Also known as Alpha Canis Majoris, Sirius is the fifth closest system to Sol, at 8.6 light-years (ly) away. It is the brightest star in the sky and is located in the north central part of Constellation Canis Major, the Larger Dog. Sirius is also the lower left member of the "Winter Triangle" of first magnitude stars, whose other components are Procyon (Alpha Canis Minoris) at upper left and Betegeuse (Alpha Orionis) at right center. A binary, the system is the title member of the Sirius stellar moving group (also know as the Sirius Super Clusteror Ursa Major star stream), which include all five stars of the Great Dipper as well as Gemma and are mostly around 490 million years old and all moving towards the galactic center.

In Star Trek, Sirius is most notable for the fact that, after the last Kzin War, in the mid-to-late 21st century, the Treaty of Sirius was signed in this system, ending the series of wars between Earth and the Kzin government. (TAS: "The Slaver Weapon"). The Sirius Cybernetics Corporation originated in this system. (TNG: "The Schizoid Man"; Star Trek: Deep Space Nine set decorations). In 2364, Sirius was noted on a large star chart at Starfleet Headquarters. Also, on several other occasions that year, viewscreen readouts aboard the USS Enterprise-D showed the location of the A and B Sirius stars, in the form of charts from the Enterprise library computer. (TNG: "The Naked Now", "The Last Outpost", "Conspiracy").

Sirius IX is the ninth planet in the Sirius system. In 2269, the planetary government of Sirius IX charged Harry Mudd with fraud and swindling. (TAS: "Mudd's Passion").

In reality, Sirius is one of the points in the Winter Triangle together with Betelgeuse in Orion and Procyon in the smaller dog, Canis Minor. Sirius is a binary star system. The primary (Sirius A, nicknamed the Dog Star) is a white A star. The companion (Sirius B, or "the Pup"), orbits with a period of 49.97 years, and was the first white dwarf to be discovered.

Alpha Canis Majoris A, the Dog Star, is the brightest star in the night sky as well as the brightest star in its constellation. Unlike Sol, it is a slightly bluish, white main sequence dwarf star of spectral and luminosity type A0-1 Vm. This relatively large star has 2.02 (+/- 0.03, Liebert et al, 2005) to 2.14 times Sol's mass and about 1.68 to 1.71 (+/- 0.013) times its diameter, but RECONS estimates a mass of 2.0 Solar based on luminosity. According to the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 2491, diameter estimates vary from 0.00560 to 0.00589". Compared to Sol, Sirius A is hotter and more than 21 times brighter. It may be only about 225 to 250 million years old (Liebert et al, 2005; and Ken Croswell, 2005), but being so much bigger and hotter than Sol, the star will exhaust its core hydrogen within only a billion years and turn into a red giant or Cepheid variable before puffing away its outer layers to reveal a remnant core as a white dwarf.

Dust has been detected in this binary system (Backman et al, 1986; and Kuchner and Brown, 2000 -- in postscript). Sirius A is rich in elements heavier than hydrogen ("metals rich"), as it has about one to 7.4 times the iron abundance of Sol (Cayrel de Strobel et al, 1991, pages 285-286). It was probably enriched by its companion star, which was once bigger and hotter than Sirius A and so evolved and "burnt out" even faster. Sirius B manufactured lots of heavier elements which it puffed out into space and onto Sirius A before becoming a white dwarf.

The distance from Sirius A where an Earth-type planet would be comfortable with liquid water is about 4.6 AUs (at about the distance of Jupiter from the Sun in the Solar System), where its orbital period would be 6.8 years long. The orbit of any protoplanet forming around Sirius A at that distance could have been disrupted by the close orbit of Sirius B, and also probably during B's mass loss when it became a nova then a white dwarf. Even if it is possible for an Earth-type planet to orbit youthful Sirius A and develop life, it is likely to be primitive single-cell, anaerobic bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O3) although Sirius A puts out a lot more hard radiation (especially ultraviolet) than Sol.

Nicknamed "the Pup" as the much smaller companion to the Dog Star, this much dimmer object is a white dwarf (DA2-5 or A2-5 VII). It is more than 8,200 times fainter than Sirius A and 360 times fainter than Sol. Compared with our Sun, it appears to have the same mass (97.8 to 103.4 percent, Hubble press release; Barstow et al, 2005; Liebert et al, 2005; and Holberg et al, 1998) but less than one percent (0.0864 +/- 0.012) of its diameter (Barstow et al, 2005; and Holberg et al, 1998). In fact, Sirius B's diameter of about 11,700 km (about 7,300 miles) is about 92 percent of Earth's. Its mass and diameter are consistent with the theoretical size for a carbon-core white dwarf, one that may have evolved from a 5.05 +0.374/-0.276 Solar-mass, B-type main-sequence star about 124 +/- 5 million years ago, after 101 to 126 million years as a giant star (Liebert et al, 2005; and Ken Croswell, 2005). Hence, Sirius B was once brighter than Regulus A, currently a B7 main-sequence star. While now tiny compared to main sequence stars, white dwarf stars are actually intensely hot, but without the internal heat of fusion to keep them burning, they gradually cool and fade away. Sirius B was the first White Dwarf to be discovered, but it took almost a century from its discovery to the wide acceptance of its nature as a stellar remnant. The star's calculated average density is 92,000 times that of Sol's, so that one cubic inch of its material would weigh about 15 tons at the surface of the Earth.