What is the brightest star in the night sky?

Sirius, in Canis Major, and it is bright mainly because it is close - about 8.6 light years, the fifth-nearest star system. It is genuinely luminous, some 25 times the Sun, but there are far brighter stars that look fainter because they are further away.

It is a double. Sirius B is a white dwarf, the burnt-out core of a star that was once more massive than Sirius A, now about the size of Earth with the mass of the Sun - a teaspoon of it would weigh several tonnes. Its existence was predicted from a wobble in Sirius A's motion before it was seen, which is the same detective method that later found exoplanets.

The ancient Egyptians ran their calendar on it. The heliacal rising of Sirius - its first reappearance in the dawn sky after weeks of invisibility - coincided with the annual Nile flood, so the star announced the event the whole agricultural year depended on. The Greeks associated its return with the hottest part of summer, which is where the dog days come from, and Roman writers blamed it for fevers and for making dogs bad-tempered.