Sol

**Sol** (The Sun) is a main-sequence yellow dwarf star at the center of the Sol system and is by far the most important source of energy for life on Terra. It is a nearly perfect spherical ball of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. Its diameter is about 109 times that of Terra, and it has a mass about 330,000 times that of Terra, accounting for about 99.86% of the total mass of the Sol system. About three quarters of the Sol's mass consists of hydrogen; the rest is mostly helium, with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.

Sol is roughly middle aged and has not changed dramatically for four billion years, and will remain fairly stable for another four billion years. However, after hydrogen fusion in its core has stopped, Sol will undergo severe changes and become a red giant. It is calculated that the Sol will become sufficiently large to engulf the current orbits of Mercury, Venus, and possibly Terra. Terra's movement around Sol is the basis of the solar calendar, which is the calendar in use on the planet.

## Characteristics

Sol has an absolute magnitude of +4.83, estimated to be brighter than about 85% of the stars in the Milky Way, most of which are red dwarfs. It is a Population I, or heavy-element-rich, star. The formation of Sol may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements in the system, such as gold and uranium, relative to the abundances of these elements in Population II, heavy-element-poor, stars. These elements could most plausibly have been produced by nuclear reactions during a supernova.

In the sky of Terra, Sol is by far the brightest object in the sky, with an apparent magnitude of −26.74. This is about 13 billion times brighter than the next brightest star, Sopdet, which has an apparent magnitude of −1.46. The mean distance of the Sol's center to Earth's center is approximately 1 astronomical unit. At this average distance, light travels from the Sol's horizon to Terra's horizon in about 8 minutes and 19 seconds.

For the purpose of measurement, Sol's radius is considered to be the distance from its center to the edge of the photosphere, the apparent visible surface of the Sol. By this measure, Sol is a near-perfect sphere. The tidal effect of the planets is negligible and does not significantly affect the shape or orbit of the Sol by a notable amount. The rotational period of the Sol at its equator is about 28 days.

## Life

### Formation

Sol was formed about 4.57 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium and that probably gave birth to many other stars. As one fragment of the cloud collapsed it also began to rotate because of conservation of angular momentum and heat up with the increasing pressure. Much of the mass became concentrated in the center, whereas the rest flattened out into a disk that would become the planets and other Sol system bodies. Gravity and pressure within the core of the cloud generated a lot of heat as it accreted more matter from the surrounding disk, eventually triggering nuclear fusion. Thus,Sol was born.

### Main sequence

Sol is about halfway through its main-sequence stage, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than four million tonnes of matter are converted into energy within Sol's core, producing neutrinos and solar radiation. At this rate, Sol has so far converted around 100 times the mass of Terra into energy, about 0.03% of the total mass of the Sol. Sol will spend a total of approximately 10 billion years as a main-sequence star. Sol is gradually becoming hotter during its time on the main sequence, because the helium atoms in the core occupy less volume than the hydrogen atoms that were fused. The core is therefore shrinking, allowing the outer layers of Sol to move closer to the centre and experience a stronger gravitational force. This stronger force increases the pressure on the core, which is resisted by a gradual increase in the rate at which fusion occurs. This process speeds up as the core gradually becomes denser. It is estimated that Sol has become 30% brighter in the last 4.5 billion years. At present, it is increasing in brightness by about 1% every 100 million years.

### Post-hydrogen

Sol does not have enough mass to explode as a supernova. Instead it will exit the main sequence in approximately 5.4 billion years and start to turn into a red giant. It is calculated that the Sol will become sufficiently large to engulf the current orbits of the Sol system's inner planets, possibly including Terra.

Even before it becomes a red giant, the luminosity of the Sun will have nearly doubled, and Terra will be hotter than Venus is in the current era. Once the core hydrogen is exhausted in 5.4 billion years, the Sol will expand into a subgiant phase and slowly double in size over about half a billion years. It will then expand more rapidly over about half a billion years until it is over two hundred times larger than today and a couple of thousand times more luminous. This then starts the red-giant phase where the Sun will spend around a billion years and lose around a third of its mass.

After passing the red giant phase, Sol has approximately 120 million years to live, but much happens. First, the core, full of degenerate helium ignites violently in the helium flash, where it is estimated that 6% of the core, itself 40% of the Sun's mass, will be converted into carbon. The Sun then shrinks to around 10 times its current size and 50 times the luminosity, with a temperature a little lower than today.

When the helium is exhausted, Sol will repeat the expansion it followed when the hydrogen in the core was exhausted, except that this time it all happens faster, and the Sun becomes larger and more luminous. Soon, Sol becomes increasingly unstable, with rapid mass loss and thermal pulses that increase the size and luminosity for a few hundred years every 100,000 years or so. The thermal pulses become larger each time, with the later pulses pushing the luminosity to as much as 5,000 times the current level and the radius to over 1 AU. For Sol, four thermal pulses are predicted before it completely loses its outer envelope and makes a planetary nebula. By the end of that phase – lasting approximately 500,000 years – Sol will only have about half of its current mass.

The luminosity stays approximately constant as the temperature increases, with the ejected half of the Sun's mass becoming ionised into a planetary nebula as the exposed core reaches 30,000 K. The final naked core temperature will be over 100,000 K, after which the remnant will cool towards a white dwarf. The planetary nebula will disperse in about 10,000 years, but the white dwarf will survive for trillions of years before fading to black. The black dwarf stage is when Sol's remnant may no longer emit any light, or heat.

## Orbit and location

Sol lies close to the inner rim of the Milky Way's Orion Arm at a distance of 7.5–8.5 kpc (25,000–28,000 light-years) from the Galactic Center. Sol is contained within the Gamma Bubble, a space of rarefied hot gas, produced a supernova. The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. Sol is found in what scientists call the galactic habitable zone.

Sol's orbit around the Milky Way is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition Sol oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. The Sun's passage through the higher density spiral arms often coincides with mass extinctions on Terra, perhaps due to increased impact events. It takes the Solar System 237.5 million years to complete one orbit through the Milky Way (a galactic year), so it has completed 20–25 orbits. It takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU.