8.60 light-years away
Sirius
Why is the brightest star in our sky also the reason we know white dwarfs exist?
The brightest star in the night sky is bright for an unglamorous reason: it is close. What makes it interesting is the faint thing going round it.
Bright because it is near
Sirius outshines everything else in the night sky, but it is not a spectacular star in itself — roughly twice the Sun's mass and twenty-five times its output, which is unremarkable among the stars you can name. At 8.6 light-years it is simply one of the closest bright stars, and proximity does the rest.
A companion found by arithmetic
In 1844 Friedrich Bessel noticed that Sirius did not move in a straight line. It wove, very slightly, with a period of about fifty years. Since nothing visible was pulling on it, he concluded that something invisible was — and calculated where it must be.
Eighteen years later Alvan Graham Clark, testing a new telescope, saw it: a companion ten thousand times fainter. Sirius B turned out to be about the mass of the Sun packed into something the size of the Earth, a density nobody had a physics for at the time. It was the first white dwarf identified, and it arrived as a prediction before it arrived as an observation.