|Absolute and relative magnitude
The brightness of any
celestial body (stars, asteroids, planets, etc) is measured by a
quantity called magnitude.
The classification of stars of different magnitudes is made first
of all, on an historically basis: the stars cataloged by Ptolemy
(2d cent. A.D.), all visible with the unaided eye, were ranked on
a brightness scale such that the brightest stars were of 1st
magnitude and the dimmest stars were of 6th magnitude.
The modern magnitude
scale was placed on a precise basis by N. R. Pogson (1856). It
was found by photometric measurements that stars of the 1st
magnitude were about 100 times as bright as stars of the 6th
magnitude, 5 magnitudes lower. For this reason, Pogson defined a
mathematical, exponential law to formalize the magnitude's scale.
This modern scale allows a precise expression of a star's
relative brightness and extends to both extremely bright and very
brightness of any celestial object depends on many parameters
such as the object's size and the distance from the
observer (a candle very near you is much brighter than a very far
-and very bright- star!). For this reason, the brightness of any
celestial body, measured directly as you can see it, is also
called relative magnitude. Since all the objects in the
solar system are moving (and changing), the relative magnitude of
an object changes in time.
It is therefore necessary to define an absolute
magnitude for every class of objects (which can be asteroid,
stars etc). An absolute magnitude is a quantity which measures a
brightness independent of the distance. Normally, it indicates
the magnitude the object would have if it were 1 AU from the Earth.
Absolute magnitude is a measure of the intrinsic luminosity of
the star, i.e., its true brightness.
This absolute magnitude can be
defined in different photometric systems. Since in
modern times magnitudes are measured with photometers and
electronic detectors, which may be more sensitive to light at one
wavelength than at another wavelength, it is necessary to specify
the method and the filter used when comparing two or more
magnitudes (click here
to know more about the different wavelengths of light). The magnitude usually referred to is the visual, or
photovisual, magnitude, measured with a photometer.