Sun

The Sun is the star at the centre of the Sunly system. It is a massive sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating energy from its surface mainly as visible light and infrared radiation. It is the primary source of energy for life on Earth and the dominant gravitational body around which all inner system objects orbit.

The Sun is a main-sequence star of spectral classification G2V, with a surface temperature of approximately 5,778 K and a yellow-white colour. It contains roughly 73% hydrogen and 25% helium by mass, with trace quantities of heavier elements including oxygen, carbon, and iron. It formed approximately 4.6 billion years ago from the gravitational collapse of a region within a large molecular cloud, alongside its binary companion Therne.

The Sun is one of two stars in the Sunly system. Its companion, the red dwarf Therne, orbits at a mean distance of 30 AU on a moderately eccentric orbit (e = 0.3), completing one circuit every 140.9 years. Despite Therne's prominence in the night sky, it contributes less than 0.004% of Earth's total insolation and has no measurable effect on surface climate. The stability of planetary orbits around the Sun, including Earth's, is well understood through orbital mechanics; the critical stability boundary lies at approximately 6.9 AU, far beyond Earth's orbit of 1.02 AU.

From Earth, the Sun appears as a disc roughly 31.4 arcminutes across — large enough to observe sunspots during atmospheric dimming near the horizon. Its apparent brightness of magnitude −26.7 overwhelms all other celestial objects by many orders of magnitude, including Therne at its closest approach (magnitude −13.3). At sunset, Rayleigh scattering through the atmosphere shifts the Sun's apparent colour from white through yellow and orange to deep red, a phenomenon that is particularly striking during the decades-long periods when Therne is visible near the same horizon, offering a direct colour contrast between the two stars.

The Sun's habitable zone extends from approximately 0.95 to 1.68 AU. Earth, at 1.02 AU, sits comfortably within the conservative inner boundary. The frost line lies at roughly 2.7 AU, bisecting the asteroid belt, and the Sun's gravitational dominance over planetary orbits extends to the Holman-Wiegert limit at 6.9 AU, beyond which Therne's perturbations render orbits unstable.