Sol

Before the Purge, no object experienced human scrutiny as thoroughly as Sol, our Sun. Networks of satellites monitored its activities from within the corona, stretching out past the heliopause. Probes with million-kilometer long cables occasionally dived into the photosphere, sending back what readings they can before becoming a part of the lower photosphere.

The photosphere itself - the 'surface' of the Sun, has an extremely low density, enabling those probes to dive very deep into the photosphere before they disintegrated. A couple of experiments even escaped the photosphere relatively intact.

The sun appears to grow and shrink slightly on a cyclical basis, although it is, actually, putting out more power from millennia to millennia. Roughly 30% brighter than its birth, the fusion reactions that drive its core are becoming more and more efficient, and will eventually drive it to swell, slowly, into a red giant. Unless somehow accounted for, this increase in Solar Output will boil away Earth's biosphere long before that, however - well within a billion years, even. This sort of window of evolution is also put forth as a reason for the radio silence of the rest of the galaxy.

Although the Sun appears yellow from the surface of Earth, it appears much more white from Space (though still slightly yellowish). This is because Raleigh scattering scatters bluer light, making the blue sky, and lets redder light pass. Any planet with a significant atmosphere will seem to have a blue sky, at least ignoring the clouds.

The Skearns

The first of the skearns appeared around Sol during the battle of Ouranos Prime. Years after the Purge, Eve warped the star away to an unknown location, and the force with which she ripped it created what is now known as the Hellskearn.

Currency

Before the Purge, the sol was the traditional unit of currency. It was energy backed, largely between a combination of solar and fusion power. One sol was worth one microgram of energy, roughly 90 megajoules, or 25 kilowatt-hours. Most residents of Earth, Luna and Mars assigned their energy income to specific tasks, rather than utilizing it themselves - supporting the Venusian terraforming project and the construction of the Terran Ring, for example.

A kilogram of stored, pure deuterium was thus worth about 3,500 Sols, and one sol was worth about 16.5 liters of water. This also set the cost of methane, common hydrogen, and highly influenced the cost of food. Since the Solar Consortium had no interest in dealing with rogue states, Ouranos Prime and the Procyon Concord used credits, which represented a gram of pure deuterium. Credits were abolished after the Battle of Ouranos Prime, after which humanity went with the sol standard completely, until the Purge.