Alpha Halo or Installation 04
A large part of the Halo ring
Alpha Halo with Basis in the background
Installation 04 is the official name of the ring-shaped space station called Halo (or Alpha Halo by Humans) in Halo: Combat Evolved.
The Station
Halo is one of seven giant space stations built by The Forerunner. It has been orbiting the gas giant, Threshold, for 101,217 local years. It is first and foremost a weapon, intended to destroy the parasitic Flood threat by wiping out all possible hosts, i.e. all intelligent life in its range. While that range is approximately 25,000 light years, the activation is thought to trigger the other six Halo stations, extending the range to the whole Milky Way. Ironically, the Flood is apparently still alive on this and other Forerunner facilities throughout the galaxy. Aside from the Flood the only other sentient being "alive" on Installation 04 is its Monitor, 343 Guilty Spark.
Astronomy
The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld. While the diameter of Niven's ringworld is close to the diameter of Earth's orbital of 300,000,000 km, the 10,000 km diameter of Halo is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our sun in its center, and Halo is orbiting a planet, but does not encircle it.
Besides the very beautifully sculpted landscape on Halo's surface, the sky is also worthy of attention. Depending on the viewpoint, you can see the star, a moon (called Basis), a gas giant (called Threshold), and the band of Halo itself, which looks like a street into the sky.
In the course of Halo 1, neither Threshold nor Basis is reachable, all action takes place on or in the Halo. In Halo 2 there is a multiplayer level on Basis, and there is a campaign level in Threshold's atmosphere. That level is set on a Forerunner gas mining facility.
Indigenous life
Installation 04 contained a huge number of indigenous life, most of which was small animals and plants. Cortana noted that the ring has thousands of typed of fauna. In addition, many small animals populate the ring, as the Master Chief noted mothlike insects as well as swamp birds near the Flood Containment Facilities. There was not time to study the ring extensively, but it is likely that there was even more life there than was observed.
ALSO SEE: Halo Dinosaurs