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

Dimmest star seen by 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

There are two kinds of magnitudes:
APPARENT MAGNITUDE, small m, refers to the brightness of objects based on what we see on Earth, with no regard to how far away the source may be.

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

Distance Modulus
Defined as the difference between the apparent magnitude, m, and absolute magnitude, M. It tells you something about the distance. For example:

1) You are told a star has an apparent magnitude equal
to its absolute magnitude. How far away is the star?

2) You are told a star has an apparent magnitude greater
than its absolute magnitude. How far away is the star?

3) You are told a star has an apparent magnitude less than
its absolute magnitude. How far away is the star?

Luminosity Once you know the distance to the star, D, you can determine its intrinsic luminosity, L, based on how bright it is (the flux, F,) on earth and the inverse square law: