In a nutshell, magnitude is used by astronomers to describe the brightness of a star as seen from Earth; negative numbers denote brighter stars compared to those with positive integers. So a star with -2 magnitude is brighter than one with +2 magnitude. Incidentally, magnitude isn’t reserved just for stars–it also applies to other celestial objects too including the Moon and the other planets in our solar system.
However, it isn’t quite that simple, as astronomers recognise two types of Magnitudes: Apparent Magnitude and Absolute Magnitude, with significant differences between the pair in how they are calculated and defined.
Absolute magnitude basically calculates how bright a star would be if plonked down at a distance of 10 parsecs (or 32.6 light years, with 1 parsec being 3.26 light years) from our sun. Ten parsecs is considered a standard distance for this, and another similar term that can be used to refer to absolute magnitude is absolute visual magnitude (Mv). Of course, to be able to define a star’s absolute magnitude, you first need to know its apparent magnitude.
Apparent magnitude, on the other hand, is a measurement of how bright a star looks from a particular location in space, such as the Earth. When we say, for example, that Sirius has a magnitude of -1.46, that’s its apparent magnitude, telling us how bright it appears from where we are. But that is not a fixed value, as another observer looking at it from, say, 100 light years from us would record a different apparent magnitude for Sirius, by virtue of being a different distance away from aforementioned star. However, were we to plonk Sirius ten parsecs from Earth, it would appear a lot less impressive (sorry Sirius!), reaching a maximum of just +1.4 (remember, positive numbers mean dimmer stars.)
Another example, one a lot closer to home, is the Sun: from where we observe it on Earth, it achieves an apparent magnitude of -26.8, but were we to punt it ten parsecs away from us, it would have an absolute magnitude significantly dimmer than even Sirius’, reaching a relatively paltry 4.83.
References:
http://astronomy.swin.edu.au/cosmos/A/Apparent+Magnitude
http://astronomy.swin.edu.au/cosmos/A/Absolute+Magnitude
https://www.space.com/21640-star-luminosity-and-magnitude.html
Seeds, M.A. 2013. Foundations of Astronomy.
chailattenebula.wordpress.com 