SATELLITE - Body in orbit around another larger body (such as a planet or star).
SCALAR - Quantity that has only magnitude as opposed to both magnitude and direction. For example, mass is scalar quantity. By convention in physics the word "speed" is a scalar quantity, having only magnitude, while the word "velocity" is used to denote both the speed and the direction of the motion and is thus a vector quantity.
SCALE HEIGHT - Perpendicular distance over which a particular physical variable drops by a factor of e (~2.71828). The scale height depends on the type of material/object observed. In an atmosphere (planetary or stellar), the scale height is the vertical distance over which the atmospheric pressure drops by a factor of e. In a spiral galaxy, it is the height above the galactic disk at which the density of a particular constituent of the disk has declined by e. This is different for different types of object (e.g. the scale height of stars is different than the scale height of HI gas in the galaxy), and generally increases with the age of the object.
For planetary atmospheres, scale height can be calculated by
where, k = Boltzmann constant = 1.38 x 10−23 J·K−1, T = mean planetary surface temperature (K), M = mean molecular mass of dry air (kg), and g = acceleration due to gravity on planetary surface (m/s2). In the Earth's atmosphere, the pressure at sea level, P0, is about 1.01 × 105 Pa, the mean molecular mass of dry air is 28.964 u and hence 28.964 × 1.660 × 10−27 = 4.808 × 10−26 kg, and g = 9.81 m/s2. As a function of temperature the scale height of the Earth's atmosphere is therefore 1.38 / (4.808 × 9.81) × 103 = 29.26 m/deg.
Given P0, the pressure at height z = 0 (pressure at sea level), atmospheric pressure at height, z, is:
SCATTERED OBJECTS - Kuiper Belt Objects (KBOs) with large orbital eccentricities (perihelia between 30 and 48 AU, aphelia >60 AU). These objects were probably flung into their current orbits early in Solar System history by gravitational interactions with the giant planets. The current population of the scattered disk is only ~1% of what it was in the early Solar System. Gravitational interactions with Neptune in particular can ejecting them from the Solar System. Alternatively, such interactions may pull scattered disk objects further into the Solar System, where they probably become Centaurs for a period of time. There are ~150 scattered disk objects and Centaurs currently known.
SCHREIBERSITE - Ni-Fe phosphide, (Fe,Ni)3P, common in iron and stony-iron meteorites.
SCHWARZCHILD RADIUS - Radius of the event horizon surrounding a nonrotating black hole. Its size is given by:
For a one solar mass star this is ~3 kilometers.
SELF-GRAVITATION - Process by which individual constituents of a large body are held together by the combined gravity of the object as a whole. Without it, stars, stellar clusters, galaxies, and groups and clusters of galaxies would all expand and dissipate. Because gravity is such a weak force, only relatively massive objects become self-gravitating.
SEMICONDUCTOR - Subclass of insulators, semiconductors are materials with conductivity that can be controlled through methods such as doping or changing the temperature. Like all insulators, their valence band is completely full in the ground state. A semiconductor has no charge carriers at absolute zero. Pure semiconductors have very few charge carriers even at room temperature. Conductivity can be increased through doping, creating either p-type semiconductors or n-type semiconductors.
SEMIMAJOR AXIS - Half of the longest diameter, a, of an ellipse. Together with the semi-minor axis (b) and eccentricity (e), it forms a set of related values that completely describe the shape of an ellipse:
SERPENTINE - A Mg phyllosilicate, Mg3Si2O5(OH)6, produced by aqueous alteration of olivine, pyroxene and amphibole. It is abundant in the matrixes of CI and CM chondrites.
There are three varieties of serpentine: antigorite and lizardite, which occur as massive aggregations, and chrysotile, which is fibrous. Chrysotile is one of the main forms of asbestos. A misfit between the octahedral and tetrahedral sheets causes bending, which is distributed into corrugations in antigorite and causes chrysotile to curl into cylindrical (often hollow) tubes.
The composition of serpentine is very uniform. The most important substitution is Fe2+ ↔ Mg2+ ↔ Ni2+ with a much less important substitution of Al3+ ↔ Si4+. Chrysotile shows least chemical deviation from ideal composition.
SHATTER CONE - Conical fragment of rock with regular thin grooves (striae) that radiate from the apex of the cone. Shatter cones range in size from less than one centimeter to more than one meter across and are formed in fine-grained brittle rocks, such as limestone or quartzite. They result from the high pressure, high velocity shock wave produced by an impacting meteorite.
SHELL BURNING - Nuclear "burning" regions that occur in shells surrounding a star's core. For example, helium burning might take place in the core (where the hydrogen has been exhausted) with a shell of hydrogen burning surrounding it. Stars may have more than one region of shell burning during their stellar evolution, each shell with its own nuclear reactions.
SHEPHERD SATELLITE - Satellite whose gravitational effect on a planetary ring helps preserve the ring's shape. Two shepherd satellites, Prometheus and Pandora, constrain or shepherd the particle of Saturn’s F ring particles to stay between their orbits. These satellites about 50 kilometers across with orbits about 1000 kilometers on either side of the F ring. Shepherd satellites are probably also responsible for the braids and kinks in the F ring.
