H-alpha line in galaxies/ hubble redshift

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As an assignment I am trying to calculate the hubble redshift by looking at spectra of galaxies...
Obviously the very first thing I want to do is find a suitable emission line...i am trying to find the halpha line in all the spectra I am analysing.
First off, there is a very large spike in the spectra I am looking at at a wavelength of around 650nm and I am guessing that this must be the halpha. But my question is, why is it that this halpha line should definitely be an emission line as opposed to an absorption line?
 

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cepheid
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As an assignment I am trying to calculate the hubble redshift by looking at spectra of galaxies...
Obviously the very first thing I want to do is find a suitable emission line...i am trying to find the halpha line in all the spectra I am analysing.
First off, there is a very large spike in the spectra I am looking at at a wavelength of around 650nm and I am guessing that this must be the halpha. But my question is, why is it that this halpha line should definitely be an emission line as opposed to an absorption line?
The Halpha line is seen in emission because it comes from H II regions. H II is weird astronomer/spectroscopist notation for ionized hydrogen. H I is neutral hydrogen. Similarly, He I = neutral helium. He II = singly-ionized helium. He III = doubly-ionized helium. But I digress.

H II regions are what are also known as emission nebulae. Sources of ionizing (e.g. UV) photons such as hot O and B type stars create shells of ionized gas around them, and this ionized gas glows as excited electrons recombine with their atomic nuclei and cascade down the energy levels (so the glowing is therefore due to atomic emission lines). Since this interstellar gas is mostly hydrogen, you see the hydrogen recombination lines, with Halpha being the most prominent of these.
 

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