The magnitude of a star is a measure of its
brightness.
In the second century BC the Greek astronomer Hipparchus devised an
approximate scale of stellar magnitudes.
Comparing the brightness of two stars
he decided that if one star was 2.5 times brighter than the other the difference of
magnitude between them was 1.
Two stars with a difference of 5
magnitudes would be 100 times brighter. The unaided human eye can just detect stars of
magnitude six in good seeing conditions.
For example: 2.5x2.5x2.5x2.5x2.5 =
2.55 = 100. [In actual fact 2.5125 = 100]
A lower intensity
means a greater positive number for magnitude. That means that a star of magnitude -1.0
is much brighter than a star with a magnitude of + 5.0. In fact a difference of magnitude of
+5 means a decrease in intensity of 100 (by definition).
How bright a star looks when viewed
from the Earth is given by its apparent magnitude. However this does not give a
true impression of the actual brightness of a star. A nearby faint star may well look brighter
than another star that is actually brighter but more distant. (A good example of this is
shown by Rigel and Sirius in the following table. Sirius looks brighter than Rigel when seen
from the Earth but it is actually fainter but much closer.)
The actual brightness of a
star is measured by its absolute magnitude. The absolute magnitude of a star is
defined as the apparent magnitude that it would have if placed at a distance of 10 parsecs
from the Earth.
| Object | Apparent magnitude | Absolute magnitude | Distance (light years) |
| Sun | -26.7 | n/a | n/a |
| Venus | -4.4 | n/a | n/a |
| Jupiter | -2.2 | n/a | n/a |
| Sirius | -1.46 | +1.4 | 8.7 |
| Rigel | -0.1 | -7.0 | 880.0 |
| Arcturus | -0.1 | -0.2 | 35.86 |
| Proxima Centauri | +10.7 | +15.1 | 4.2 |
| Vega | 0.0 | +0.5 | 26.4 |
| Betelgeuse | +0.4 | -5.9 | 586 |
| Deneb (a Cygni) | +1.3 | -7.2 | 1630 |
| Andromeda galaxy | 5 | -17.9 | 2 200 000 |
| Our galaxy | n/a | -18.0 | n/a |