Solakku

Solakku

Solakku, viewed through a solar filter
Characteristics
Evolutionary stage	Main Sequence
Spectral type	kA5hA8mF4
B−V color index	0.27
Details
Mass	1.549 M☉
Radius	1.559 R☉
Luminosity	8.166 L☉
Temperature	7803 K
Metallicity [Fe/H]	0.178 dex
Age	2.6 billion years
Orbit
Mean distance from Milky Way core	8200 light-years
Galactic period	45 million years
Velocity	340 km/s about Galactic Center

Solakku is the star at the centre of the avali home system. It is chemically peculiar and roughly classifiable as an A-type main-sequence star. It orbits the galactic enter at a distance of 8200 light-years on average and is approximately 6.4 astronomical units (960,000,000 km) away from Avalon. This corresponds to about 53 light-minutes.

Solakku forms a binary solar system together with its red dwarf companion Crest. Alone, Solakku contains 91.29% of the total mass of this system and together with Crest, the two stars hold 99.77% of the total mass.

Like other main-sequence stars, Solakku produces energy through nuclear fusion of Hydrogen into Helium and emits most of this energy through light, in its case mostly visible light and infrared.

Composition

Solakku consists mainly of hydrogen and helium, though the composition is different between what is observable in the photosphere and what is present in the core. As it is already 77% through its total lifespan, Solakku’s core consists of 82% helium, with the remaining 18% being mostly hydrogen.

The measured photosphere composition is 78.3% hydrogen and 19.1% helium. Metals account for the remaining 2.6%, most notably 0.25% iron and traces of Calcium. This is an unusual high metallicity for an A-type star, making it an Am-type chemically peculiar star. This makes classification difficult. Visually, Solakku is an A8 star, but the calcium indicates A5 and the metallic lines F4, leading to its unusual spectral type kA5hA8mF4.

As elements heavier than hydrogen usually sink into the star over time due to gravity as the density of the core increases, it can generally be assumed that the metallicity of Solakku in its inner layers is even higher than what the photospheric composition would suggest.

Structure

Structurally, Solakku consist of several zones, separated by short transition layers. The main layers are the core, convective zone, radiative zone and atmosphere, which is itself split into the photosphere, chromosphere and corona.

Core

The core of Solakku makes up about 20% of its radius and is the only place inside the star where nuclear fusion is possible due to the immense pressures and temperatures, which are estimated to be as high as 21 million kelvin.

Fusion takes place through both the proton-proton chain as well as the CNO cycle. Both of these processes convert hydrogen into helium at a combined rate of 5.068×10¹² kg/s, of which 3.47×10¹⁰ kg/s (0.7%) are converted into energy and 7.095×10⁹ kg/s (0.0014%) are released as neutrinos, the mass of which is equal to roughly 2% of Solakku’s total energy output.

Convective zone

Illustration of different stars’ internal structure based on mass. Solakku on the left has an inner convective zone and an outer radiative zone.

As the CNO cycle requires higher temperatures and produces more energy, it only occurs closer to the center of the core and produces more heat, while the proton-proton chain dominates the outer core, but produces less heat.

This, combined with the generally high energy output of the core, generates a temperature gradient which is steep enough to cause convection within the star, which extends up to 50% of Solakku’s radius. This process aids in heat transfer, moving energy away from the hot core and towards the upper layers of the star.

Radiative zone

Further away from the core, temperatures and pressures fall rapidly, away from what is required to sustain convection currents. The rapid change in density in particular is what ultimately makes convection impossible. The result is a layer which is in thermal equilibrium and posseses a much more gradual temperature gradient.

This layer extends up to the surface of Solakku and transfers energy passively through thermal conduction or radiative diffusion, the latter of which gives this layer its name. Energy is only moved slowly through these processes, arriving at the final, observable surface temperature of Solakku.

Atmosphere

The atmosphere of Solakku consists of a photosphere, chromosphere and corona. The photosphere is defined as the visible surface of the star, or the deepest layer that is no opaque to visible light. Thermal photons produced here are able to escape to become sunlight. This zone is usually several hundred to a few thousand kilometres thick, a tiny fraction of Solakku’s full radius.

The spectrum of the light emitted here can be approximated as that of a black-body radiating at 7803 K, the measured surface temperature of Solakku.

Particle densities drop of rapidly in the Chromosphere and Corona, though temperatures increase to over 20,000 K in the Chromosphere and millions of K in the Corona. It is possible to briefly observe the Corona once a day on Avalon, but only from the Valaya-facing side, when Solakku itself has just been eclipsed by the ice giant, with the Corona appearing as a faint white whisp trailing the star.

The radius of the Corona, and thus the Atmosphere as a whole, is variable from point to point, depending on speed and particle density of the solar winds making up the Corona, but can be up to 24 times the radius of Solakku, past the orbit of Infernum.

Solar radiation

Sunlight

Solakku mostly emits light of higher wavelengths due to its high temperature, leading to an apparent color of blue-tinged white. As well as providing visibility during daytime, this light is the primary energy source for life on Avalon, directly powering photosynthesis.

However, Solakku also radiates ultraviolet photons, which are attenuated by Avalon’s ozone layer. UV Radiation that reaches the surface can have positive biological effects, but higher wavelengths of it can be dangerous and can be considered mutagens in some contexts. These emissions are therefore directly responsible for a number of biological adaptions seen particularly in lower latitudes on Avalon, where light has a more direct path through the ozone layer.

The light energy emitted by Solakku equals a total output power of 3.125×10²⁷ watts, but due to the inverse-square law, only about 269.9 W/m² reaches Avalon. This value is known as the solar flux and represents the amount of energy reaching Avalon’s position. The exact amount that reaches the surface may be lower and depends on atmospheric factors and latitude, but also on the exact positions of Valaya and Avalon in their respective orbits.