SHERGOTTITE (SHE) - Most abundant type of SNC meteorites believed to have come from Mars, with 17 known examples by mid-2002; the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or hypabyssal (shallow plutonic) origin, and resemble terrestrial rocks more closely than do any other achondrite group. They all have exceptionally young crystallization ages of 150-200 Ma, and usually show signs of severe shock metamorphism (typically plagioclase has been converted to maskelynite). Shock metamorphism probably occurred when the shergottites were blasted off the Martian surface.
SHIELD VOLCANO - Volcanic construction of a central type formed by repeated effusion of fluid (usually basaltic) lava. The shield is characterized by a shallow flank, the steepness of which can vary from a high of 7-8°, to a low of ~1°, but with typical values of ~5°. There often are one or more caldera craters near the summit, like broad saucer-shaped cavities with steep walls. One of the largest shield volcanoes in the solar system is Olympus Mons on Mars, with a diameter greater than 500 km and total relief of 25 km (below). The largest shield volcano on Earth is Mauna Loa on the Big Island of Hawaii.
SHOCK MELTING - Complete melting of shocked material at pressures in excess of 90 GPa at which instantaneous temperature increases exceed 1500 K.
SHOCK METAMORPHISM - Metamorphism produced by hypervelocity impact between objects of substantial size moving at cosmic velocity (at least several kilometers per second). Kinetic energy is converted into seismic and heat energy almost instantaneously, yielding pressures and temperatures far in excess those in normal terrestrial metamorphism. On planetary bodies with no atmosphere, smaller impacting bodies (even micrometeorites) can produce shock metamorphic effects as observed in meteorites. Observed effects include planar deformation features (PDFs), diaplectic glass, and melting. Effects are observed in different minerals at different pressures.
SHOCK STAGE - A petrographic assessment, using features observed in minerals grains, of the degree to which a meteorite has undergone shock metamorphism. The highest stage observed in 25% of the indicator grains is used to determine the stage. Also called "shock level".
|Shock Stage||Minimum T Increase (K)||Features|
|S1: unshocked (<5 GPa)||10||Sharp optical extinction viewed in microscope; small number of irregular fractures|
|S2: very weakly shocked (5-10 GPa)||20||Undulatory extinction; irregular fractures|
|S3: weakly shocked (1-20 GPa)||100||Olivine: planar fractures, undulatory extinction, irregular fractures; Plagioclase: undulatory extinction|
|S4: moderately shocked (30-35 GPa)||300||Olivine: mosaicism (weak), planar fractures; Plagioclase: undulatory extinction, isotropic in places, planar deformation features.|
|S5: strongly shocked (45-55 GPa)||600||Olivine: mosaicism (strong), planar fractures and planar deformation features; Plagioclase: maskelynite (isotropic feldspar)|
|S6: very strongly shocked (75-90 GPa)||1500||Olivine: solid state recrystallization and staining, presence of ringwoodite, local melting; Plagioclase: shock melted|
SHOCK WAVE - Abrupt perturbation in the temperature, pressure and density of a solid, liquid or gas, that propagates faster than the speed of sound.
SHORT-LIVED RADIONUCLIDES (SLRs) - Radioactive isotopes with half lives less than ~20 Ma. SLRs are also called "extinct isotopes" because they have decayed completely into their daughter products, and are detectable only by isotopic excesses in daughter isotopes. SLRs include: 26Al (t½ = 0.73 Ma), 41Ca (t½ = 0.1 Ma), 53Mn (t½ = 3.7 Ma), 60Fe (t½ = 1.5 Ma), 182Hf (t½ = 9 Ma) and 129I (t½ = 16 Ma). Assuming an initially homogeneous distribution of SLRs, the isotopes potentially provide high-resolution chronometers of early solar system events. The resulting SLR ages are presented relative to a given sample - often CAIs for meteorites - the absolute age of which must be determined using long-lived radionuclide dating.
SIDEREAL TIME - Time measured in relation to the fixed stars: the length of a sidereal day is 23 hr, 56 min, 4.09 sec of mean solar time.
SIDERITE - An obsolete term for an iron meteorite.
SIDEROLITE - An obsolete term for a stony-iron meteorite.
SIDEROPHILE ELEMENT - Literally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on Earth's surface.
SIEGBAHN NOTATION - Nomenclature for characteristic x-rays introduced by Manne Siegbahn in 1923. This system is based on relative intensity of lines from different series. Characteristic X-rays named according to the shell being filled (K, L, M …) and the number of shells changed by electron (α = 1 shell, β = 2 shells, γ = 3 shells, etc.). Because this spectrographic nomenclature was developed before the electronic structure of atoms was well-understood, there are inconsistencies. These inconsistencies were eliminated in the IUPAC (International Union of Pure and Applied Chemistry) notation.
SILICA - Silicon dioxide, SiO2.
