The origin of this concept can
be traced back to Ptolemy, the astronomer of yore hailing from Alexandria.
In the course of his studies of the stars he divided all the visible ones
into six different categories on the basis of their brightness, the first
magnitude containing stars which were the brightest and the second, third,
fourth, fifth and sixth containing stars according to the diminishing level
of their brightness. Those in the sixth magnitude were barely visible to the
human eye. Later astronomers improved upon Ptolemy’s classification, as
better tools to study the mysteries of space were accessible to them.
Astronomers in the 17th Century used the telescope and
classified more stars that were hitherto unknown, owing to their not being
visible to the naked eye.
Then as the need for
improving the system was felt, a standard system of magnitudes was adopted
in the 19th Century. According to this system, a star would be
2.512 times brighter than a star of the next magnitude. There is a lot of
mathematics involved here too. Do some mental sums and you will see that
2.512 is the fifth root of 100. So when you look at it the other way round,
a star in the first magnitude is 100 times brighter than a star in the sixth
There are twenty stars in
the first magnitude and they have a magnitude of 1.5. These are the
brightest stars that are visible from the earth. The higher the magnitude
attributed to a star is, the dimmer it is when seen from the earth. Until
the nineteenth century, magnitude was the only way to measure a star’s
brightness. Astronomers now have high precision instruments that help them
measure even minute differences between the magnitudes of different stars.
Now astronomers are able to measure the actual amount of light from a star
that reaches the earth.
– absolute, apparent and visual
Absolute magnitude refers to the
magnitude of a star when viewed from a standard distance of about 32 light
years. Unless otherwise specified the given magnitude of a star is its
apparent magnitude. When a star is studied with the help of a telescope, it
is studied to photographic film. Photographic film is more sensitive to blue
light whereas the human eye is more sensitive to yellow light. So the
perception of brightness will differ. Hence, a classification of visual
magnitude was found necessary to indicate this difference.
stars and their magnitudes
The sun has a magnitude of
-26.7 and is 10 million times as bright as Sirius.
Sirius, the brightest star outside the solar system, has a magnitude of -1.6.
Alpha Centauri the third brightest star has a magnitude of -0.1.
Arcturus, the fourth brightest star in the sky, has a visual magnitude of