The magnitude is the measure of the brightness of a celestial object. Since it entirely depends on the distance towards this object, the term apparent magnitude is preferred in astrophotography, and refers to the magnitude as seen from the Earth.

The apparent magnitude uses a logarithmic scale (+1 in magnitude ≈ x2.5 in brightness), and is based on the brightness of the star Vega (magnitude = 0). A negative value means that the object is brighter, and a positive value means that this object is darker than Vega.

Apparent Magnitude Scale
The apparent magnitude scale (credit: ESA)