When we spread out the light from a source into a rainbow, we can reveal information about its chemical makeup. This is how we understand the spectral signatures that reveal that stars have different temperatures. But to learn about the objects that we study in space, whether they be stars, interstellar gas, or galaxies, we first have to know something about the chemical properties of the elements that make up these objects. And one of these elements is, by far, the most important to study: Hydrogen.
Why is hydrogen the most important element to study in astronomy? Primarily because it is the most abundant. If you count the number of hydrogen atoms in all of space (stars, gas, and galaxies), it can be shown that nine out of ten atoms in space are hydrogen. The second-most abundant is helium, which makes up almost one out of ten atoms in space, and every other element is present in only trace amounts — which is not to say that they are unimportant! But in this post, we are going to focus on the big one.
Hydrogen is also the simplest element on the Periodic Table, as the above diagram shows — one lone proton being orbited by one electron. And like all elements, hydrogen is able to absorb and emit light of certain wavelengths. If the electron is hanging out in the ground state (the n=1 state), it can absorb photons that will shimmy it to the n=2, n=3, n=4, etc. state (and there are an infinite number of these states). Likewise, if the electron begins in the n=2 state, then the atom can absorb photons of light to push it into the n=3, n=4, n=5, etc. state.
When a hydrogen atom is in one of these “excited” states (i.e., n ≠ 1), it also has the opportunity to emit a photon and travel back down to a lower energy level. The photons absorbed have the same wavelengths as the photons emitted, so that they always appear in the same place in a spectrum. In the following illustration, the first four energy levels of the hydrogen atom are shown. Three commonly-studied transitions between different energy levels are named, along with their absorption/emission wavelengths in units of Ångströms (= 10-10 m). The colors of the line are the approximate colors that they might appear to your eye — with the exception of the Lyman-α transition, which emits in the ultraviolet and is therefore invisible to the human eye.
When studying spectra from space, it is common to study either absorption spectra (spectra with lines that show that atoms are absorbing photons) or emission spectra (spectra with lines that show that atoms are emitting photons). The absorption process is the most common when studying stellar spectra. And for many stars, it is the hydrogen lines that gives us a first indication about the physical properties of the stars. Here, for instance, is the spectrum of an A-type star, i.e., one with strong Hydrogen absorption features:
Galaxies, which are large conglomerations of stars, can also show hydrogen absorption features. But many galaxies, like spiral galaxies or else irregular galaxies with ongoing star formation, actually produce strong emission features. This is because the hydrogen gas that exists between the stars in these galaxies is heated by the stars, so that individual atoms are excited to higher energy levels. A great example is the spectrum of the irregular galaxy NGC 6052, shown below:
You might have noticed in this galaxy spectrum that these hydrogen emission lines appear to sit on top of what look like hydrogen absorption features. The absorption features, as mentioned above, come from the stars in the galaxy, whereas the emission features come from the gas between the stars.