Image from Bernd Helfert

Colonized space stretches thousands of light years from Earth in every direction and is expanding outwards at close to the speed of light as further worlds are settled on the Periphery. This region, known as the Terragen Sphere, occupies billions of cubic light years, from the densely settled, industrialised, and historically rich Inner Sphere to the more sparsely populated Outer Volumes.

Most of the colonised and inhabited worlds are found within the thick disk of the Milky Way Galaxy, centred on the Orion Arm or Spur, a relatively minor arm. However, Terragen expansion extends towards the centre of the galaxy, into the nearby Sagittarius Arm and also into the Perseus Arm towards the rim. Although star systems above and below the galactic disk are generally few and far between, enterprising colonists can be found even here, with inhabited space extending out to a distance of several thousand light-years.

Many of the most important systems in the innermost parts of the Terragen sphere are linked by a nexus of wormholes stabilised by exotic energy; however the outermost parts of colonised space are generally not yet connected to this network.

Scattered through the Terragen Sphere can be found a number of worlds inhabited by intelligent aliens (known as xenosophonts), most of which are confined to only one, or a few systems. However beyond the edge of explored space there are indications of more extensive and powerful xenosophont civilisations, and Terragen expansion may eventually become restricted in some directions as the outermost edges approach these ancient empires.

Galactographic regions

Major cultures in the Terragen Sphere

Terragen Metaempires

There are four main cultural divisions in the Orion's Arm Civilisation:
Sephirotics, Ahumans, Nonaligned, and Xenosophonts.


Ahumans (transapients that do not associate closely with humans or their sibling clades)


Cooperative Treaties


  • 'Terragen Expansion'  - Text by Anders Sandberg amended by Steve Bowers
    The expansion of the Terragen Sphere over time.
  • Astrography  - Text by Anders Sandberg
    Interstellar cartography, determining one's position in space, naming of interplanetary and interstellar bodies and regions, the science of interplanetary and interstellar map-making.
  • Backscattering Waves  - Text by M. Alan Kazlev and Steve Bowers
    Some colonies themselves send out waves of colonsation, which may come into conflict with other waves from the original source.
  • Beyond, The - Text by M. Alan Kazlev
    Popular term for that volume of the galaxy outside of the Terragen Sphere.
  • Bubble (galactography)  - Text by M. Alan Kazlev
    The extent to which a starfaring civilization has expanded through interstellar space.
  • Cartography  - Text by M. Alan Kazlev, modified from the original write-up by Robert J. Hall
    The making and reading of maps.
  • Civilization  - Text by M. Alan Kazlev
    Sophont or transingularitan collective society, usually with development of agriculture, arts, sciences, technology, sentient rights, infrastructure, social services, and so on.
  • Core Worlds - Text by M. Alan Kazlev
    The Sol system and neighboring systems which were the first colonized by
  • Coreward - Text by M. Alan Kazlev
    [1] In astrogation the direction of the Sagittarius arm and the centre of the galaxy.
    [2] In some informal usages, the direction of Sol (more properly termed Solward).
  • Epoch (Astronomy)  - Text by Steve Bowers
    A point in time from which astronomical quantities are measured.
  • Galactic Directions  - Text by Anders Sandberg, amended by Steve Bowers
    Planetary terms like north, south, east, west, are insufficient for referring to directions within the galaxy. Instead, the a different set of conventions have achieved widespread acceptance when referring to direction.
  • Glossary  - Text by M. Alan Kazlev and Steve Bowers
    Words used in the Orion's Arm scenario.
  • Hazard Rating  - Text by Stephen Inniss and M. Alan Kazlev
    It is common for short summaries of worlds, habitats, stellar systems, or polities to include a Hazard Rating. The standard modern version is a ten point scale.
  • Local Arm - Text by M. Alan Kazlev
    The arm of the Milky Way Galaxy where most of Terragen civilization is still located, and which includes the Sol System. It is also called the Orion Arm.
  • Master Star List  - Text by Mark Ryherd, John M. Dollan and Steve Bowers
    The stars of the Inner Sphere and their colonies.
  • Old Earth  - Text by M. Alan Kazlev
    Generic term used to designate Earth before the Great Expulsion. As well as the literal meaning, and the reference to Terragen origins and ancient baseline history, the term has various other associated nuances, from the ridiculous or exaggerated to the romantic or sublime, including lost paradise, fool's opportunity, pre-archaiocractic human supremacy, unmoderated baseline civilization (positive or negative, depending on context and clade), natural evolution, and so on.
  • Polity  - Text by M. Alan Kazlev
    An organized society; a state or quasistate; any form of politico-social organization.
  • Stars near Sol - Exploration and Colonisation  - Text by Arik
    Early exploration efforts in the Solar Neighborhood.
  • Technosphere - Text by Mitchell Porter in Anders Sandberg's Transhuman Terminology
    [1] An expanding sphere of civilization/technology, spreading outwards using von Neumann Probes or simple colonization. Judging from how most life behaves, it will gradually restructure matter and energy inside itself in various ways.
    [2] The totality of AI-ruled space.
  • Terragen  - Text by M. Alan Kazlev
    Any biont, ai, or any other entity, clade, species, or group that either originated on Earth or, more generally, can trace eir ultimate origin and ancestry back to Earth; or any civilization, empire, polity or organization created or maintained by terragen sentients.
  • Terragen Sphere  - Text by Stephen Inniss
    The region of the galaxy that has been colonized or explored by Terragen sophonts.
  • Wormhole Gazetteer   - Text by Graham Donald
    Wormholes listed in the Encyclopedia Galactica (current list).
Development Notes
Text by Steve Bowers and M. Alan Kazlev

Initially published on 13 March 2001.