SO: A transition type of galaxy between ellipticals and spirals; has a disk but no arms.
scraps: Lines of cliffs; found on Mercury, Earth, The Moon and Mars.
scattered: Light absorbed and then reemitted in all directions.
Schmidt telescope (Schmidt camera): A telescope that uses a spherical mirror and a thin lens to provide photographs of a wide field.
Schwarzschild radius: The radius that, according to Schwarzschild’s solutions to Einstein’s equations of the general theory of relativity, corresponds to the event horizon of a black hole.
scientific method: No easy definition is possible, but it has to do with a way of testing verifying hypotheses.
scientific notation: Exponential notation.
secondary cosmic rays: High-energy particles generated in the Earth’s atmosphere by primary cosmic rays.
secondary distance indicators: Ways of measuring distance that are calibrated by primary distance indicators.
sedimentary: Formed from settling in a liquid or from deposited material, as for a type of rock.
seeing: The steadiness of the Earth’s atmosphere as if it affects the resolution that can be obtained in astronomical observations.
seismic waves: Waves traveling through a solid planetary body from an earthquake or impact.
seismology: The study of waves propagating through a body and the resulting deduction of the internal properties of the body. “Seismo-” comes from the Greek for earthquake.
semimajor axis: Half the major axis, that is, for an ellipse, half the longest diameter.
semiminor axis: Half the minor axis, that is, for an ellipse, half the shortest diameter.
Seyfert galaxy: A type of galaxy that has an unusually bright nucleus and whose spectrum shows broad emission lines that cover a side range of ionization stages.
Shapley-Curtis debate: The 1920 debate (and its written version) on a scale of our galaxy and of “spiral nebulae.”
shear wave: A type of twisting seismic wave.
shock wave: A front marked by an abrupt change in pressure caused by an object moving faster that the speed of sound in the medium through which the object is traveling.
shooting stars: Meteors.
shower: A time of many meteors from a common cause.
sidereal: With respect to the stars.
sidereal day: A day with respect to the stars.
sidereal rotation period: A rotation with respect to the stars.
sidereal time: The hour angle of the vernal equinox; equal to the right ascension of objects on your meridian.
sidereal year: A circuit of the Sun with respect to the stars.
significant figure: A digit in a number that is meaningful (within the accuracy of the data).
singularity: A point in space where quantities become exactly zero or infinitely large; one is present in a black hole.
slit: A long, thin gap through which light is allowed to pass.
SOHO: The solar and Heliospheric Observatory, a joint NASA and European Space Agency mission to study the Sun. It is 1 million kilometers toward the sun from Earth, a location from which it can view the Sun constantly with its dozen instruments, which include coronagraphs, cameras with filters that show the corona, and particle detectors.
solar activity cycle: The 11-year cycle with which solar activity like sunspots, flare, and prominences varies.
solar atmosphere: The photosphere, chromosphere, and corona of the Sun.
solar constant: The total amount of energy that would hit each square centimeter of the top of the Earth’s atmosphere at the Earth’s average distance from the Sun.
solar day: A full rotation with respect to the Sun.
solar dynamo: The generation of sunspots by the interaction of convection, turbulence, differential rotation, and magnetic field.
solar flares: An explosive release of energy on the Sun.
solar mass: The mass of the Sun, 1.99 X 1033 grams, about 330,000 tons the Earth’s mass.
solar rotational period: The time for a complete rotation with respect to the Sun.
solar time: A system of timekeeping with respect to the Sun such that the Sun is overhead of a given location at noon.
solar wind: An outflow of particles from the Sun representing the expansion of the corona.
solar year: (tropical year) An object’s complete circuit of the Sun; a tropical year in between vernal equinoxes.
solstice: The point on the celestial sphere of northernmost or southernmost declination of the Sun in the course of a year; colloquially, the time when the Sun reaches that point.
space velocity: The velocity of a star with respect to the Sun.
spallation: The break-up of heavy nuclei that undergo nuclear collisions.
special theory of relativity: Einstein’s 1905 theory of relative motion.
speckle interferometry: A method obtaining higher resolution of an image by analysis of a rapid series of exposures that freeze atmospheric blurring.
spectra classes: See spectrum.
spectral lines: Wavelengths at which the brightness is abruptly different from the brightness at neighboring wavelengths.
spectral type: One of the categories O, B, A, F, G, K, M, L, into which stars can be classified from study of their spectral lines, or extensions of the system. The above decreasing sequence of spectral types corresponds to a sequence of surface temperature.
spectrograph: A device to make and record a spectrum.
spectrometer: A device to make a electronically measure a spectrum.
spectroscopy: The use of spectrum analysis.
