The radius of the event horizon around a black hole.
second of arc (arcsecond)
Measure of angle. A full circle equals 360 degree. 1 degree equals 60 arcminutes.
1 arcminute equals 60 arcseconds.
seeing
Atmospheric condition on a given night. When the atmosphere is unsteady, producing blurred images, the seeing is said to be poor.
SETI
The Search for Extraterrestrial Intelligence.
SIM
Space Interferometery Mission
solar system
The Sun and its planets, astroids, comets and so on.
solar wind
Rapidly moving atoms and ions that escape from the solar corona and blow outward through the solar system.
spectral class or type
A star's position in the temperature classification system O, B, A, F, G, K, M. Based on the appearance of the spectrum.
spectrograph
A device that separates light by wavelengths to produce a spectrum.
spectroscopic binary
A star system in which the stars are too close together to be visible separately. We see a single point of light, and only by taking a spectrum can we determine that there are two stars.
spectroscopic parallax
The method of determining a star's distance by comparing its apparent magnitude with its absolute magnitude as estimated from its spectrum.
spiral galaxy
A galaxy with an obvious disk component containing gas; dust hot, bright stars; and spiral arms.
stellar parallax
A measure of stellar distance (see parallax).
supernova remnant (SNR)
The expanding shell of gas marking the site of a supernova explosion.