SILICA GROUP - Mminerals formed exclusively from silica. There are ten known silica polymorphs, two of which are synthetic. Five of the naturall polymorphs are related by reconstructive transformations and can exist metastably: stishovite, coesite, quartz, tridymite, and cristobalite. Conditions to form coesite and stishovite are attained only during meteoroid impacts where there are extremely high shock pressures.
Three of these have polymorphs related by displacive transformations between low- and high-temperature forms (α and β): quartz (shown above), tridymite, and cristobalite. The low temperature forms have lower degrees of crystal symmetry than the high temperature forms.
SILICATE - The most abundant group of minerals in Earth’s crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the tetrahedra.
All tetrahedral structures have net negative charge (except for the silica polymorphs) and cations required to balance charges. Cations also serve to bond negatively charged structural elements such as halogens and water. One complicating factor is that Al3+ commonly substitutes for Si4+ in chain, sheet and framework silicates.
SILCON CARBIDE - Presolar interstellar dust grain found in CM and E chondrites; its formula is SiC.
SIMPLE CRATER - Smallest impact features, consisting of a bowl-shaped interior with smooth walls, an elevated rim, and a shallow inclination hummocky exterior. The interior wall is usually inclined at 20-40° to horizontal, whereas the exterior wall is inclined at 5-15° to horizontal. At larger diameters, simple craters become more complex with transitional forms developing in the 15-25 km diameter range on the Moon, 4-10 km range on Mars and 2-6 km range on Earth. Transitional craters have landslides on the interior wall and a hummocky floor. The photograph shows Moltke on the Moon, a typical simple crater. The crater diameter (measured across the top of the rim) is 6.5 km and the crater depth (measured from the top of the rim to the centre of the floor) is 1.3 km.
SIMPLE CUBIC PACKING - Way in which atoms (considered as hard spheres) pack together to fill space. In simple cubic packing, all layers are identical and atoms are lined up in stacks and rows. Each sphere is touched by six neighbors, four in the same layer, one directly above, and one directly below.
SIMPLE SUBSTITUTION - Substitution in which the cations or anions replacing one another have the same charge and similar radii (within ~15%). Simple substitutions may result in complete or partial solid solution. The following simple substitutions are commonly complete: Fe2+ (0.78 Å) ↔ Mg2+ (0.72 Å), Fe2+ (0.78 Å) ↔ Mn2+ (0.83 Å), and Br- (1.96 Å) ↔ Cl- (1.81 Å). In contrast, the following substitutions are usually partial because they significantly differ in ionic size: Na+ (1.18 Å) ↔ K+ (1.51 Å), Mg2+ (0.72 Å) ↔ Mn2+ (0.83 Å), I- (2.20 Å) ↔ Cl- (1.81 Å).
SINGULARITY - Regions of space where the density of matter, or the curvature of space-time, becomes infinite and the concepts of space and time cease to have any meaning. Singularities are predicted to occur in all black holes and in certain models of the Universe. For example, open Friedmann models of the Universe possess a singularity in the finite past, while the closed models have both an initial and final singularity. In general, cosmic censorship hides singularities behind event horizons, the exception being the initial singularity of the Big Bang.
SMALL MAGELLANIC CLOUD - Smaller of the two irregular galaxies that make up the Magellanic Clouds. These two galaxies orbit the Milky Way once every 1,500 million years, and each other once every 900 million years. Located 200,000 light years away, the SMC has a mass of ~3 billion Msun. Some observations suggest that it might be a barred disk galaxy, deformed by gravitational interactions with the Milky Way and the Large Magellanic Cloud (LMC). The Magellanic Stream, a tail of gas that has been stripped from the SMC/LMC system, provides further evidence for interactions between these three galaxies.
SNELL’S LAW - For a wavefront traveling through a boundary between two media, the first with a refractive index of n1, and the other with one of n2, the angle of incidence, θ, is related to the angle of refraction, φ, by:
SOLAR ABUNDANCES - Amount of elements in the Sun as determined by spectral line intensities. Approximately 60 elements have been identified; the most abundant are listed in the table.
SOLAR APEX - Imaginary point in the constellation Hercules, near the bright star Vega, towards which the Sun is moving. The Sun's velocity relative to nearby stars is ~19.7 km/s. The point on the opposite side of the sky from which the Sun appears to be moving away is called the antapex.
SOLAR COSMIC RAYS – Cosmic rays with energies ~107 to 1010 eV, primarily ejected solar flares and coronal mass ejections (CME). Their composition is similar to that of the Sun, and are produced in the corona by shock acceleration, or when part of the solar magnetic field reconfigures itself.
SOLAR ENERGETIC PARTICLE (SEP) NOBLE GASES - Gases more deeply implanted than solar wind noble gases; probably of solar origin, but particles of interstellar origin may contribute as well.
