Short explanation: Lyman α
(or Lyα) is a kind of ultraviolet light, originating from
hydrogen.
As yet, the only way of receiving information from outer space is through
radiation, i.e. light. Radiation consists of light particles, or
photons, which are characterized by their energy E, their
frequency
ν (nu), or their wavelength λ (lambda). The higher the
energy, the higher the frequency, but the shorter the wavelength, and vice
versa. The different terms are just different ways of talking about the
light.
Astronomers often like to measure the wavelength in Ångström (Å), i.e.
100 millionth of a cm.
If the wavelength of the radiation lies between approximately 4000 Å and 7000 Å,
the light is visible to the human eye.
The shorter the wavelength, the more blue the light is,
while it becomes more red for longer wavelengths. If λ <
4000 Å, we call it ultraviolet (UV) light, and for even shorter wavelengths
we have X-rays and gamma rays. If λ > 7000 Å, we call it
infrared (IR) light, microwaves, and radiowaves.
One special type of UV light is particularly interesting to astronomers,
namely photons with a wavelength of 1216 Å. Radiation consisting of photons
with this wavelength is called Lyman α radiation, or simply
Lyα.
Lyα traces hydrogen
The reason that Lyα is so interesting is that it is created by hydrogen,
and hydrogen makes up 90% of all the elements in the Universe. The energy of
a Lyα photon corresponds the energy difference between the ground state
and the 1st excited state of the hydrogen atom. This means that a Lyα
photon hitting a hydrogen atom in the ground state will excite it to the
1st state. After a while (100 millionth of a second) the atom de-excites
back to the ground state, re-emitting the Lyα photon
(or another). The photon is said to be
scattered.
On the other hand, if a so-called Lyβ photon hits the hydrogen atom,
it will excite it to the 2nd state. From here, the atom may de-excite
directly to the ground, re-emitting a Lyβ photon, but it may also go
first to the 1st state, and then to the ground state, emitting
two photons (the sum of energy of which equals the Lyβ photon),
in which case the Lyβ photon is destroyed.
Only if the energy of the photon matches very closely the energy difference
between the ground and the 1st state, if it is in resonance, will it
interact with the atom. Thus, we also say that the Lyα radiation is
resonant scattered.
This phenomenon is what makes Lyα so special:
it is one of the most commonly
produced photons, and even though it is produced deep inside a hydrogen
cloud, it may scatter its way through the gas and escape the cloud without
being absorbed, making us able to detect the source.
Hence, Lyα is one of our main windows to, in particular, the very
distant Universe. Since the very distant Universe also means the very
early Universe, observations of Lyα radiation is a very efficient
way of learning about how the galaxies formed.