectral Spatial Imager on ATLAS 1

[3] Variability of the mesospheric nightglow sodium D2/D1 ratio

Have a great day!