The Lyman Alpha Series is a method used by physicists
and cosmologists to detect the presence of hydrogen, usually in
the form of vast clouds, in the universe and Quasars, or quasi-stellar
radio sources. These facts aid cosmological research by giving one
an idea about where the quasars are located, how quickly they are
receding, the boundaries of the universe and more. First, though,
one must be familiar with the properties of hydrogen.
Hydrogen is the lightest and most abundant element in the universe.
It is composed of one proton and one electron. As explained in the
article on atoms, electrons have energy levels starting from n=1,
the lowest. When energy is applied to them, they basically “jump
up” to another level, n>1, depending on the amount of energy
added. When light at a wavelength of 1216 Å (Angstroms) hits
a hydrogen atom, the electron becomes excited, absorbing this energy
and jumping to the next energy level.
Scientists use this fact to determine the size and location of hydrogen
clouds. If light is plotted on a graph, and there is a “dip”
on the graph that corresponds to a wavelength of 1216 Å, then
this means that hydrogen is present somewhere between the Earth
and the Quasar. We can derive the location of the cloud by determining
the how much the light has been stretched. This stretching occurs
because the universe is expanding. Scientists take the wavelength
of light received on Earth and extrapolate backwards until they
obtain a value of 1216 Å. More extrapolation can be done in
order to determine the location of the Quasar that has emitted the
light. As said before, when light at a wavelength of 1216 Å
strikes a hydrogen atom, the electron absorbs the energy and jumps
up to a higher energy level.
The electron, however very quickly
moves back down to n=1 and emits a photon. In other words, when
the electron is boosted to a higher energy level, it does not stay
there; it comes back down quickly. Thus, the extra photons that
are emitted from these clouds create a bulge in the graph. The combination
of absorption and emission lines on a graph are what scientists
call the Lyman Alpha Forest.