Page 4 - 83 basic knowledge of astronomy

2 Various Classifications of Astronomical Ra-
dio Sources

There are several ways to classify astronomical radio sources. Typical clas-
sifications are:

• – Thermal source
Source emitting via a thermal mechanism (e.g. blackbody radia-
tion),
– Non-thermal source
Source emitting via a non-thermal mechanism (e.g. synchrotron
radiation, inverse Compton scattering, annihilation radiation, maser
emission, etc.),

• – Continuum source
Source emitting over a broad range of frequencies,

– Spectralline source
Source emitting in narrow lines at specific frequencies,
and

• – Galactic source
Source inside our Milky Way Galaxy,
– Extragalactic source
Source outside our Galaxy.

3 Spectra of Typical Continuum Radio Sources

Figure 2 shows typical spectra of continuum radio sources.
These spectra are characterized by a quantity called ‘spectral index’ α, de-
fined by the formula: S ν ∝ ν −α , where S ν is a quantity called the ‘flux
−2
−1
density’ [unit: Jy (Jansky) ≡ 10 −26 Wm Hz ], which is a measure of the
strength of the radiation from a source.
Thermal and non–thermal sources usually show different spectral indices,
namely:
∼
α = −2 for thermal sources,
and
α ≥ 0 for non-thermal sources.

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