S0 galaxy Galaxy which shows evidence of a thin disk and a bulge, but which has no spiral arms and contains little or no gas.

SB0 galaxy S0-type galaxy whose disk shows evidence of a bar.

scarp Surface feature on Mercury believed to be the result of cooling and shrinking of the crust, forming a wrinkle on the face of the planet.

Schwarzschild radius The distance from the center of an object such that, if all the mass 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.

scientific method The set of rules used to guide science, based on the idea that scientific laws be continually tested, and modified or replaced if found inadequate.

seasonal cap Portion of Martian polar ice caps that is subject to seasonal variations, growing and shrinking once each Martian year.

seasons Changes in average temperature and length of day that result from the tilt of Earth's (or any planet's) axis with respect to the plane of its orbit.

secondary atmosphere The chemicals that composed Earth's atmosphere after the planet's formation, once volcanic activity outgassed chemicals from the interior.

seeing A term used to describe the ease with which good telescopic observations can be made from Earth's surface, given the blurring effects of atmospheric turbulence.

seeing disk Roughly circular region on a detector over which a star's pointlike images is spread, due to atmospheric turbulence.

seismic wave A wave that travels outward from the site of an earthquake through the Earth.

semi-major axis One half of the major axis of an ellipse. The semi-major axis is the way in which the size of an ellipse is usually quantified.

semi-major axisx Type of active galaxy whose emission comes from a very small region within the nucleus of an otherwise normal-looking spiral system.

Seyfert galaxy Type of active galaxy whose emission comes from a very small region within the nucleus of an otherwise normal-looking sprial system.

shepherd satellite Satellite whose gravitational effect on a ring helps preserve the ring's shape. Examples are two satellites of Saturn, Prometheus and Pandora, whose orbits lie on either side of the F ring.

shield volcano A volcano produced by repeated nonexplosive eruptions of lava, creating a gradually sloping, shield-shaped low dome. Often contains a caldera at its summit.

shock wave Wave of matter, which may be generated by a star, which pushes material outward into the surrounding molecular cloud. The material tends to pile up, forming a rapidly-expanding shell of dense gas.

sidereal day The time needed for a star on the celestial sphere to make one complete rotation in the sky.

sidereal month Time required for the Moon to complete one trip around the celestial sphere.

sidereal year The time required for the constellations to complete once cycle around the sky and return to their starting points, as seen from a given point on Earth.

singularity A point in the universe where the density of matter and the gravitational field are infinite, such as at the center of a black hole.

solar constant The amount of solar energy reaching Earth per unit area per unit time, approximately 1400 W/m2.

solar core The region at the center of the Sun, with a radius of nearly 200,000 km, where powerful nuclear reactions generate the Sun's energy output.

solar cycle The 22-year period that is needed for both the average number of spots and the Sun's magnetic polarity to repeat themselves. The Sun's polarity reverses on each new 11-year sunspot cycle.

solar day The period of time between the instant when the Sun is directly overhead (i.e. at noon) to the next time it is directly overhead.

solar eclipse Celestial event during which the new Moon passes directly between the Earth and Sun, temporarily blocking the Sun's light.

solar interior The region of the Sun between the solar core and the photosphere.

solar maximum The starting point of the sunspot cycle, during which only a few spots are seen. They are generally confined to narrow regions, one in each hemisphere, at about 25-30 degrees latitude.

solar minimum The starting point of the sunspot cycle, during which only a few spots are seen. They are generally confined to narrow regions, one in each hemisphere, at about 25&151;30 degrees latitude.

solar nebula The swirling gas surrounding the early Sun during the epoch of solar system formation, also referred to as the primitive solar system.

solar neutrino problem The discrepancy between the theoretically predicted numbers of neutrinos streaming from the Sun as a result of fusion reactions in the core and the numbers actually observed. The observed number of neutrinos is only about half the predicted number.

solar system The Sun and all the bodies that orbit it—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their moons, the asteroids, and the comets.

solar wind An outward flow of fast-moving charged particles from the Sun.

south celestial pole Point on the celestial sphere directly above the Earth's south pole.

spectral class Classification scheme, based on the strength of stellar spectral lines, which is an indication of the temperature of a star.

spectrometer Instrument used to produce detailed spectra of stars. Usually, a spectrograph records a spectrum on a photographic plate, or more recently, 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 whose spectral lines show back-and-forth Doppler shifts as two stars orbit one 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 brightnesses of the star give the star's distance from Earth.

spectroscopy The study of the way in which atoms absorb and emit 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.

spin-orbit resonance State that a body is said to be in if its rotation period and its orbital period are related in a simple way.

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 ringlets. (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.

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.

Standard Solar Model A self-consistent picture of the Sun, developed by incorporating the important physical processes that are believed to be important in determining the Sun's internal structure, into a computer program. The results of the program are then compared with observations of the Sun, and modifications are made to the model. The Standard Solar Model, which enjoys widespread acceptance, is the result of this process.

star A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core.

star cluster A grouping of anywhere from a dozen to a million 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 Earth.

starburst galaxy Galaxy in which a violent event, such as near-collision, has caused a sudden, intense burst of star formation in the recent past.

Stefan's law Relation that gives the total energy emitted per square centimeter of its surface per second by an object of a given temperature. Stefan's law shows that the energy emitted increases rapidly with an increase in temperature, proportional to the temperature raised to the fourth power.

stellar nucleosynthesis The formation of heavy elements by the fusion of lighter nucleii in the hearts of stars. Except for hydrogen and helium, all other elements in our universe result from stellar nucleosynthesis.

stellar occultation The dimming of starlight produced when a solar system object such as a planet, moon, or ring, passes directly in front of a star.

stratosphere The portion of Earth's atmosphere lying above the troposphere, extending up to an altitude of 40 to 50 km.

strong nuclear force Short-range force responsible for binding atomic nuclei together. The strongest of the four fundamental forces of nature.

subgiant branch The section of the evolutionary track of a star that corresponds to changes that occur just after hydrogen is depleted in the 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.

summer solstice Point on the ecliptic where the Sun is at its northernmost point above the celestial equator, occurring on or near June 21.

sunspot An Earth-sized dark blemish found on the surface of the Sun. The dark color of the sunspot indicates that it is a region of lower temperature than its surroundings.

sunspot cycle The fairly regular pattern that the number and distribution of sunspots follows, in which the average number of spots reaches a maximum every 11 or so years, then falls off to almost zero.

supercluster Grouping of several clusters of galaxies into a larger, but not necessarily gravitationally bound, unit.

supergiant A star with a radius between 100 and 1000 times that of the Sun.

supergranulation Large-scale flow pattern on the surface of the Sun, consisting of cells measuring up to 30,000 km across, believed to be the imprint of large convective cells deep in the solar interior.

supernova Explosive death of a star, caused by the sudden onset of nuclear burning (type I), or an enormously energetic shock wave (type II). One of the most energetic events of the universe, a supernova may temporarily outshine the rest of the galaxy in which it resides.

supernova remnant The scattered glowing remains from a supernova that occurred in the past. The Crab Nebula is one of the best-studied supernova remnants.

synchrotron radiation Type of nonthermal radiation caused by high-speed charged particles, such as electrons, as they are accelerated in a strong magnetic field.

synchronous orbit State of an object when its period of rotation is exactly equal to its average orbital period. The Moon is in a synchronous orbit, and so presents the same face toward Earth at all times.

synodic month Time required for the Moon to complete a full cycle of phases.