Milky Way

The Milky Way is the galaxy where the humans and the former Covenant species, such as the sangheili and the jiralhanae, reside, and is approximately 100,000 light years in diameter. It is a barred spiral galaxy that lies within the local group of galaxies neighborhood, which is just a very small portion of the universe. Visible from Earth as a band of light in the night sky, the Milky Way is just one of many galaxies in the observable universe, although the sphere of influence of humanity and the other modern spacefaring civilizations is limited to only small portions of the Milky Way. The Milky Way is orbited by two small satellite galaxies known as the Magellanic Clouds.

Description
The Milky Way is a barred spiral galaxy of either SBb or SBc classification on the Hubble Sequence. It has two major arms, each sporting several minor arms that stem off of the major arm. One of these major arms, the Carina and Sagittarius Arm, is home to the Orion Arm. Splotches of intense star birth and formation can be seen in the arms. These areas are pink/red in hue and appear as wisps or clouds situated in the galactic plane.

The Orion Arm
Main article: Orion Arm The Orion Arm is the origin of humanity and their home planet Earth, with at least 800 worlds inside human territory at the height of human colonial expansion. It is also the point of origin for one of the former Covenant races, the kig-yar.

The Orion Arm was the setting for the Human-Covenant War of the 26th Century and the ancient Forerunner-Flood War, witnessing at least two massive wars. It is also the home of a Halo installation and a slipspace portal to the Ark.

Other Galactic components
The Milky Way is composed of a disk, in which all the arms and the majority of galactic matter lies, and the galactic bulge, essentially the center of a galaxy. The bulge is home to the galactic nucleus, the dead center of a galaxy and home to an extremely dense clustering of ancient stars. The nucleus is also thought to support a supermassive black hole. Additionally, most known galaxies possess a halo of old and dying star clusters left over from the galaxy's formation. These clusters are situated up to 100,000 light years from the galactic bulge. Habitable planets may certainly lie within the halo, and some clusters may escape the range of the Halo Effect given their distance from the galactic center.

Prehistory
100,000 years before the present day, the previous galactic community was dominated by the forerunners, a now extinct species that possessed extremely advanced technology and an empire that spanned a significant portion of the galaxy. Millions of years before the rise of the forerunners, an even more advanced civilization known as the xel'naga controlled the Milky Way, having taken control of it after defeating the "lesser" species known as the sharquoi. Having reached the pinnacle of technological advancement, they were believed to have significantly contributed to the evolution of life across the galaxy, even created several species, such as forerunners and humans. Over ten million years before the present, however, the forerunners rose up against their creators in defiance of their decision to bestow their Mantle on humanity, and seemingly ended their rule in the galaxy after destroying their last remnants in the Andromeda galaxy.

The forerunners, originating from Ghibalb in the Sagittarius Complex of the Sagittarius Arm, ruled a large portion of the galaxy for millions of years, affecting the development of other species as they saw fit, such as the protoss species, which they fought for hundreds of years with no end in sight. Around 150,000 BCE, humanity, still composed of many species, arose from their homeworld Earth, known as "Erde-Tyrene" at the time. They mastered interstellar travel and began to move their civilization along the Orion Arm. They later allied with the san'shyuum species, forming a prosperous interstellar empire known as the Cara, based on the planet Charum Hakkor. Around 107,445 BCE, humanity and their san'shyuum allies came into contact with the flood, a sapient parasite supposedly originating from the Andromeda galaxy. Hundreds of systems were lost to the parasite in the resulting conflict. Meanwhile, humanity's aggressive expansion and the perceived violations of the Mantle prompted the forerunners to take action and begin a war against humanity. Although the flood later withdrew and seemingly died out from the galaxy, it had already weakened the humans enough to allow the forerunners to emerge victorious.

Following the war in approximately 106,445 BCE, the forerunners dismantled humanity's civilization, regressing them into a Tier 7 species. The remaining humans were devolved and exiled to their homeworld, where they slowly began to redevelop a primitive civilization under the tutelage of the forerunner Lifeshaper, the Librarian.

Activation of the Halo Array
10,000 years after humanity's conflict with the flood, the parasite resurfaced and attacked worlds of the forerunners' domain. Encounters with the flood led to the Forerunner-Flood War, with the flood consuming or killing the great majority of the galaxy's sapient species as the forerunners first studied them, and then tried to stop them. Eventually, the forerunners were forced to activate the ultimate weapon, the Halo Array, in order to stop them - at the same time, killing its builders. The forerunners built safe havens, including the shield worlds and the Ark, to save the remaining ten sapient species in a plan known as the Conservation Measure, before activating the Array, scouring the galaxy of all sapient life forms and killing the flood except for samples kept on the Halos, then releasing the saved species from their havens to their home worlds.
 * Main article: Forerunner-Flood War