Sagittarius A*
Saha ionization equation
Scale factor (cosmology)
Second law of thermodynamics
Sersic profile
Seyfert galaxy
Shape of the Universe
Sloan Digital Sky Survey
Sloan Great Wall
SN 1054
SN 1987A
Solar luminosity
Solar mass
Solar System
Soltan argument
Spectroscopic parallax
Spectroscopy
Speed of light