Sun

It is the Sun, drawn as a circle with a central dot. The sign is the standard shorthand for the Sun in astronomy and astrology. Astronomers hang it on quantities measured against the Sun: M☉ is one solar mass, L☉ one solar luminosity, R☉ one solar radius, so "10 M☉" means a star ten times as heavy as ours. Astrologers put it in a horoscope to mark where the Sun stood at someone's birth: "☉ in Leo" is a Leo sun sign.

The dotted circle as a sun is far older than this sign. Egyptian hieroglyphs wrote the sun and the sun god Ra with a disc carrying a central dot, and some early Chinese forms of 日 ("sun, day") were an outline with a mark inside; these are parallels, not ancestors of ☉. The compact sign Europeans use today became standard in early modern astronomy and astrology, when the planets got the small marks still printed in charts; earlier European pictures of the Sun varied and often showed rays.

Why does ☉ also stand for gold?

Alchemists paired each metal with a planet, and gold belonged to the Sun. Alchemy, the pre-modern craft of transforming substances, matched the seven metals it knew to the seven classical "planets": gold to the Sun, silver to the Moon, iron to Mars, lead to Saturn and so on. In an alchemical recipe ☉ therefore means gold, and alchemists often just called the metal Sol, Latin for Sun. Unicode's own notes for the character record this gold meaning alongside the Sun.

What does it mean when someone sends ☉?

Usually the Sun, from one of three worlds. From a science friend it is shorthand for a solar unit; from an astrology friend it is their sun sign; from anyone else it is a small, stylish sun or simply a decorative dot-in-circle, which some people also use as a bullet point. It is a plain text character, so it appears in whatever font the reader has and may look small or faint on a phone. If you want a sunny mood rather than a scholarly nod, ☀️ reads instantly; ☉ reads as learned.