Vector Shift — Kethra Star System#
Draft • 18 September 2026 • Worldbuilding in development
Status#
Approved by Briana, 18 September 2026: Kethra (called Selenne in the capital) orbits one star of a binary pair. The second star, the companion, lies far outside Kethra's orbit. Briana chose this arrangement ("Option B") over a planet orbiting both stars. See the planet naming decision.
Everything else in this document is a proposal. None of it is canon until Briana approves it. The proposals are based on real astrophysics and work at the level of design. Their purpose is to make the setting consistent, not to supply exact figures.
Unit used below: AU, the distance from Earth to our Sun.
The system at a glance (proposal)#
| Element | Proposal | Why it follows |
|---|---|---|
| Primary star | Kethra's sun. A Sun-like star; its type and colour are still open. | Kethra orbits it inside the habitable zone. |
| Companion star | A smaller, dimmer star on a stretched orbit. At its closest it comes about 40–60 AU from the primary; at its farthest, about three times that. | Close enough to shape the system; far enough that the primary keeps a giant planet. |
| Inner system | Compact: Kethra, perhaps one or two rocky neighbours, and one gas giant. | The companion cuts the young planet-forming disk short. Planets form only within roughly a third of its closest approach. |
| The Cinderbelt | A permanent rubble belt at the system's outer edge. | Near the cut-off, the companion stirs the rubble so hard that pieces shatter each other instead of merging, so no planet ever formed there. The belt exists because of the second star. It is the physical home of the Cinderbelt Coalition's industry. |
| The companion's worlds | The companion has its own small planetary system: a sibling system across the gap. | Each star in a wide binary keeps its own disk and can form its own planets. |
| Deep history | A bombardment era long before humans arrived. | A close pass by the companion can knock outer rubble and comets inward. |
Why the companion's orbit takes centuries#
An earlier conversational estimate said decades. That was wrong. The working:
- Closest approach 50 AU, farthest 150 AU, so the average distance (the semi-major axis) is 100 AU.
- Assume both stars together weigh about 1.5 times our Sun.
- Orbital period in years = √(average distance³ ÷ combined mass) = √(100³ ÷ 1.5) = √666,667 ≈ 820 years.
The companion moves fastest near its closest point. So the "bright era" is a comparatively short stretch of a few generations, followed by centuries of a dimmer, more distant night-star.
What people on Kethra see (proposal)#
The night-star. At its closest, the companion is a brilliant point of light. This estimate assumes it gives off half the primary's light:
- At 50 AU: it receives 0.5 ÷ 50² = 1/5,000 of the primary's brightness. That is about 9 magnitudes (the astronomers' brightness scale) fainter than the sun, or roughly 80 times as bright as Earth's full moon. Bright enough to read by and to cast shadows.
- At 150 AU: 0.5 ÷ 150² = 1/45,000. About 12 magnitudes fainter than the sun, or roughly 9 times as bright as Earth's full moon.
Bright nights and dark nights. As Kethra goes around its sun each year, the companion is sometimes on the opposite side from the sun (up all night, so nights never get properly dark) and sometimes on the same side (up during the day, so nights are fully dark). The difference between the two halves of the year is strongest during the bright era.
Eras of the night-star. Across the 800-year cycle, the night-star grows brighter and then fades. Chronicles, calendars and religions could mark the bright era as an age in its own right.
Long climate cycles. The companion's pull slowly stretches and tilts Kethra's orbit. That drives ice ages and warm periods more strongly than on Earth. The landscape shows the evidence: old ice lines, drowned coasts, settlements abandoned as the climate shifted.
Two kinds of light. If the two stars differ in colour, plants may have evolved darker or multi-coloured leaves to use both.
Hooks for the story (proposal)#
- Calendars as politics. A world with two overlapping cycles needs a decision about which calendar is official. The capital (Selenne) and the frontier (Kethra) could keep different calendars, in the same way they use different names.
- The sibling system. The companion's worlds are visible and reachable, a natural frontier beyond the frontier.
- The Cinderbelt. A rubble belt rich in raw material, created by the second star and always in dispute.
Still open (Briana's decisions)#
- Names for the primary star and the companion, possibly one capital name and one frontier name for each.
- The type and colour of each star.
- Kethra's distance from its sun, its day length, year length, axial tilt and seasons.
- Kethra's moons, if any.
- Whether this is humanity's homeworld or a world humans settled, and how humans came to be in Andromeda.
- Where in the night-star's 800-year cycle the story takes place.
- Where the bombardment era sits in deep time, and what it left behind.
Guidance for AI actors#
- Treat only the Status section as canon. Everything else is a working proposal. Do not state it as established fact in-world unless Briana approves it.
- Characters would not know the astrophysics. They would know what they can see: the night-star, bright and dark nights, the belt, old ice lines. Their explanations may be religious, folk or scientific, depending on the character.
Prepared by Claude.