Solar activity

Solakku generates a constant stream of charged particles at variable rates, known as solar wind, which makes up the corona. This solar wind, driven by radiative pressure, is not uniform, but varies in strength over latitude and longitude above Solakku. It consist primarily of electrons, protons and alpha particles which have been accelerated to up to 800 km/s, but also contains hydrogen atoms and atomic nuclei of various metals.

Otherwise, Solakku is particularly quiescent. As heat transfer is radiative in its outer zone, there are no convection cells to shift the plasma making up Solakku’s surface. The complex magnetic fields formed as a result are usually the driving force of powerful events such as solar flares in lower-mass stars, but this does not occur on Solakku.

Instead, interactions between Infernum and Solakku have a profound effect. Due to Infernum’s low orbit and high mass, its gravity will pull at the plasma below it, creating a tidal wave trailing Infernum. As this wave moves faster than the rotation period of Solakku, it crashes into the slower-moving plasma and drags it along, leaving behind a trail of vortices and other shock-induced currents.

These plasma currents in turn create magnetic fields, which may combine to create further observable effects, such as plasma loops, which occur when plasma is confined within one of these fields and lifted above Solakku’s surface.

These rogue fields inevitably reconnect with each other, or the magnetic field of Solakku as a whole, releasing bursts of energy in the form of radiation all across the electromagnetic spectrum, up to ultraviolet light and X-Rays. These solar flares occur regularly on Solakku’s surface, directly below Infernum’s orbit. However, they are relatively weak compared to ones on lower-mass stars, where they are driven by much stronger convection currents.

Effects

Solar wind is almost entirely warded off by the combined magnetospheres of Avalon and Valaya, though these same fields may concentrate the charged particles received from the solar wind into Van Allen belts, which block spacecraft from parking inside whole ranges of orbits. Other celestial bodies with magnetospheres also exhibit these effects. On bodies without a magnetic field, the solar winds may reach the surface and chemically transform it, such as with the formation of Tholins.

Avalon’s atmosphere, especially the ozone layer, is responsible for attenuating or even completely filtering the dangerous ultraviolet and X-Ray radiation of solar flares. Additionally, Valaya’s orbital inclination currently situates it and Avalon slightly away from Infernum’s orbital plane. The majority of energy released by these flares is thus only rarely aimed directly at it. This also true to varying degree for most planets within the Solakku system, with Magnus and Solvis being the most at risk.

Both types of emissions have a profound impact on space travel, however. Spacecraft situated close to Avalon, Valaya or any celestial body with or within a magnetosphere are still protected from solar winds, but interplanetary space possesses a dangerously high background radiation count caused by solar wind. Both computer systems and crewed modules aboard spacecraft traversing this space must thus be built to mitigate or block the effects of solar wind emissions.

Solar flares pose an acute danger within a specific area, aligned with Infernum’s orbital plane. If one strikes a spacecraft, it may cause electrical malfunctions on space probes and severe radiation exposure to lifeforms on crewed vessels, especially on closer orbits, such as around Solvis. The energies of solar flares are not inhibited by Solvis’ magnetosphere and can thus reach any spacecraft outside of its atmosphere.

However, the possible damage they may be caused in these scenarios is significantly less than flares in lower mass stars, such as [Crest]. Most modern crewed spacecraft are constructed to handle other stars’ solar activity as well, meaning they can easily protect against Solakku’s flares. Historically, however, solar wind and solar flares lead to major difficulties in non-FTL deep-space exploration.

Life phases

Formation

Solakku formed approximately 2.6 billion years ago through gravitational collapse of a molecular cloud, beginning its life cycle. This occurred most likely at the same time as Crest’s formation, as they are projected to have very similar ages. However, Crest does not share Solakku’s chemical peculiarities, so the two stars may have formed in different regions of their molecular cloud. Another theory posits that Crest was captured by Solakku and the overlapping ages are merely a coincidence.

Some meteorites orbiting Solakku and captured for study were found to contain traces of decay products of short-lived nuclei which only form in the extreme conditions of supernovae. The amount indicates that a number of these took place near this molecular cloud, the shockwaves of which would’ve triggered the formation of Solakku and enriched the environment with additional metals, potentially explaining Solakku’s chemical peculiarity.

As Solakku was forming, a disk of gas and cloud, known as a protoplanetary disk would’ve also accumulated around it. The particles making it up then slowly accreted into larger chunks over millions of years, which could repeatedly collide and combine to form planets.

Closer in to Solakku, this process would’ve stopped at creating silicate and metal rich terrestrial bodies, such as Magnus and Solvis, but beyond the frost line, temperatures would’ve been cold enough for volatile molecules to freeze into solids and further accumulate on any forming planets, pushing them past the mass required to collect thick envelopes of lighter gases, primarily hydrogen, forming the giant planets Valaya, Edith and, to a lesser extent, Frost.

The planets then slowly migrated from their initial positions over time due to gravitational interactions, most prominently Infernum, which was transferred from its position beyond the frost line to a close orbit around Solakku. After just a few million years, solar winds from Solakku would’ve blown all remaining dust and gas away, completing the process.

Main Sequence

Position of Solakku within the Hertzsprung-Russell Diagram, showing it is currently a main-sequence star.

Solakku is just over three quarters through its main-sequence stage, during which hydrogen in its core fuses into helium. Approximately 3.6 billion years will have passed between its formation and transition into the red giant phase.

During its main-sequence phase, Solakku has gradually become cooler in its core and surface, but larger in radius, with the luminosity only slowly increasing.

Location

TODO: section on location and celestial neighbourhood

Solar system

TODO: section on planetary system with relevant links to individual bodies

TODO: embed relevant links to wikipedia articles in text