Astronomy is an ancient science. As such, it carries with it some very archaic conventions. The magnitude system, invented in 150 B.C., is one of them.

The BRIGHTEST stars, are called FIRST magnitude. Stars that appear half as bright are SECOND magnitude, those dimmer still are THIRD, etc.

On such a scale, the LARGER the number, the DIMMER the star. Objects BRIGHTER than the brightest star (1st magnitude), are expressed as negative values. Each magnitude corresponds to a factor of 2.5 in brightness.

There are two kinds of magnitudes:

Apparent and Absolute

APPARENT MAGNITUDE: is a way of comparing the brightness of objects based on what we see on Earth, with no regard to how far away the source may be.

ABSOLUTE MAGNITUDE: the magnitude of an object, assuming it is placed at a distance of 10 parsecs (about 32 light years). This is thus a measure of an intrinsic property of the star or object, and is thus a distance-independent measurement.

1. Example Apparent Magnitudes

Dimmest star seen by the Hubble Space Telescope	30.0
Dimmest star seen with an amateur telescope	15.0
Dimmest star the human eye can see	6.0
Brightest star in the sky (Sirius)	-1.5
Venus at its brightest	-4
Brightness of the Full Moon	-11
Brightness of the Sun	-27