spectrum (plural: spectra): A display of electromagnetic radiation spread out by wavelength or frequency.
speed of light: By Einstein’s special theory of relativity, the speed at which all electromagnetic radiation travels, and the largest possible speed of an object moving through space.
spherical aberration: For an optical system, a deviation from perfect focusing by having a shape that is too close to that of a sphere.
spicule: A small jet of gas at the edge of the quiet Sun, approximately 100 km in diameter and 10,000 km high, with a lifetime of about 15 minutes.
spin-flip: Transition in the relative orientation of the spins of an electron and the nucleus it is orbiting.
spiral arms: Bright regions looking like a pinwheel.
spiral density wave: A wave that travels around a galaxy in the form of a spiral, compressing gas and dust to relatively high density and thus beginning star formation at those compressions.
spiral galaxy: A class of galaxy characterized by arms that appear as though they are unwinding like a pinwheel.
spiral nebula: The old name for a shape in the sky, seen with telescopes, with arms spiraling outward from the center; provided in the 1920s actually to be a galaxy.
sporadic: Not regularly.
sporadic meteor: A meteor not associated with a shower.
spring tides: The tides at their highest, when the Earth, Moon, and Sun are in a line (from “to spring up”)
stable : Tending to remain in the same condition.
star: A self-luminous ball of gas that shines or has shone because of nuclear reaction in its interior.
star clusters: Groupings of stars of common origin.
stationary limit: In a rotating black hole, the radius within which it is impossible for an object to remain stationary, no matter what it does.
steady-state theory: The cosmological theory based on the perfect cosmological principle, in which the Universe is unchanging over time.
Stefan-Boltzmann law: The radiation law that states that the energy emitted by a black body varies with the fourth power of its surface temperature.
stellar atmosphere: The outer layer of stars not completely hidden from our view.
stellar chromosphere: The region above a photosphere that shows an increase in temperature.
stellar corona: The outermost region of a star characterized by temperatures of 106 K and high ionization.
stellar evolution: The changes of a star’s properties with time.
stelliferous era: The current era, with lots of stars in the Universe.
stones: A stony type of meteorite.
stratosphere: An upper layer of a planet’s atmosphere, above the weather, where the temperature begins to increase. The Earth’s stratosphere is at 20–50 km.
streamers: Coronal structures at low solar latitudes.
string theories: See superstring theories.
strong force: The nuclear force, the strongest of the four fundamental forces of nature.
strong nuclear force: The strong force.
subtend: The angle that an object appears to take up in your field of view; for example, the full Moon subtends ½º.
sunspot: A region of the solar surface that is dark and relatively cool; it has an extremely high magnetic field.
sunspot cycle: The 11-year cycle of variation of the number of sunspots visible on the Sun.
superbolt: Giant Lightning.
supercluster: A cluster of clusters of galaxies.
supercooled: The condition in which a substance is cooled below the point at which it would normally make a phase change; for the Universe, the point in the early Universe at which it cooled below a certain temperature without breaking its symmetry; the strong and electroweak forces remained unified.
supergiant: A post-main-sequence phase of evolution of stars of more that about 8 solar masses. They fall in the upper right of the temperature-luminosity diagram; luminosity class I.
supergranulation: Convection cells on the solar surface about 20,000 km across and vaguely polygonal in shape.
supergravity: A theory unifying the four fundamental forces.
superluminal speed: An apparent speed greater than that of light.
supermassive black hole: A black hole of millions or billions of times the Sun’s mass, as it found in the center of galaxies and quasars.
supermassive star: A stellar body of more than about 100 M
supernova (plural: supernovae): The explosion of a star with the resulting release of tremendous amounts of radiation.
supernova remnants: The gaseous remainder of the star destroyed in a supernova.
superstring (string) theories: A possible unification of quantum theory and general relativity in which fundamental particles are really different vibrating forms of a tiny, one-dimensional “string” instead of being localized at single points.
symmetric: A correspondence of shape so that rotating or reflecting an abject gives you back an identical form; symmetry of forces in the early Universe corresponds to forces acting identically that now act differently as the four fundamental forces; gravity, electromagnetism, weak nuclear force, and strong nuclear force.
synchronous orbit: An orbit of the same period; a satellite in geostationary orbit has the same period as the Earth’s rotation and so appears to hover.
synchronous rotation: A rotation of the same period as an orbiting body.
synchronous radiation: Nonthermal radiation emitted by electrons spiraling at relativistic velocities in a magnetic field.
synodic: Measured with respect to an alignment of astronomical bodies other than or in addition to the Sun or the stars.
syzygy: An alignment of three celestial bodies.