Darkstar

The darkstar is a companion star to the sun of Opelyx. The darkstar and the sun are both main sequence stars but the darkstar is considerably smaller and dimmer. It has a diameter that is 72% of the sun's diameter but is only 13% as luminous. The darkstar orbits the sun once every 30.85 days. The darkstar is visible during the day but it is frequently challenging to see due to its proximity to the much brighter sun. At its maximum visual distance from the sun, the two bodies are separated by seven degrees of arc, or just over five times the apparent diameter of the sun. It is generally easily visible either at dusk or dawn most days, depending on its position in orbit. At its maximum separation from the sun, it is above the horizon for half an hour longer than the sun.

The darkstar-sun system orbits in the same orbital plane as Opelyx, meaning the stars eclipse one another regularly. This leads to frequent days of reduced brightness. When the sun eclipses the darkstar, the luminosity of the system drops to 88% of normal. When the darkstar eclipses the sun, the luminosity drops to 55% of normal. This darkening effect lends the star its name.

The moon is visually larger than the sun in the sky and can fully eclipse either the sun or the darkstar. The moon lies in a different orbital plane, however, so such eclipses only occur irregularly. On average, total eclipses of either star happen roughly every 60 years. The darkstar is most visible during total solar eclipses.