Sagittarius Galaxy
The Sagittarius Dwarf Elliptical Galaxy (also called the Sagittarius Dwarf Spheroidal Galaxy or Sagittarius I) is a satellite galaxy of the Milky Way.
History
Sagittarius was the first galaxy visited by Destiny and her seed ships, at least 30 and possibly more than 50 million years ago. At this time Sagittarius was considered part of "known space" by the Ancients, but proved to be a good test of the ships' automated systems. A Destiny Stargate was later repurposed for the Bifrost outpost, providing a Stargate connection to the Asgard in the Ida Galaxy. During the Ancient Diaspora 5 to 10 million years ago, when the Ancients planned to rewrite all life in the Milky Way with the Dakara superweapon to wipe out a devastating plague, at least one city-ship, Babel, deviated from the Ancients' plan to relocate their civilization to the Pegasus Galaxy, instead settling in Sagittarius to "watch over" the reemergence of life in the Milky Way.
Prior to his rise to Supreme System Lord, the Goa'uld Ra established a foothold territory in Sagittarius, finding several habitable worlds beyond the conventional reach of the Stargate Network. After the discovery of the Tau'ri, several cultures were relocated to Sagittarius by ship: a population of Humans hidden away from the rest of the Goa'uld. Osiris would later rule Sagittarius on Ra's behalf from the planet Aaru, until it was conquered by Anubis during his coup against the System Lords. Following his defeat, Osiris would become Pharaoh of Earth, with Sagittarius ruled instead by Ba'al, from the planet Hara. Originally seen as a dominion of prestige, the ever-increasing dependence of Goa'uld society on the Stargate Network slowly turned Sagittarius into an inconvenient backwater. Eventually, Goa'uld occupation in the region ended entirely, when the Asgard negotiated to deny the Goa'uld access to Sagittarius - which they called the Passage of Nilor - under Addendum 10815 of the Protected Planets Treaty. This remained the case until the clause was withdrawn in 1999, in exchange for Earth's admittance to the Treaty.
Following Anubis' return as a partially Ascended being, he once again established a foothold in Sagittarius. He planned to use the galaxy as a refuge for his loyalists when he activated the Dakara weapon to recreate the Milky Way in his own image; a plan that Ba'al later coopted, stealing Stargates from worlds in the Milky Way with the intention of setting up a new network for his dominion in Sagittarius, out of sight and mind of the Ori.
During the Battle of PB3-865, an unfinished BC-304 being constructed at the Beta Site experienced a hyperdrive overload while trying to flee from the Ori attack. This overload, amplified by an experimental Naquadria generator being used to overcharge the shields, propelled the BC-304 into deep space. Unable to use their hyperdrive without the Naquadria augmentation, the escapees aimed for Sagittarius rather than the Milky Way, hoping the shorter jump and smaller quantity of stellar objects to collide with would make the effort safer.
Astronomy
Sagittarius is located around 50,000 lightyears from the core of the Milky Way, in a looping polar orbit that spirals through the disk of our galaxy. Sagittarius has passed through the Milky Way several times over a span of billions of years, the gravitational interactions stretching it into an elongated ellipse, and stripping stars, clusters, and stellar material from Sagittarius into a belt of stars sometimes called the Sagittarius Stream. While the Sagittarius Galaxy (as the name suggests) appears to be in the constellation Sagittarius when viewed from Earth, the cluster Palomar 12 is now found in Capricornus, having been captured from the Sagittarius galaxy around 1.7 billion years ago. The galactic core was catalogued as the globular cluster Messier 54 in 1778, but scientists did not recognise Sagittarius as an actual galaxy until 1994, before the Stargate Program became operational.
At 50,000 lightyears from the black hole at the center of the Milky Way — or between 70,000 and 81,000 lightyears from Earth, depending which part of the galaxy you measure to — the distance to Sagittarius is less than the diameter of the Milky Way, and thus close enough to be within range of the Stargate Network without requiring the use of a ZPM. However, because Sagittarius lies outside the Milky Way, it would be impossible to construct a standard seven symbol address gate address based on constellations from within our galaxy. Thus, Stargates located in Sagittarius would require an eight symbol address to be accessible from the Milky Way, and visa versa.
The distance to Sagittarius is a fraction of that to other galaxies: three times closer than the Large Magellanic Cloud and sixty times closer than the Pegasus Galaxy. Under normal circumstances, the intergalactic hyperdrive on a BC-304 could complete the trip in around 11 hours, while a modern Asgard vessel could be there in 5 minutes. The less sophisticated interstellar hyperdrives found aboard Goa'uld vessels, however, are far slower: a Tel'tak would require 47 days to complete the voyage, and a Ha'tak nearly twice as long. This "just out of reach" proximity, accessible for advanced civilizations but inconvenient for more primitive races, has led both the Asgard and the Ancients to establish outposts there.
The intermediate-mass black hole at the center of M54 was/will be identified by Earth astronomers in 2009. Because of its close proximity, Sagittarius has been of particular interest to astronomers studying the shape of the Milky Way's dark matter halo, and was/will be extensively mapped as part of the European Space Agency's Gaia mission, which was in active development by 2006, and operational from 2013–2022.