The Sun is the star around which Earth and the other planets (and asteroids, and comets) revolve as part of our Solar System. It is the brightest object in the sky, as seen from Earth, but is actually a very typical star when compared to all the other stars in the universe.
After the Copernican Revolution, in which scientific consensus finally recognized that the Earth revolved around the Sun and not the reverse, it was still widely believed that the Sun was, nevertheless, the center of the universe. We now know, thanks to scientific astronomy, that the Sun is only one of billions of stars in the Milky Way galaxy — and is, in fact, more or less on the outskirts of that galaxy, almost midway between two of its major "arms" (see Wikipedia:Milky Way and Wikipedia:Orion Arm) — and that the Milky Way itself is only one of billions of galaxies in the observable universe. Furthermore, modern cosmology holds that there is no "center of the universe" (an implication of Einstein's theory of relativity).
The sun in religion
"Overnight I became a sun-worshipper. Well, not overnight, you can't see the sun at night. But first thing the next morning, I became a sun-worshipper. Several reasons. First of all, I can see the sun, okay? Unlike some other gods I could mention, I can actually see the sun."
The sun is enormously influential. Even primitive people can see that life on Earth depends on the sun. There is nothing quite like the sun in human experience. Therefore, a link is suspected between sun worship and monotheism. Many ancient religions from Europe and Asia taught that the sun rides through the sky in a chariot every day. Somehow the sun returns to the east to rise again the following morning. Different religions teach that the sun is male or female. Norse mythology imagined that the chariot with the sun runs through the sky with wolves chasing her. Nordic views on the sun. In the sky the sun looks small enough to fit in chariot and ancient people had no way of telling that the sun is very much bigger. The sky is often seen as a vault with the sun and moon running across it. That is how things look so Bronze Age people and Iron Age people did not know any better.
As the days become shorter in autumn primitive people may get fearful. They do not know the geometry of the Solar System. Therefore there is always the possibility that the days will keep on getting steadily shorter till the sun never rises again and life on Earth ends. During autumn there may be festivals with fires to strengthen the weakening sun and prevent the above calamity. A few days after the Winter Solstice or shortest day the Romans held the festival of Sol Invictus or unconquered sun. They celebrated because the sun had overcome the forces that shorten the days and was becoming stronger again. Coincidentally or not Christians celebrate Christmas just then.
Huitzilopochtli was an Aztec sun god. The Latin Americans had neither chariots nor wheels. They had no mythology about any chariot but their sun god needed regular human sacrifices. Without human sacrifices the sun might refuse to move through the sky and might not have the strength to prevent the end of the world which was liable to happen once every 52 years.
Some apologetic arguments are based on a misunderstanding of the role of the sun, such as the argument from the second law of thermodynamics.
Some Christian apologists claim that solar fusion is a myth.