Sirius

Sirius (Alpha Canis Majoris) is the brightest star in the night sky — by a wide margin. At apparent magnitude -1.46, it shines nearly twice as brightly as the next-brightest star (Canopus). Sirius is easy to find on winter evenings — follow the line of Orion's belt down and to the left, where the brilliant white star will be unmistakable.

The star is relatively close to Earth at just 8.6 light-years, making it one of our nearest stellar neighbors after Alpha Centauri, Barnard's Star, and a handful of other dim red dwarfs. Sirius A is a hot main-sequence A-type star roughly twice the mass of the Sun and 25 times more luminous. Most of Sirius's extreme apparent brightness comes from its proximity rather than intrinsic luminosity — many vastly more luminous stars appear much dimmer because they are much farther away.

Sirius is actually a binary system. Its companion, Sirius B (nicknamed "the Pup"), is a white dwarf — the dense, hot corpse of a once-massive star that has burned through its hydrogen fuel and collapsed. Sirius B has the mass of the Sun packed into a volume the size of Earth, making it incredibly dense; a teaspoon of white dwarf matter would weigh several tons.

The ancient Egyptians built much of their religious calendar around Sirius. The star's annual "heliacal rising" — the first morning of the year when Sirius is visible in pre-dawn light after a period of being lost in the Sun's glare — coincided with the Nile flood, the central event of the Egyptian agricultural year. The star was associated with the goddess Sopdet (later identified with Isis), and many Egyptian temples are oriented to face Sirius's heliacal rising point on the horizon.

The Dog Star's appearance during the hottest part of the summer (when Sirius rises with the Sun and is technically up during the day, even though invisible) gave us the phrase "dog days of summer" — a direct etymological link from Sirius the Dog Star to American slang about hot weather.