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.

This is the visible spectrum of the light from the sun. And if you have played with white light and prisms before, it might come as a huge surprise to you to know that the spectrum from the sun is actually not continuous.

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Why is it not ? The dark patches in the above spectrum arise from gas at or above the
Sun’s surface absorbing sunlight emitted below.

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                                               Source

Since there are different types of gases that compose the sun, there are numerous wavelengths of light that get absorbed by these gases.

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                                         Source: xkcd

How do we know which line corresponds to which ? Well, it’s because we have a periodic table, and we know the spectrum of all the elements in it:

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                                               Source

And then it’s a matter of solving the jigsaw puzzle of fitting the spectrum with the tiles that you have. When we do so, we obtain the following composition of elements:

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                                   Source: Earth Blog

We can even take it one step further by finding the composition of other neighboring stars as well.

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                                  Source: Potsdam University 

All of this information about the star can be captured from a simple spectrum. And this is why one of the most important tool that an astronomer has about an object is it’s spectrum.

Have a good one!