Astronomy Principles and Practice Fourth Edition.pdf

Standard magnitude systems 307
S(λ)
1 . 0
λ
Figure 19.1. An example of the relative spectral response of a detector system normalized so that its maximum
equals unity.
Let us suppose that we have been given some equipment and we use it to measure the brightnesses
of stars. By defining a standard star, or a set of standard stars (see discussion relating to Pogson’s
relationships (see equation (15.10)–(15.13)), a magnitude scale can be set up and by comparing the
response of the system to the standard star and all of the stars in turn, each star can be assigned
a magnitude. If the same stars now have their brightnesses measured by a second detector system,
providing a different passband and different equivalent wavelength, it will be found, in general, that
the stars do not appear on this second scale at the identical positions at which they appeared on the first
scale. This is a reflection of the fact that each star has an energy envelope defined by its temperature
and that the signal from the detector is a function of the amount of energy that falls on it within the
spectral range to which it responds. Thus, each detector system has its own magnitude scale.
19.4 Standard magnitude systems
In order to allow direct comparisons between observing stations, internationally accepted detector
systems and magnitude scales are used. An early system was obviously one related to the sensitivity of
the eye and is known as the visual system (m v ). It has now been virtually replaced by the photovisual
system (m pv ). By using orthochromatic plates which are sensitive up to about 5900 Å and by using a
yellow filter to cut out the response below 5000 Å, the spectral response roughly matches that of the
eye and the two magnitude scales correspond closely.
The photographic system (m pg ) is obtained by using ordinary plates whose spectral sensitivities
have not been deliberately extended and cut off at about 5000 Å. The sensitivity is, therefore, limited
to the blue part of the spectrum down to the ultraviolet, where a cut-off is provided by the Earth’s
atmosphere or by the telescope optics. The zero point of the photographic scale is chosen so that stars
of a certain temperature and colour, corresponding to certain types of star (spectral type A5), have
identical magnitudes on the visual and photographic scales when the visual magnitude is equal to 6·0.
Stars chosen as standards are found in a catalogue known as the North Polar Sequence (NPS).
More modern magnitude systems depend on the combination of the spectral sensitivity of the
cathodes of photoelectric multipliers and specially chosen filters. In the U, B, V system developed by
Johnson and Morgan, three bands are used with equivalent wavelengths of 3650 Å (U), 4400 Å (B) and
5500 Å (V ), corresponding to the ultraviolet, blue and yellow regions of the spectrum (see figure 19.2).
Magnitudes measured in the V system correspond closely to those measured on the photovisual scale.
Similar passbands to UBV have been established using CCDs and selected colour filters. Many other