Glossary -- S

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Schwarzschild radius

the distance from a black hole's center at which the escape velocity equals the speed of light (same as the event horizon).

season

approximately three-month period bounded by an equinox and a solstice.

seeing

a measure of the amount of turbulence of the air. When the seeing is ``good'', the amount of turbulence is small and the images are steady (little twinkling). ``Poor'' seeing occurs when the atmosphere is turbulent so the images shimmer and dance around (more twinkling).

seismology

the study of a planet's interior from observations of how seismic waves (``earthquake waves'') travel through the interior.

semi-major axis

the distance between the center of an elliptical orbit and one end of the orbit along the long dimension of the elliptical orbit. It equals the average distance between two orbiting objects.

Seyfert galaxy

a spiral galaxy with a compact, very bright nucleus that produces a non-thermal continuous spectrum with broad (fat) emission lines on top.

shell burning

nuclear fusion that is occurring in a layer outside a star's core instead of inside the core as the core compresses. The fusion rate is faster than before and the outer layers are pushed outward to form a red giant.

short period comet

a comet with an orbit period less than about 200 years long that comes from the Kuiper Belt.

sidereal day

time between successive meridian crossings of a star. It is the true rotation period of a planet (on Earth, one sidereal day = 23 hours 56 minutes 4.09 seconds). Rotation rate of the Earth = 1° every 4 minutes (actually 3.989 minutes). The Earth's sidereal day is four minutes shorter than the solar day our clocks are based on so a star crosses the meridian 4 minutes earlier than it did the previous night.

sidereal period

the period of revolution of one object around another measured with respect to the stars (e.g., for the Moon, it is 27.3 days).

sidereal year

the time required for the Earth to complete an exactly 360° orbit around the Sun as measured with respect to the stars = 365.2564 mean solar days (contrast with tropical year).

solar day

time between successive meridian crossings of the Sun. Our clocks are based on this interval of time (on Earth, one solar day = 24 hours on average).

solar eclipse

when the shadow of the Moon hits the Earth at exactly new phase. The Moon covers up part or all of the Sun.

solar neutrino problem

the number of neutrinos observed to be coming from the Sun's core is significantly less than what is predicted by solar interior models.

solar wind

fast-moving, charged particles (mostly protons, electrons, and helium nuclei) flowing outward from the Sun's upper atmosphere, the corona.

solstice

point on the sky where the ecliptic is furthest from the celestial equator by 23.5°. When the Sun is at the solstice point we have either the longest amount of daylight (summer: June 21 for northern hemisphere) or the shortest amount of daylight (winter: December 21 for northern hemisphere).

south celestial pole

(SCP): projection of the Earth's south pole onto the sky. The SCP altitude = the observer's southern latitude.

speckle interferometry

method that compensates for atmospheric turbulence by taking many fast exposures of an object to freeze the effect of seeing. Computer processing of the multiple exposures removes atmospheric and instrument distortions to produce high-resolution images at the telescope's theoretical resolving power.

spectral type

the classification of a star according to its temperature as measured from the strengths of its spectral lines. In order of temperatures from hottest to coolest the spectral types are O B A F G K M. This is also the order of luminosity and mass (most luminous and most massive to dimmest and least massive).

spectroscopic binary

two stars orbiting a common point at too great a distance away from us to resolve the two stars individually, but whose binary nature is indicated in the periodic shift of their spectral lines as they orbit around each other.

spectroscopic parallax

a method of determining distances to stars from knowledge of the luminosity of their spectral types and measurement of their apparent brightness. The distances are derived from the inverse square law of light brightness.

spectroscopy

the analysis of an object from its spectrum.

spectrum

display of the intensity of light at different wavelengths or frequencies.

spherical aberration

a defect seen in images that is caused by the objective not being exactly shaped (e.g., an objective mirror not being exactly parabolic) so that not all of the light is focussed to the same point.

spiral galaxy

a highly flattened galaxy with a disk and a central bulge. The disk has a spiral pattern with slightly more stars and gas than in the rest of the disk. A slow, steady star formation rate means that they still have gas and dust left in them from which stars are still forming. The star orbits are constrained to stay within a small distance from the mid-plane of the disk and have small eccentricities.

spring tide

tide that has a large change between low and high tide. It occurs at new and full phase, when the Moon's tidal effect is aligned with the Sun's tidal effect.

standard candle

luminous objects of a known luminosity used to measure large distances via the inverse square law of light brightness.

starburst galaxy

a galaxy undergoing a large burst of star formation usually as a result of a collision or merger of two galaxies. It can produce as much light as several hundred ``normal'' undisturbed galaxies.

Stefan-Boltzmann law

relation between the amount of energy emitted by a unit area on an object producing a thermal spectrum and its temperature: energy in Joules emitted by one square meter = 5.67× 10^-8 × temperature⁴. The temperature is in Kelvin.

stellar nucleosynthesis

the creation of more massive nuclei from the fusion of less-massive nuclei inside stars. Just about all of the elements heavier than helium on the Earth were originally created via stellar nucleosynthesis.

subgiant

the stage in a star's life between the main sequence and the red giant stages. The helium core shrinks and the hdyrogen shell layer outside the core undergoes nuclear fusion. The energy of the shell burning is great enough to push the outer hydrogen layers outward and they cool off. During the subgiant stage, the expansion is such that the luminosity remains essentially constant as the outer layers expand.

sublime

the turning of a solid directly into a gas without going through the intermediate liquid phase, e.g. the vapor of ``dry ice'' (the sublimation of frozen carbon dioxide).

sunspot

cooler region on the Sun's surface that is a region of intense magnetic fields and is associated with solar activity. Because a sunspot is 1000 to 1500 K cooler, it is dimmer than the surrounding surface. The number of sunspots is greater when the Sun is more active.

supercluster

a grouping of galaxy clusters pulled together by their mutual gravitational attraction to produce long, thin structures up to a few hundred megaparsecs long with large voids devoid of galaxies between the superclusters.

supergiant

a dying star of extremely high luminosity and relatively cool surface temperature. Their diameters are over 100 times that of the Sun.

supernova

for Type II supernova: final huge mass-loss stage for a dying high-mass star where the outer layers are ejected during the core's collapse to form a neutron star. A Type I supernova is the result of enough hydrogen accreted onto a white dwarf's surface to put the white dwarf beyond the Chandresekhar limit. The white dwarf collapses and the super-rapid fusion blows the white dwarf apart (contrast with a nova). The luminosity of a supernova can temporarily be as much as an entire galaxy of billions of stars.

synodic period

the time required for a planet or moon to go from a particular configuration with respect to the Sun back to that same configuration (e.g., for the Moon, it is the time to go from a given phase back to the same phase---29.5 days).

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last update: 06 August 1999