SOLAR FLARE - Sudden eruptions from the surface of the Sun. Flares typically last a few minutes and can release energies equivalent to millions of hydrogen bombs. Flares become frequent near sunspot maximum, when smaller flares can occur daily and large flares can occur about once a week. During a flare the material in the flare may be heated to temperatures of 10 million K; matter at these temperatures emits copious amounts of UV and X-Ray, as well as visible light. In addition, flares tend to eject matter, primarily in the form or protons and electrons, into space at velocities that can approach 1000 km/second. These latter events are coronal mass ejections, and produce bursts in the solar wind that influence much of the rest of the Solar System, including the Earth. The observation of a large flare on the surface of the Sun is usually a signal for increased auroras and related activity several days hence when the ejected burst reaches Earth. Flares are also observed on other stars.
SOLAR LUMINOSITY (Lsun) - Solar luminosity is 4 × 1033 ergs per second.
SOLAR MASS (Msun) - Mass of the Sun, Msun; equal to 1.99 × 1030 kg.
SOLAR WIND - Supersonic flow of high-speed charged particles continuously blowing off a star (mostly e- and p+). When originating from stars other than the Sun, it is sometimes called a "stellar" wind. The solar wind may be viewed as an extension of the corona into interplanetary space. The solar wind emanates radially from all parts of the Sun: the fast wind originates from the coronal holes and the quiet Sun, whereas the slow wind arises from the coronal streamers. Active regions, which lie under closed magnetic loops, are also a source of the slow wind.
The solar wind contains roughly equal number of electrons and protons, along with a few heavier ions, and blows continuously from the surface of the Sun at an average velocity of ~400 km/s. The Sun's solar wind leads to a loss of ~10-14 Msun per year. The solar wind is fractionated from the photosphere by the forces that accelerate the ions off of the Sun. This fractionation appears to be ordered by the first ionization potential (FIP) of the elements (low-FIP elements tend to be over-abundant and high-FIP elements tend to be under-abundant).
SOLAR WIND (SW) NOBLE GASES - Noble gases implanted by the solar wind into mineral grain surfaces that have been part of an asteroidal regolith; present in "gas-rich" meteorites, but also in lunar soils from the lunar regolith. Elemental abundances in most minerals are moderately fractionated compared to solar, favoring heavy elements.
SOLID ANGLE - Fraction of the surface of a sphere covered by a particular object, as seen by an observer at the sphere's center. For a small region with area, Δa, the numerical value of the solid angle is:
where r is the radius of the sphere. Although the solid angle is a dimensionless quantity, it is usually expressed in units of steradians (sr). The solid angle is often a function of direction.
SOLID SOLUTION - Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with the entire range of compositions possible) or partial (in which only part of the range of potential compositions occurs). For example, olivine shows a complete range of compositions between the end-members forsterite (Mg2SiO4) and fayalite (Fe2SiO4) produced by the substitution Mg2+ → Fe2+.
SOLSTICE - Position of the sun when farthest north (summer solstice) or farthest south (winter solstice). The solstices occur because the rotation axis of the Earth is tilted by an angle of 23.5° from the vertical. If the Earth's rotation axis were perpendicular to the plain of its orbit, there were be no solstice days and no seasons. The Sun attains its most northerly declination (+23.5°) around 21 June; this is the northern summer solstice and marks the longest day of the year for northern hemisphere observers. This is also the date of the southern winter solstice (shortest day of the year for southern hemisphere observers). Six months later, the Sun reaches its most southerly declination (-23.5°) and the solstices are reversed in each hemisphere.
SPACE GROUP - Group or array of operations consistent with an infinitely extended regularly repeating pattern. It is the symmetry of a 3D structure. There are 230 space group symmetries possible (only 65 for biological structures).
SPECIFIC GRAVITY - Ratio of the density of a substance to the density of a standard substance. The standard is usually liquid water for solids and liquids and air for gases. The density of liquid water under typical conditions is ~1000 kg/m3. The density of air at room temperature near the surface of the earth is approximately 1.2 kg/m3. Specific gravity is a unitless quantity.
SPECTRAL CLASS - Classification scheme, also called the "Harvard Spectral Sequence," based on the strength of stellar spectral lines, which indicate a star's temperature. Each category in this classification can be subdivided into 10 subclasses using numbers from 0 to 9. For example, the sequence for the O and B subclasses is O0, O1, O2, O3 ... O8, O9, B0, B1...
The data column in the table below gives (in order): temperature (K), mass divided by Msun (M), radius divided by Rsun (R), luminosity divided by Lsun (L), and main-sequence lifespan (T).
