Astronomy has used a number of symbols (astronomical symbols), such as an asterisk-like symbol (but often with five points) for a star and a circle with a dot in the center for the Sun. Those two and the symbol for Earth are those that remain the most common, most often as subscripts. Some common ones:
Note: the above renderings are by your browser: the browser and its font may give you a rendering other than what I aimed for. See the further reading for images of the symbols.