Page 30 - 83 basic knowledge of astronomy

incident radiation

particle
induced emission

Figure 24: A schematic view of maser amplification. Induced emission causes
new induced emission, initiating an “avalanche” of maser emission photons
all traveling in a common direction parallel to the incident photon.

Usually, masers are strong radio sources. For example, the brightness
temperature of some H 2 O masers is as high as 10 14 K.

3. In the purely thermal equilibrium case,
we obtain from the detailed balance:

ν
−κ ν I ν + ν = 0, and therefore I ν = ,
κ ν
and from the Planck spectrum:

2hν 3 1
I ν = B ν (T) ≡ ,
c 2 e kT − 1
hν
where B ν (T) is defined to be the Planck function. From the above
equations, we have a relationship between the emissivity and opacity,
which is called Kirchoff’s law:
ν
= B ν (T) . (44)
κ ν

4. Local thermodynamic equilibrium (LTE)
In a fairly wide range of real circumstances in interstellar gas media, and
in laboratory conditions, at a given temperature T, there is the case
when Kirchoff’s law ν /κ ν = B ν (T) holds to a good approximation,
but the radiation intensity I ν is not equal to the Planck function

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