|O||Ionized He lines strong; multiply ionized metal lines; weak H lines||K = 28,000-50,000; M = 20-60; R = 9-15; L = 90,000-800,000; T = 1-10 Ma||ζ Orionis|
|B||Neutral He lines moderate; singly ionized metal lines; H lines moderate||K = 10,000-28,000; M = 3-18; R = 3.0-8.4; L = 95-52,000; T = 11-400 Ma||Rigel (B8), Spica|
|A||Neutral He lines very faint; Balmer H lines dominant, singly-ionized metal lines||K = 7,500-10,000; M = 2.0-3.0; R = 1.7-2.7; L = 8-55; T = 400-3,000 Ma||Vega (A0), Sirius (A1), Deneb|
|F||H lines moderate, neutral and singly ionized metal lines||K = 6,000-7,500; M = 1.1-1.6; R = 1.2-1.6; L = 2.0-6.5; T = 3-7 Ga||Procyon, Canopus (F0)|
|G||Singly ionized and neutral metal lines; H lines faint||K = 5,000-6,000; M = 0.85-1.1; R = 0.85-1.1; L = 0.66-1.5; T = 7-15 Ga||Sun (G2), α Centauri (G2), Capella|
|K||Singly ionized and neutral metals lines strong; molecular bands begin to appear; H lines faint||K = 3,500-5,000; M = 0.65-0.85; R = 0.65-0.85; L = 0.10-0.42; T = 17 Ga||Aldebaran (K5), Arcturus (K2)|
|M||Ti oxide molecular lines; neutral metal lines strong; molecular lines moderate; H lines very faint||K = 2,000-3,500; M = 0.08-0.65; R = 0.17-0.63; L = 0.001-0.08; T = 56 Ga||Antares, Betelgeuse|
|L||Strong metal-hydride molecular bands (CrH, FeH), and neutral metals; TiO and VO bands are nearly absent.||K = ~1300-2500; M < 0.09; L = 10-5-10-6||brown dwarf|
|T||Strong bands of methane (CH4) - like spectrum of Jupiter||K = 1500-2000; M < 0.09; L = 10-6||cool brown dwarf|
SPECTROSCOPY – Technique of splitting electromagnetic radiation (light) into its constituent wavelengths (a spectrum), in much the same way as a prism splits light into a rainbow of colors. Spectra are not smooth but punctuated by 'lines' of absorption or emission caused by interaction with matter. The energy levels of electrons in atoms and molecules are quantized, and the absorption and emission of electromagnetic radiation only occurs at specific wavelengths. Spectra contain an abundance of information.
The precise position (wavelength) at which known emission and absorption lines are detected can be used to measure the redshift of the observed object. For example, if the spectral line of Hβ (486.2 nm) is detected at 487.8 nm, one can calculate that the object has a recession velocity of 1,000 km/sec. This type of analysis can also be used to detect spectroscopic binaries and extra-solar planets. Spectral line broadening of absorption and emission lines can be used to measure the internal velocity dispersion of complex objects (e.g. the average velocity of stars within galaxies). Most absorption and emission lines are produced by metals. The depths or heights of these lines can be used to estimate the abundances of the metals responsible. In stars, these also permit the measurement of the temperature and pressure of the stellar atmosphere.
SPEED – Scalar quantity, the magnitude of the vector velocity:
Where r(t) is the three-dimensional vector displacement. The speed can be also be specified as an average speed or an instantaneous speed. Speed is expressed in units of [distance/time], often m/s or km/h.
SPEED OF LIGHT – Speed at which electromagnetic radiation propagates in a vacuum. Although referred to as the speed of light, this should be more properly called the 'speed of a massless particle’ as it is the speed at which all particles of zero mass (not only photons, but gravitons and massless neutrinos if they exist) travel in a vacuum. Einstein's theory of relativity makes several statements about the speed of light:
SPICULES - Spikes of gas that rise through the chromosphere (right). Spicules are rising jets of gas that move upward at ~30 km/sec and last only ~10 minutes.
SPINEL - Mg-Al oxide, MgAl2O4, found in CAIs.
SPIN-FLIP TRANSITION - Origin of the 21-cm emission line that originates with a neutral 1H atom. The proton and the electron each have a quantum “spin,” which points either “up” or “down.” Spins can be parallel (both of them “up” or “down”), or antiparallel (opposite states). The antiparallel state has slightly less energy than the parallel state, so if an atom in the parallel state changes to antiparallel, a 21-cm radio photon is emitted. This transition occurs every ~10 million years. Although the probability of detecting cold hydrogen gas through this mechanism is very small, the relative rarity of the transition is more than compensated for by the superabundance of neutral 1H in the interstellar medium; at any one time some fraction will be in the slightly excited state.
The spin-flip transition can only be used to trace the distribution of neutral hydrogen in the Universe. For regions rich in molecular hydrogen (e.g. molecular clouds), astronomers must use a different tracer. This is generally the carbon monoxide molecule (CO) which has a characteristic emission at the shorter wavelength of 2.6 mm.
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 - Characteristic feature of all spiral galaxies, a concentration of stars sweeping out from either the central bulge or the ends of the galactic bar. Grand design spirals (e.g., M51) have long, narrow, well defined arms, while those galaxies with short, fragmented arms are termed flocculent spirals. The persistence of galactic spiral arms indicates that they are neither rigid nor permanent structures, because they have not been wound up by differential rotation of the galaxy. Some galaxies have a well-organized spiral structure (grand design spirals), while others are patchy (flocculent spirals). The arms may be wound tightly around the galaxy or may be more open. Some spiral arms originate at the end of bars, others directly from the galactic bulge.
