Glossary of astronomy - S

Semi-major axis is half the maximum length of an ellipse. It is used to give a physical dimension to a two-body Keplerian orbit, such as for a binary star system. However, when the distance to the system is unknown, the semi-major axis may be given as an angle.

Solstice occurs when the Sun reaches its most northerly or southerly point in the Sky as seen from Earth. It occurs around June 20th and December 21st each year. The Summer Solstice has the longest duration of daylight and the shortest period of night time, while the reverse is true for the Winter Solstice.

Spectroscopic binary is a type of binary star system where the individual components have not been resolved with a telescope. Instead, the evidence for the binarity comes from shifts observed in the spectrum. This is caused by the Doppler effect as the radial velocity of the components change over the course of each orbit.

Starburst galaxy is a galaxy that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as a merger.

Starfield or star field refers to a set of stars visible in an arbitrarily-sized field of view of a telescope, usually in the context of some region of interest within the celestial sphere.

Stellar atmosphere, or stellar envelope, is the outermost region of a star. Although it forms only a small portion of the star's mass, for some evolved stars the stellar envelope can form a significant fraction of the radius.

Stellar classification is the categorization of stars based upon their spectrum. The modern MK spectral classification scheme is a two-dimensional classification based on temperature and luminosity.

Superior planet is an archaic term that is sometimes used to refer to planets that orbit further from the Sun than the Earth, such as Saturn. The name originated from geocentric cosmology of Ptolemy.

Synodic period is the time taken for a given object to make one complete orbit about another object, as calculated respect to the background stars.

Syzygy is the straight-line configuration of three celestial bodies in a gravitational system.