- "In its hubris, mankind claims dominion over the galaxy. However, their realm is naught but a few flickering candles in a vast and hungry darkness."
- —A Treatise on the End of the Imperium, denounced and burned in 800.M41
The Milky Way Galaxy, or more commonly known as the "Milky Way", 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, 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.
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