All spiral arms lie within the thin disk of a galaxy, delineated by dark dust lanes and highlighted by young, blue stars and luminous nebulae (HII regions). They range from being tightly wound, faint and smooth in galaxies with little gas and dust, to more open, brighter and clumpy in galaxy with higher percentages of interstellar material. Most young stars form within spiral arms. The more gas and dust available, the more of a galaxy will be involved in star formation. Galaxies with high rates of star formation tend to be dominated by their spiral arms and have relatively small central bulges. Spiral galaxies with very little gas and dust are dominated by their central bulge of older stars and generally have fainter spiral arms. In these galaxies, star formation is not vigorous enough to form clusters of luminous stars so their arms also tend to be smoother.
Two main models exist to explain the origin of spiral arms. In the density wave model, the spiral arms represent regions in the galactic disk that are denser than average. As the dust and gas piles up in these overdense regions, star formation is triggered, resulting in the neat, ordered arms of grand design spirals. In the self-propagating star formation model, regions of star formation are stretched into spiral patterns by the differential rotation of the galaxy. These regions of star formation are only temporary, appearing and disappearing on timescales much shorter than the life of the galaxy. This model is complimentary to the density wave model and is successful in explaining the origins of the more patchy flocculent spirals. Astronomers now think that a combination of both models may be at work.
SPIRAL GALAXY - Galaxy with spiral arms. These are classified based on their appearance in optical light, into those in which the arms radiated from a central bulge (classic spirals, S), and those where the arms radiated from a central bar (barred spirals, SB). Both types of spiral galaxies have a central bulge of old stars surrounded by a flattened disk of young stars, gas and dust. These regions are obvious in color images of face-on spirals: the central bulge or bar is yellow (older stars), whereas bright nebulae and young blue stars trace out the spiral arms within the disk. Dust is also visible in edge-on spirals as dark lanes, similar to the dark lanes we see in our own Milky Way when we observe the night sky.
Spiral galaxies are classified using the Hubble Classification Scheme (q.v.). Classes include Sa/SBa, Sb/SBb or Sc/SBc (classic/barred) according to the tightness of their spiral, the clumpiness of their spiral arms, and the size of their central bulge. These differences reflect the relative amounts of gas and dust contained within them. Only ~2% of the mass of Sa spiral galaxies is in the form of gas and dust. Since these are essential ingredients in the formation of new stars, this means that a relatively small proportion of Sa galaxies are have active star formation. Consequently, these galaxies are dominated by large bulges of old stars and relatively small disks containing faint, smooth, tightly wound arms. In contrast, Sc spirals contain ~15% gas and dust; a relatively high proportion of the galactic mass is involved in star formation. Sc galaxies have small bulges and are dominated by loosely wound arms that are often resolved into clumps of stars and HII regions. The proportion of young stars increases from Sa to Sc galaxies.
Spiral galaxies range in size from 5–100 kpc across, have masses of 109-1012 Msun, and luminosities of 108-1011 Lsun. Most spiral galaxies rotate in the sense that the arms trail the direction of the spin. The measuring rotation curves of spiral galaxies indicate that the orbital speed of material in the disk does not decline as expected were most of a galaxy's mass concentrated near its center. This indicates the visible portion of spiral galaxies contains only a small fraction of the total mass of the galaxy, and that spiral galaxies are surrounded by an extensive halo consisting mostly of dark matter.
S-PROCESS - Slow neutron capture by nuclei in massive stars. In the s-process, one starts with existing iron-group nuclei. Therefore, it would only be expected to take place in second-generation stars that collapsed out of the residue of a previous supernova explosion. The flux of neutrons is small enough that rate of neutron capture by atomic nuclei is slow relative to the rate of radioactive beta-decay. These neutrons come from various reactions in the He-burning region of a red giant star. Hundreds to thousands of years may pass between successive neutron captures. In this situation, a seed nucleus will slowly capture neutrons, for example 56Fe → 57Fe → 58Fe → 59Fe, followed by 59Fe → 59Co (β decay). This process builds nuclei by climbing the line of stability, until 208Pb and 209Bi are reached. Beyond this point, no nuclei are stable enough to allow neutron capture to operate: actinides cannot be synthesized by the s-process.
STANDARD CANDLE - Astronomical object with a known absolute magnitude. They are extremely important to astronomers since by measuring the apparent magnitude of the object we can determine its distance. The most commonly used standard candles are Cepheid variable stars and RR Lyrae stars. In both cases, the absolute magnitude can be determined from its variability period. Type Ia supernovae are also considered standard candles, but more correctly they are standardizible candles since they do not all have the same peak brightness. However, differences in peak luminosities correlate with how quickly the light curve declines after maximum light via the Type Ia supernova luminosity-decline rate relation, and can be made into standard candles by correcting for this effect.
STANDARD PRESSURE AND TEMPERATURE (STP) - Standard conditions of 273.15 K and 1 atm (0°C at sea level) used as a baseline for calculations involving quantities that vary with temperature and pressure.
