Being an amateur astronomer myself and not long into the subject I found myself asking this question. 'Magnitude' is a scale used to dictate the brightness of any given space object, this is easier said than done due to the way the human eye responds to light. It takes a difference in light levels of around 2.5 times before we can determine that an object is "twice as bright".
Stars are measured using this logorithmic relationship. A brighter object ha s a lower magnitude than a feinter one and the brightest objects in space have a magnitude which can be below zero. A magnitude difference of 5 is equivalent to a 'true' difference of 100, so an object with a magnitude of '0' will be 100 times brighter than an object with a magnitude of '5'. The feintest objects viewable by the naked eye are around 6.0 while our own Sun has a magnitude of -26.7.
The magnitude of an object is usually indicated by the brightness of the light reaching earth, which of course can be reduced by dust clouds and particles and also the distance from the Earth so this magnitude is called 'apparent magnitude'. This can be compared to 'absolute magnitude' which is measured from a standard distance from the object of 32.6 light years.
Although brightness of a star is one important characteristic it's colour is also important as this can indicate its surface temperature, in the same way heated metal glows dependant on it's temperature (red hot, yellow hot, white hot and blue hot). Knowing the colour and energy output of a star can be paramount to discovering it's size, as a luminous but cool star will have a larger surface area as the energy will be spread out.
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