25.04 light-years away
Vega
Why was one particular star chosen as the zero point for brightness?
Every magnitude quoted on this site is, in the end, a comparison with this star. It was chosen for the least romantic reason available: it is bright, it is northern, and it is convenient.
Zero by definition
The magnitude scale is ancient and awkward — smaller numbers mean brighter objects, a hangover from Greek catalogues that sorted stars into six classes by eye. When the scale was put on a mathematical footing, it needed an anchor, and Vega was assigned magnitude zero across the visible bands.
Modern calibrations have drifted slightly from Vega itself, and the star is now known to be a little variable and viewed nearly pole-on, which complicates its use as a standard. But the convention stuck, and the numbers on the plate in this tool still trace back to it.
First photographed, and ringed with dust
In 1850 Vega became the first star other than the Sun to be photographed, in a daguerreotype made at Harvard. In 1983 the IRAS satellite found it was radiating far more infrared than a bare star should, which meant it was surrounded by dust — the first debris disc found around another star, and the beginning of a whole field of study.
It also spins so fast that it is measurably flattened, its equator bulging and cooler than its poles. Around thirteen thousand years from now, Earth's axial wobble will make it the pole star again, as it was around 12,000 BC.