STAR - Self-luminous object held together by its own self-gravity. Often refers to those objects which generate energy from nuclear reactions occurring at their cores, but may also be applied to stellar remnants such as neutron stars.
STARBURST GALAXY - Galaxies observed to be forming stars at an unusually fast rate (about 103 times greater than in a normal galaxy). At such high levels of star formation, the supply of gas and dust within the galaxy would be exhausted within about 108 years. This indicates that these episodes of intense star formation started relatively recently and perforce will end relatively soon.
The areas of high formation rates may be spread throughout a galaxy, but most starbursts are observed in a small region around the nucleus. Star formation is probably triggered by tidal interactions during galactic encounters, galactic mergers, or due to the presence of a galactic bar, all of which result in the accumulation of substantial amounts of gas and dust in the central regions of the galaxy. Massive stars form from the available enshrouding gas and dust, which emit large amounts of UV radiation. This radiation is absorbed by the surrounding dust and reemitted at IR wavelengths, making starburst galaxies among the most luminous IR objects in the Universe. Supernova explosions and stellar winds from the massive stars eventually sweep the gas from the galaxy and halt further star formation. Starburst galaxies appear to be more prevalent in the early universe than they are now. These galaxies, ~12 billion light years distant, appear to have characteristics similar to nearby starbursts and indicate that galaxy interactions were much more common in the past.
STARDUST MISSION - Space mission to study Comet Wild 2 (see http://stardust.jpl.nasa.gov/home/index.html). During the encounter, Stardust performed a variety of tasks including making counts of particles encountered by the spacecraft and real-time analyses of the compositions of these particles and volatiles. It also captured cometary particles using Aerogel and stored them for return to Earth. Results indicate that comets are not composed entirely of volatile rich materials but rather are a mixture of materials formed at all temperature ranges, at places very near the early sun (olivine) and at places very remote from it (ices).
STEFAN-BOLTZMANN LAW - Law of blackbody radiation that states that the amount of energy given off by a blackbody per second per unit area (flux) is proportional to the fourth power of the temperature of the blackbody. For flux (power per area) expressed as J/m2s with σ = 5.6703 x 10-8 W/m2K4:
If R is the radius of a star, its luminosity is:
STELLAR ASSOCIATION - Loose group of stars moving in the same direction but not gravitationally bound to each other. The most highly visible type of stellar association is dominated by young, massive stars and is called an OB association.
STELLAR BLACK HOLE - One of the possible evolutionary endpoints of high mass stars. Stellar black holes have masses <100 Msun. Once the core of the star has completely burned to Fe, energy production stops and the core collapses resulting in a supernova explosion. If the core is greater than ~2.3 Msun, the maximum mass of a neutron star, collapse continues and a stellar black hole is formed. These black holes are generally modeled as Kerr black holes, because the original rotation of the massive star would be conserved during the collapse. Stellar black holes are therefore most easily discovered in X-ray binary systems, where gas from a companion star is being pulled into the black hole. X-rays are produced by this gas which is heated to ~107 K as it spirals towards the black hole via an accretion disk. The mass of the black hole, typically 3–20 Msun, by observing its gravitational effect on the companion star.
About 20 X-ray binary systems are thought to contain stellar black holes. One of the best candidates is LMC X-3 in the Large Magellanic Cloud. Here the x-ray source is associated with a main sequence B3 star whose shape has been severely distorted by the gravitational field of its companion. The binary system has an orbital period of 1.7 days. The mass of the compact object probably is considerably higher than >3 Msun. The Cygnus X-1 binary system, discovered in 1965, consists of a O9.7 Iab type supergiant and a compact object orbiting with a period of 5.6 days. The mass of the unseen companion is ~10 Msun. Cygnus X-1 is one of the brightest X-ray sources in the sky. A jet from the Cygnus X-1 source has ionized the interstellar medium, producing a spectacular bubble-like x-ray feature.
STELLAR EVOLUTION - Changes in a stars luminosity and temperature over its lifetime; conventionally, plotted on an Hertzsprung-Russell (HR) diagram. All stars, irrespective of their mass spend most of their lifetime on the main sequence. The more massive a star, the more luminous and hotter it is. As all stars age, they enter a giant phase (their brightness remains constant, but the effective surface temperature decreases, 7 → 9). This reflects a change in the fusion processes at work within the star: outer layers expand and are no longer of sites nuclear burning. As these layers cool, the star drifts towards the right side of the HR diagram. Subsequent evolution depends on the mass of the star.
For a ~1 Msun star, when H burning ends, the core temperature is insufficient for He burning to occur. With no source of energy production in the core there is no longer any outward radiative pressure to resist gravitational collapse, and the outer regions of the star start to collapse (9 → 10). Collapse raises the temperature in the H shell and H fusion occurs. Luminosity increases as the core continues to collapse and the temperature in the H shell keeps increasing. The burning shell also provides pressure on the outer layers of the star and causes them to expand. As the layers expand they cool and the star appears to become redder.
