Sol

No object experiences human scrutiny as thoroughly as Sol, our Sun. Networks of satellites monitor its activities from within the corona, stretching out past the heliopause. Probes with million-kilometer long cables occasionally dive 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 these probes to dive very deep into the photosphere before they disintegrate. A couple of experiments have actually 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 millenia to millenia. 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.

Naturally, no one lives on the Sun, or even near it, though a few fools have tried.

Currency

Money is energy-backed. One sol is worth one milligram of energy, roughly 90 gigajoules, or 25 megawatt-hours. Most residents of Earth, Luna and Mars assign their energy income to specific tasks, rather than utilize it themselves - supporting the Venusian terraforming project and the construction of the Clarke Ring, for example. Coins are dimes (tenth pieces) and sols (figure it out). Bills are actually electronic writs, able to have any value, and are more like modern-day credit or debit cards in form and function. Some are more secured than others.

A kilogram of stored, pure hydrogen is thus worth about 5,000 Sols, and a liter of water about 250. In the latter case, water is 'purchased' and wastewater 'reclaimed', bought back by sanitary systems. People sometimes make money by hosting bring-your-own-beverage parties. This has increased the relative cost of food, of course, necessitating the use of credits on Ouranos Prime.