Dyson sphere

Diagram of a Dyson Sphere

A Dyson Sphere/Shield World (or "shell" as it appeared in the original paper by Freeman Dyson) is a hypothetical mega structure that was originally described as a system of orbiting solar power satellites meant to completely encompass a star and capture its entire energy output. Dyson speculated that such structures would be the logical consequence of the long-term survival of technological civilizations, and proposed that searching for evidence of the existence of such structures might lead to the detection of advanced intelligent extraterrestrial life. A 3 Meter thick Dyson Sphere with an Earth orbit radius would require an estimated 2.12057325 x 10^23 square meters of material.

Since then, other variant designs involving building an artificial structure — or a series of structures — to encompass a star have been proposed in exploratory engineering or described in science fiction under the name Dyson Sphere. Most fictional depictions of a Dyson sphere describe a solid shell of matter enclosing a star, which is considered the least plausible variant of the idea.

While it is believed that some of these design variants are impractical, if not physically impossible, some designs do not require any major breakthroughs in our basic scientific understanding for their construction.

In Halo

In the book Halo: Ghosts of Onyx, it is disclosed and revealed that at the core of Onyx there is a possible Slipspace rift that shunts those that pass through it to a structured Dyson Sphere, dubbed beforehand by the Forerunners as a Shield World. It is separated from a normal space-time symmetry by advanced Forerunner Slipspace technology. Although Dr. Halsey calls it the Micro Dyson Sphere, the Sphere is 150 million kilometers (or two astronomical units) in diameter. The Micro Dyson Sphere has a spectra and energy output slightly smaller than the dimensions of Sol. This was going to be the haven where the Forerunners were to seek refuge when Halo activated, but no one knows what went wrong with their alleged plan, as the expanse appeared to be completely deserted when UNSC forces arrived. In Halo 3, these circumstances were revealed to be the result of the betrayal of the Forerunner AI Mendicant Bias, whose defection to the Flood forced the Forerunners to abandon their plans to evacuate into the shield installations. The only way out is to shut down or destroy the sphere. Basically, it fulfills its function as a "Shield World" perfectly. It protects the occupants of it and gives them a huge "world" to live in. Potentially, an entire new civilization could be constructed in a Dyson Sphere.

After their arrival in the Forerunner-constructed Dyson Sphere, Dr. Halsey explains the structure. She calls and entitles it a Micro Dyson Sphere because it is in fact very small, or perceived to be small when looked at from normal space.

The Micro Dyson sphere is located in a factually compressed Slipstream space. In other terms, any object inside might appear to be a certain size, but inside the Slipstream bubble it is much, much larger. They never left Onyx, they simply entered a Slipspace bubble.