The Milky Way Galaxy
The Milky Way Galaxy, more commonly known as the "Milky Way," or just "the galaxy," is the galaxy that Mankind calls home and in which the Imperium of Man and all of the other starfaring intelligent species known to Humanity are located. The Milky Way is a barred spiral galaxy that is part of the Local Group of galaxies.
It is one of hundreds of billions of galaxies in the observable universe. Its name is a translation of the Latin term Via Lactea, in turn translated from the Greek Galaxias, referring to the pale band of light formed by stars in the galactic plane as seen in the night skies of Holy Terra.
Some sources hold that, strictly speaking, the term Milky Way should refer exclusively to the band of light that the galaxy forms in the Terran night sky and the skies of many other settled planets, while the galaxy should receive the full name Milky Way Galaxy, or alternatively the Galaxy. However, it is unclear how widespread this convention is, and the term Milky Way is routinely used in either context.
The stellar disk of the Milky Way Galaxy is approximately 100,000 light-years in diameter, and is considered to be, on average, about 1,000 light years thick. It is estimated to contain at least 200 billion stars and possibly up to 400 billion stars with an estimated average of 300 billion, the exact figure depending on the number of very low-mass, or dwarf stars. This can be compared to the one trillion stars of the neighbouring Andromeda Galaxy.
The stellar disc does not have a sharp edge, a radius beyond which there are no stars. Rather, the number of stars drops smoothly with distance from the centre of the galaxy. Beyond a radius of roughly 40,000 light years, the number of stars drops much faster with radius, for reasons that are not understood.
Extending beyond the stellar disk is a much thicker disk of gas. The gaseous disk at the centre of the Milky Way has a thickness of around 12,000 light years. The Galactic Halo extends outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose perigalacticon is at about 180,000 light years. At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds, and the objects would likely be ejected from the vicinity of the Milky Way.
Following the birth of the Chaos God Slaanesh at the end of the Age of Strife in the early 30th Millennium A.D., the galaxy was rent by the implosion of the Aeldari Empire. Where the core worlds of that ancient stellar realm had been in the galactic northwest, a vast new Warp rift spilled out into realspace -- the Eye of Terror, originally classified as stellar anomaly "Cygnus X-1" by Imperial astrographers.
The strains on the fabric of local space-time created by Slaanesh's birth in the Immaterium also weakened the barriers between the Warp and realspace across the galaxy. The result was the creation of smaller, permanent Warp rifts across the Milky Way beyond the Eye of Terror, including such anomalies as the Maelstrom. At the same time, Warp Storms grew in size and frequency across the galaxy in the ten thousand Terran years since the birth of the Eye of Terror.
These strains reached a breaking point following the fall of the world of Cadia to the forces of Abaddon the Despoiler's 13th Black Crusade in ca. 999.M41. In the wake of the destruction of Cadia and the Necron-built Cadian Pylons on that world that had long kept the growth of the Eye of Terror in check, the Cadian Gate was overwhelmed by the forces of Chaos, and the Eye of Terror actually began to expand across the galaxy.
Eventually, the galaxy was bisected by a massive new Warp rift called the Great Rift, or Cicatrix Maledictum, spreading from its heart in the Eye of Terror all the way to the Hadex Anomaly of the Jericho Reach in the Eastern Fringes of the galaxy.
Geography
The vast spiral of the galaxy stretches across over 100,000 light years and swells to 12,000 light years thick at its hub. It contains hundreds of billions of stars and untold millions of habitable planets. Through long ages the galaxy has seen successive civilisations rise and fall without end. Some never reached other stars and died stillborn on their birth worlds, while others rose to dominate vast swathes of the galaxy before collapsing back into dissolution and anarchy.
Localized Phenomena
The Immaterium, also known as the Warp, exists as a parallel dimension to the physical plane, known as Realspace. Each point in Realspace corresponds to a unique point in the Warp, which is exploited as a means of travel many times faster than the speed of light. It is unknown if the Warp is a phenomenon unique to this particular galaxy, or if the frothing seas of the Empyrean stretch on ad infinitum.
History
Little of the vast history of the galaxy has been revealed to Mankind. What is known beyond the dimly-remembered confines of even humanity's own story has largely been drawn by various human scholars and Adepts of the Cult Mechanicus from ancient xenos ruins scattered across the stars and the dying Infinity Circuits of shattered Eldar Craftworlds.
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