information on the halo array

The Halo Array is a network of seven ring-shaped artificial worlds created by the Forerunners in order to kill all sentient life within range of the array, virtually the entire galaxy. Rather than as a weapon of war, these were used as a last resort against the Flood, a parasitic extra-galactic species that threatened to infest every lifeform in the galaxy. By activating the rings, the Forerunners denied the Flood access to these lifeforms, effectively starving the species to virtual extinction, though it clung to life in a number of Forerunner facilities, including some Halo installations.The two known Halo installations, referred to as Alpha Halo and Delta Halo respectively, orbit gas giants, though it is not known whether this is the case of all the Halos, and Alpha Halo maintained an atmospheric mining facility until it was destroyed by the Covenant. Though separated by thousands of light-years, the seven installations are networked together and capable of remote activation at the Ark, an installation outside the galaxy. Each Halo installation has a maximum effective range of 25,000 light-years. The exact means the array uses to conduct this "mass sterilization protocol" are unknown, though it is assumed to utilize energy in some type of form.

The Halo Array in its entirety has been fired once in known history, approximately 100,000 years ago, by the Didact in order to stop the Flood from overwhelming the galaxy. Despite being the network's builders, the Forerunners were killed in this last suicidal attempt to save the rest of the universe from a worse fate. The array has nearly been activated three times since; Alpha Halo was almost activated by the Master Chief before being deactivated by Cortana, while Delta Halo was activated by Commander Miranda Keyes, forced by the Jiralhanae Chieftain Tartarus, and Sergeant Johnson was forced by the Prophet of Truth to activate the Ark. None of these attempts were successful. A replacement Halo for the destroyed Alpha Halo was activated outside of the array's range, eliminating the freed Flood, but did not activate the rest of the array.

Each Halo Installation is given a designation number, from 01 to 07, and is overseen by a Monitor. The Monitors are given control of the Installations' Sentinels,Constructors and Enforcers, and are responsible for containing the Flood test subjects in their research facilities and protecting the Halo from intruders.

The Array also encompasses an installation known as The Ark or Installation 00, from which the array can be remotely and safely activated while out of firing range, and able to survive the effects.The Forerunners also built active Shield Worlds, Micro Dyson Spheres encased in a slipspace field with their own star in the center and a habitable environment, as a means to escape the effect without having to leave the galaxy.I have got this information from halopedia.com