The star Arcturus (inside constellation Bootes) is mag -0.04, and Vega (in the constellation Lyra) is mag +0.03. I don't know that any space object can be zero because all stars vary in brightness, even if it's only for a few "sunspots" darkening a bit of their surface. Planets, asteroids and such can't be references either because their everchanging distance alters their luminosity a lot. If memory serves, Arcturus or Vega was once the reference but then it was remeasured when instruments became better.
Thus the "0" is only in the lab, I assume. It's probably the brightness of some very accurate and stable light source that's used to calibrate other instruments. Mag +1 is 2.5 times fainter than mag 0, mag +2 is 2.5 times fainter than mag +1, and so on. Which means mag +2 is 2.5 x 2.5 = 6.25 times fainter than mag 0. Mag -1 is 2.5 brighter than mag 0, and so on.
Some find it counterintuitive that dimmer things have a larger number but it's the same in races: the 1st is greater than the 2nd, which is greater than the 3rd, etc.
As rule of thumb:
- a 1 mag difference means a 2.5 times brightness difference (a 40W bulb versus a 100W bulb)
- a 2 mag difference means a 6 times (roughly) brightness difference (10W vs 60W)
- a 3 mag difference means a 15 times diff
- a 4 mag difference means a 40 times diff
- a 5 mag difference means a 100 times diff, convenient value that makes comparisons easier
Pretty easy numbers to remember. A 70mm scope gathers about 100 times as much light as the eye, thus it gains 5 magnitudes. These figures are the basic rule of thumb for using magnitudes in practical observation.
EDIT: Moderators will likely move this to another section of the forum since magnitudes are not for the solar system only.
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