Vector Shift — Kethra Star System (v2)#
Draft v2 • 18 September 2026 • Worldbuilding in development
This version replaces draft v1, which is left unchanged as a record. The companion star is different in this version, and its distances and orbit have been recalculated. Two estimates given in conversation are corrected under Corrections from earlier estimates.
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 ("Option B"). See the planet naming decision.
- The primary star is a G-class yellow-white sun like ours.
- The companion is a cool giant star that is red-orange, "more red" than orange.
Briana first described the companion as "like a Betelgeuse, more orange than red", at about 3,750. Betelgeuse is a supergiant, and a supergiant cannot share a system with a living world (see below). Claude therefore proposed a smaller giant star with the colour Briana wanted, and Briana accepted it: "perhaps more red in it, but yes".
Everything else below is a proposal, including the exact temperature, distances and orbit. The figures are design-level estimates grounded in real astrophysics, not precise values.
Unit used below: AU, the distance from Earth to our Sun. Star temperatures are in kelvin (K). 3,600 K is about 3,330 °C.
Why a giant, not a supergiant#
Stars in a binary form together, so they are the same age. A Betelgeuse-type supergiant is 15 to 20 times the Sun's mass. It lives only about 10 million years, is roughly 100,000 times as bright as the Sun, and ends in a supernova. Kethra would still be molten at that age, would be sterilised by the light, and would lose its atmosphere in the explosion.
A giant avoids all of this. The companion was born slightly heavier than Kethra's sun. A heavier star burns through its fuel faster, so the companion has already swollen into a giant while the G-type sun is still in middle age. Both stars, and Kethra, are a few billion years old, which is enough time for life. The giant will eventually shrink quietly into a white dwarf. It will not become a supernova.
The two stars (proposal)#
| Primary: Kethra's sun | Companion: the giant | |
|---|---|---|
| Type | G-class, like our Sun | Red giant, early M-class |
| Colour | Yellow-white | Red-orange |
| Surface temperature | about 5,800 K | about 3,600 K |
| Mass | about 1 Sun | about 1.2 Suns |
| Width | about 1 Sun | about 40–45 Suns |
| Total light output | 1 Sun | about 290 Suns (a large share of it is infrared) |
| Life stage | Middle age | Late life, slowly growing larger |
For colour, a real red giant such as Gacrux in the Southern Cross is a reasonable guide.
The companion's orbit (proposal)#
- Closest approach: about 150 AU. Farthest point: about 450 AU.
- Orbital period: about 3,500 years. Working: average distance 300 AU; combined mass about 2.2 Suns; √(300³ ÷ 2.2) ≈ 3,500 years.
- This is much farther out than in v1, because the giant is far brighter than the small companion assumed there. At v1's 50 AU, it would add about 12% more heat than Kethra's own sun provides.
The system's layout (proposal, carried over from v1 and rescaled)#
| Element | Proposal |
|---|---|
| Inner system | Kethra, perhaps one or two rocky neighbours, and one gas giant. The companion now truncates the primary's planet-forming disk at roughly 50 AU. That leaves room for a normal inner system and at least one outer giant planet. |
| The Cinderbelt | A permanent rubble belt at the system's outer edge. The companion's pull stirs the material there so strongly that it never came together into a planet. This is the physical home of the Cinderbelt Coalition's industry. |
| The companion's worlds | Any planets that formed around the companion would have been scorched or disrupted as it swelled. They could now be burnt, hollowed or strange worlds: ruins in the sky. This replaces v1's "sibling system" and is a new proposal. |
| Deep history | A bombardment era from a past close pass. The companion's swelling also changed the outer system relatively recently, in astronomical terms. |
What people on Kethra see (proposal)#
A second, smaller sun. At closest approach the giant is a visible red-orange disk about a third the width of the sun. Working: a star about 0.4 AU across, seen from 150 AU, spans about 0.15°. The sun spans about 0.5°. At its farthest point it is a small disk or a very bright point.
Red nights. For roughly half of each year, the giant is up during the night. During the rest of the year it shares the daytime sky with the sun, and nights are fully dark.
| Giant in the night sky | At closest approach (150 AU) | At farthest point (450 AU) |
|---|---|---|
| Visible light | about 400 lux: a lamp-lit room, or a deep red-orange dusk lasting all night | about 50 lux: early twilight |
| Compared with Earth's full moon | about 1,700× brighter | about 200× brighter |
Working, at closest approach: total light 290 ÷ 150² ≈ 1.3% of the sun's. A red giant emits a large share of its energy as infrared, which the eye cannot see. As a result, only about a third of that 1.3% reaches the eye as light, compared with sunlight, giving about 0.4% of daylight, or roughly 400 lux. These are estimates, good to within about a factor of two.
Eras. Across the 3,500-year orbit, the red nights grow brighter and then fade. The bright stretch around closest approach lasts a few centuries. Chronicles, calendars and religions would treat the eras as ages in their own right.
Hot eras. At closest approach the giant adds about 1.3% to the heat Kethra receives. Even that small increase is several times stronger than the effect of doubling carbon dioxide on Earth. So each close pass brings a warm era: seas rise and growing regions shift toward the poles. As the giant recedes, the climate cools. Kethra's own orbital cycles add ice ages and warm periods on top.
A slow clock. The giant is still growing, over millions of years. No character would notice it. Scholars could know it, and faiths could have views about it.
Two kinds of light. Plants may have evolved darker or multi-coloured leaves to use both the yellow-white sun and the red giant.
Corrections from earlier estimates#
- Companion orbital period. In conversation Claude first said the orbit took "decades". v1 corrected this to about 820 years for its smaller companion. v2's orbit is about 3,500 years.
- Night brightness. In conversation Claude stated that the orange-giant version gave about 4,000 times full moonlight. That figure treated all of the giant's energy as visible light. After correcting for infrared, the orange version gives about 2,000×, and the redder v2 companion about 1,700× at closest approach.
Hooks for the story (proposal)#
- Calendars as politics. The capital (Selenne) and the frontier (Kethra) could keep different calendars, in the same way they use different names for the planet.
- Red-night seasons. Work, crime, worship and warfare could all change when the nights stay red and lit.
- The dying star. A slowly swelling giant is a ready symbol for anyone preaching an ending, or a beginning.
- The Cinderbelt. A rubble belt rich in raw material and always contested.
Still open (Briana's decisions)#
- Names for the sun and the giant, possibly a capital name and a frontier name for each.
- Kethra's distance from its sun, day length, year length, axial tilt and seasons.
- Kethra's moons, if any.
- Whether this is humanity's homeworld or a settled world, and how humans came to be in Andromeda.
- Where in the giant's 3,500-year orbit the story takes place: a bright, warm era or a dim, cool one.
- Where the bombardment era falls 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.
- Characters know only the sky they can see: the sun, the red giant, red nights and dark nights, the belt, and old coastlines and ice lines. Depending on the character, their explanations may be religious, folk or scientific.
Prepared by Claude.