☉Sun

The Sun is a giant star that holds 99.86% of the entire Solar System's mass all by itself, shining with the energy released as it fuses more than 600 million tons of hydrogen into helium in its core every second. Its core reaches about 15 million degrees, its surface (the photosphere) about 5,500 degrees, and yet the atmosphere above it, the "corona," is a mystery hotter still at over 1 million degrees. The light it gives off reaches Earth in about 8 minutes and 20 seconds. Now a middle-aged star that has lived about 4.6 billion years, in some 5 billion years it will swell into a red giant, then cast off its outer layers and cool into a white dwarf.

☀ Sunspots · The dark, cooler patches

Look closely at the Sun's surface and you will see blotches that appear darker than their surroundings: "sunspots." A sunspot is a region where the Sun's intense magnetic field bunches up and blocks the flow of energy to that spot, making it a "relatively cool" area about 1,500 degrees lower than its surroundings (around 5,500 degrees). It only looks dark; in reality it is still blindingly bright. The number of sunspots rises and falls in a cycle of about 11 years (the solar cycle), and when sunspots are plentiful, eruptions on the Sun's surface (flares) and huge ejections of material grow more frequent, sometimes sparking beautiful auroras on Earth and sometimes disrupting communications and satellites. By watching sunspots drift across the disk through his telescope, Galileo was the first to reveal that the Sun, too, rotates.

The Sun was worshipped as the most important deity in nearly every civilization. In Greek and Roman mythology it was the sun god Helios (the Roman Sol), who drove a chariot of fire drawn each day by four winged horses across the sky, and who was later identified with Apollo, god of light and prophecy. In Egypt, the sun god Ra was believed to sail across the sky by day and pass through the underworld by night. In East Asia the Sun was called "hae" and likened to the king and to the yang principle, and a three-legged crow, the Samjogo, was said to dwell within it. Astronomically, the symbol for the Sun, ☉ (a dot within a circle), is thought to derive from ancient Egypt.