The brightness of celestial objects is expressed in magnitudes with each magnitude change equal to the 5th root of 100 or about 2.5119. Also, the brighter an object the SMALLER the magnitude. That is to say, a first magnitude object is 100 times brighter than an object of sixth magnitude. Some extremely bright objects (the Sun, the Moon, Venus, Sirius and others) have negative magnitudes. A magnitude of -4 is 10,000 times brighter than magnitude 6.

1) Here's an example for using this calculator's BRIGHTNESS function:
The Sun has a magnitude of -26.72 and the Moon's magnitude is -12.6. How much of a brightness difference is this?
Putting -26.72 and -12.6 into the appropriate boxes and then clicking on CALCULATE shows that this is a magnitude difference of 14.12 and a brightness difference of 444,631.267469.

2) So far, all we have been discussing is APPARENT (or visual) magnitude, but another type is ABSOLUTE MAGNITUDE. From example 1, we saw the Sun has an extremely bright magnitude of -26.72. However, this is due to the fact that the Sun is very close to Earth. If all stars were to be seen at the same distance, then the ONLY factor affecting its magnitude would be its intrinsic brightness. Astronomers have chosen the distance of ten parsecs (32.59 Light Years) as the arbitrary point at which all stars would be compared. So, to use the ABSOLUTE MAGNITUDE calculator function:
The apparent magnitude of the star Sirius is -1.46 and is at a distance of 9.0 light years. What is its absolute magnitude?
Putting these numbers into the calculator we find that Sirius has an absolute magnitude of 1.33.

LUMINOSITY is a comparison of a star's intrinsic brightness compared to the Sun (where Sun's luminosity = 1). Therefore Sirius is about 26 times brighter than the Sun.