The sun

The sun of Realmspace, rarely also called Sol, was an enormous sphere of fire at the center of that crystal sphere, and the source of light and heat of the eight planets circling around it. In fact, the sun was so hot that any closer inspection was impossible for creatures not naturally immune to fire, even if magical aid was employed.

Features
The sun's surface was made up of varying areas of molten earth and fluid flame, a distinction which only was useful for its fire-loving inhabitants. Some areas called sunspots were somewhat cooler than their surroundings and could be recognized by their darker color. At other places, pillars and loops of flame could violently spurt outward from the sun's surface for millions of miles.

The surface also featured hundreds of portals to the Elemental Plane of Fire, allowing access to the natives of that place.

Another dreaded feature could be found 40 million miles (64 million km) away from the sun: like balls on the spokes of a wheel, twelve spherical dead-magic zones with a diameter of 100,000 miles (160,000 km) each orbited Realmspace's central body. They were called the sargassos and were feared by every spelljammer crew, which depended on magic for movement after all.

Inhabitants
The sun was inhabited by many creatures originally native to the Elemental Plane of Fire: efreet, fire elementals, firenewts, fire minions, helians, lavaworms, and salamanders had been recognized. It has also been surmised that inhabitants of other fiery places on the planes made their home there.

Only a few attempts by the sun's denizens to leave their home have been registered, but any spelljamming ship that got close to the sun was violently attacked by them, especially the helians.

History
The wildspace of the Realmspace crystal sphere was unusually warm compared to others. This led sages to speculate that the sun and Realmspace itself were the oldest among the generally known crystal spheres.

It was believed by sages and priests that the sun was created when the goddess Chauntea begged for a source of warmth, so that she could sustain the life flowering on Toril. The goddess Selûne ignited one of the heavenly bodies floating in Realmspace with pure flame from the Elemental Plane of Fire, creating the sun. This act started the enmity between Selûne and Shar that had lasted ever since.

According to old sarrukh legends and Abeiran traditions, during the cosmic cataclysm known as the Dawn War, the primordial known as Dendar the Night Serpent devoured the sun, starting the time period known as the Shadow Epoch. It was believed that a new sun was created to support life after the war was over.

Portfolio
Among the deities, Amaunator took responsibility for the sun within the Faerûnian pantheon; first Re and then Horus-Re in the Mulhorandi pantheon; and Utu within the Untheric pantheon. In the Maztican pantheon, the sun lay in the domain of Tezca. In Kara-Tur, it was believed that the Moon Women of the Celestial Bureaucracy were responsible for refilling the great oil lamps that made the Sun shine. Among the giant gods, Stronmaus had the sun within his portfolio.

Trivia

 * A golden sun on a chalice or a circlet was the symbol of the deity Siamorphe.
 * The sun was depicted on one of the cards usually contained within a deck of many things. It granted the person drawing it insight as well as a random wondrous item.