satellite A body that orbits around a planet, such as the Moon (a natural satellite) or the Chandra X-ray Observatory (an artificial telescope), both of which orbit around the Earth.
SO Galaxy Galaxy which shows evidence of a thin disk and a bulge, but which has no spiral arms and contains little or no gas.
SBO Galaxy An SO-type galaxy whose disk shows evidence of a bar.
Schwarzschild radius The distance from the center of a non-rotating black hole such that, if all the mass were compressed within that region, the escape velocity would equal the speed of light. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible. See event horizon.
scientific method The set of rules used to guide science, based on the idea that scientific "laws" be continuously tested, and replaced if found inadequate.
second of arc See "arc second."
Seyfert galaxy Type of active galaxy that exhibits intense energetic activity from a small region within the nucleus of an otherwise normal-looking spiral galaxy.
shock waves A wave front marked by an abrupt change in pressure caused by an object or material moving faster than the speed of sound. For example, a sonic boom produced by an aircraft going faster than the speed of sound.
singularity A point in the universe where the density of matter and the gravitational field are infinite, such as the center of a black hole.
(SIRTF) Space Infrared Telescope Facility NASA's Great Observatory for infrared astronomy, later renamed Spitzer, in honor of Lyman Spitzer, Jr.
solar flare An outburst caused by the sudden release of energy that heats and accelerates matter in the solar atmosphere, and produces a sudden brightening over a wide range of wavelengths.
south celestial pole Point on the celestial sphere directly above the Earth's south pole.
spacetime A synthesis of the three dimensions of space and of a fourth dimension, time; a hallmark of relativity theory.
spectral class Classification scheme, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.
spectral line A radiative feature observed in emission (bright) or absorption (dark) at a specific frequency or wavelength.
spectrometer Instrument used to produce detailed spectra of cosmic objects. Usually, a spectrometer records a spectrum in electronic form on a computer.
spectroscope Instrument used to view a light source so that it is split into its component colors.
spectroscopic binary A binary-star system which from Earth appears as a single star, but is known to contain more than one star because of the back-and-forth Doppler shifts that are observed as the two stars orbit one another. another.
spectroscopic parallax Method of determining the distance to a star by measuring its temperature and then determining its absolute brightness by comparing with a standard H-R diagram. The absolute and apparent brightness of the star gives the star's distance from Earth.
spectroscopy The study of the way in which atoms absorb and emit light electromagnetic radiation. Spectroscopy allows astronomers to determine the chemical composition of stars.
speed of light The fastest possible speed, according to the currently known laws of physics. Electromagnetic radiation exists in the form of waves or photons moving at the speed of light.
spiral arm Distribution of material in a galaxy in a pinwheel-shaped design apparently emanating from near the galactic center.
spiral density wave (i) a wave of matter formed in the plane of planetary rings, similar to ripples on the surface of a pond, which wrap around the rings forming spiral patterns similar to grooves in a record disk. Spiral density waves can lead to the appearance of rings. (ii) A proposed explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation.
spiral galaxy Galaxy composed of a flattened, star-forming disk component which may have spiral arms and a large central galactic bulge.
Spitzer Space Telescope NASA's Great Observatory for infrared astronomy was launched in August 2003. Formerly named SIRTF (Space Infrared Telescope Facility), it was renamed in honor of Lyman Spitzer, Jr.
S process "Slow" nuclear fusion that occurs in highly evolved stars when a neutron is slowly captured by a nucleus.
standard candle Any object with an easily recognizable appearance and known luminosity, which can be used in estimating distances. Supernovae, which all have the same peak luminosity (depending on type) are good examples of standard candles and are used to determine distances to other galaxies, see also Cepheid variable.
star A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core. [More Info: Field Guide]
star cluster A grouping of anywhere from a dozen to a million of stars which formed at the same time from the same cloud of interstellar gas. Stars in clusters are useful to aid our understanding of stellar evolution because they are all roughly the same age and chemical composition, and lie at roughly the same distance from the Earth.
stellar evolution The changes experienced by stars as they originate, mature, and grow old. [More Info: Field Guide]
stellar nucelosynthesis The process by which heavier elements are formed from lighter ones by nuclear fusion in the cores of stars.
stellar occulation The dimming of starlight produced when a solar system object such as a planet, moon or ring, passes directly in front of a star.
strong force See "nuclear force."
subgiant branch The section of an evolutionary track of a star that corresponds to changes that occur just after hydrogen is depleted in its core, and core hydrogen burning ceases. Shell hydrogen burning heats the outer layers of the star, which causes a general expansion of the stellar envelope.
supercluster Grouping of several clusters of galaxies into a larger, but not necessarily gravitationally bound, unit.
supergiant star An extremely luminous, massive star with a radius between 100 and 1000 times that of the Sun.
supermassive black hole A black hole with a mass much greater than the most massive stars (100 solar masses). The central regions of virtually every galaxy are thought to contain a supermassive black hole of a million solar masses or more. [More Info: Field Guide]
supernova Explosive death of a star, caused by the sudden onset of nuclear burning in a white dwarf star (Type Ia), or gravitational collapse of the core of massive star followed by a shock wave that disrupts the star(Type II, Type Ib, Ic). A supernova is one of the most energetic events of the universe and may temporarily outshine the rest of the galaxy in which it resides. [More Info: Field Guide]
synchrotron radiation Type of nonthermal radiation caused by high-speed charged particles, such as electrons, emitting radiation as they are accelerated in a magnetic field. [More Info: Field Guide]
synchronous orbit State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is in synchronous orbit, and so presents the same face toward Earth at all times.