Astronomy Principles and Practice Fourth Edition.pdf

20 The nature of the observables
Figure 4.1. The spectrum of electromagnetic radiation.
Thus, any radiation has a strength which can be measured. Quantitative observations of this
property can give us information about the source or about the medium through which the radiation
has travelled after leaving the source.
Experiments in the laboratory have shown that all electromagnetic radiations have the same type
of wave nature. When any radiation passes through a medium, its velocity is reduced by a certain
fraction and the wavelength as measured within the medium also reduces by the same fraction. If v is
the measured velocity and λ the measured wavelength, their relationship may be written as
v = νλ (4.1)
where ν is a constant of the particular radiation and is known as its frequency.
Thus, the electromagnetic spectrum covers an extremely wide range of frequencies. According
to the value of the frequency of the radiation, it is convenient to classify it under broad spectral zones,
these covering γ -rays, x-rays, ultraviolet light, visible light, infrared radiation, microwaves and radio
waves. The spectrum of electromagnetic radiation is illustrated in figure 4.1.
The velocity of any electromagnetic disturbance in free space (vacuum) is the same for radiations
of all frequencies. In free space, the fundamental parameter frequency, ν, is related to the wavelength,
λ c , of the radiation and its velocity, c, by the expression:
c = νλ c . (4.2)
The velocity of electromagnetic radiation in free space has been measured in the laboratory over a wide
range of frequencies and, in all cases, the result is close to c = 3 × 10 8 ms −1 .
Wavelengths of electromagnetic radiation range from 10 −14 mforγ -rays to thousands of metres
in the radio region. At the centre of the visual spectrum, the wavelength is close to 5 × 10 −4 mm or
500 nm. In the optical region, the wavelength is frequently expressed in Ångström units (Å) where
1 Å = 10 −7 mm. Thus, the centre of the visual spectrum is close to 5000 Å.
If the strength of any radiation can be measured in different zones of the spectrum, much
information may be gleaned about the nature of the source. In fact, it may not be necessary for
measurements to be made over very wide spectrum ranges for the observations to be extremely
informative. For example, as we shall see later, measurements of stellar radiation across the visual
part of the spectrum can provide accurate values for the temperatures of stars.
Partly for historic reasons, experimenters working in different spectral zones tend to use different
terms to specify the exact positions within the spectrum. In the optical region the spectral features
are invariably described in terms of wavelength; for radio astronomers, selected parts of the spectrum
are normally identified by using frequency, usually of the order of several hundred MHz. By using
equation (4.1), it is a simple procedure to convert from wavelength to frequency and vice versa.