After just a few million years the H shell runs out of fuel. Once again the star contracts under its own weight. The compact core may flash into life for a short period and He be fused into C (10). The energy released in the He flash reaches the outer layers and the star becomes a red supergiant (11). Up to half its mass is thrown out into space and seen as a planetary nebula (12) leaving a white dwarf behind.
Stars with a mass of between 8 and 20 Msun have a more complex evolution. Initially, they evolve in the same way as low mass stars, turning into red giants and undergoing a core He burning phase. However, He burning is no longer the end phase of stellar evolution. When He in the core is exhausted, the additional mass allows stellar collapse to take place and the outer layers to reignite. A cross section through the star at this point would show an outer shell of H burning, an inner shell of He burning, and the core where C burning is taking place. Once the C supply is exhausted, O begins to fuse into Ne; the He shell becomes a C burning shell, the H shell a He burning shell and a new outer layer of H burning forms. Subsequently, Ne can fuse into Mg, into Si, and so on to Cr and Fe.
Each of these stages produces less energy than the previous one and lasts for a shorter. During these final stages the star expands to thousands of times the diameter of the Sun, becoming a red supergiant like Betelgeuse. Iron is the end of the exothermic fusion road: to fuse iron into heavier elements is an endothermic reaction. Fission of Fe into lighter elements also requires an input of energy. The core cools, drawing heat from its surroundings to power the fusion; the outward radiative pressure, which had supported the star for many millions of years, ceases and the star undergoes free fall gravitational core collapse until it reaches nuclear densities (~1014 g/cm3). The core, which represents a large percentage of the stellar mass, exceeds the 1.44 Chandrasekhar limit for a white dwarf. Protons and electrons in the core are compressed into neutrons, yielding a sphere the size of a large city and the density of an atomic nucleus, held up by neutron degeneracy pressure: a neutron star. Core collapse produces a shock wave that blasts out through the star releasing an enormous amount of energy in a few seconds, equivalent to ~1028 Mton of TNT. The outer layers of the star become superheated plasmas with temperatures high enough to fuse Fe and heavier elements. These outer layers brighten rapidly and are ejected into the interstellar medium at speeds approaching the speed of light. Such an event is a Type II supernova.
Stars with over 20 Msun evolve in the same way as their slightly less massive companions dying in a supernova explosion, but the core becomes a black hole. A neutron star can mass up to around 3 Msun. After this point neutron degeneracy pressure is no longer sufficient to prevent core collapse. With nothing left to resist collapse the core condenses into an infinitely small, infinitely dense point called a black hole (singularity).
STELLAR HALO - Essentially spherical population of stars and globular clusters thought to surround most disk galaxies and the cD class of elliptical galaxies. Only ~1% of a galaxy's stellar mass is in its halo, and consequently observation of halos of other galaxies is extremely difficult. In contrast to the thin and thick disks of disk galaxies (q.v.), the halo generally has no net rotation and is supported almost entirely by velocity dispersion.
Halo stars in the Milky Way are generally old, most with ages >12 Ga. These ages are similar to those of bulge and globular cluster stars, indicating that halo stars were among the first Galactic objects to form. The halo stars are generally metal-poor, with a metallicity distribution peaking at ~1/30 of the solar value. The lowest metallicity star found in the Milky Way is a halo star with a metallicity of ~1/200,000 of the solar value.
STELLAR JETS - Linear streams of matter arising from a number of different sources (T Tauri stars, planetary nebulae, and compact objects). Stellar jets from T Tauri stars are common in star forming regions. These are called Herbig-Haro objects (q.v.). As material falls onto a protostar from the surrounding accretion disk, interactions between the magnetic fields of the rotating star and the accretion disk eject it from the stellar magnetic poles. However, this accretion process is not smooth, and there may be sudden increases or decreases in the rate of accretion onto the protostar. A sudden variations in the accretion rate produce "bullets" of denser material in the jets (HH34).
Stellar jets interact with the surrounding interstellar medium. T Tauri stars occur in star forming nebulae and their jets interact with gas remaining from the formation of the star itself, effectively sweeping out the interstellar material in their path. The swept out material accumulates at the head of the jet causing increased resistance. This eventually forms a shock front, with the jet having to force its way through the accumulated gas and dust, ionizing it.
It is not only young stars that exhibit stellar jets. Jets are also observed in planetary nebulae, which form during the final stages of evolution of stars similar to (or a little more massive than) the Sun. The jets of planetary nebulae have speeds similar to those of T Tauri jets, but are generally less well collimated (broader for their length). They form when material flowing out of the star is diverted into a jet. The collimation is usually caused by a confining equatorial disk that blocks equatorial outflow and directs the material into polar jets. These disks are often found in binary systems. For example, the star at the heart of the planetary nebula M2-9 (below) is a spectroscopic binary (a pair of stars orbiting so close to each other that we are unable to separate them in images). The closeness of the two stars causes a ring of gas to be thrown off. This encircles the pair of stars, blocking the equatorial ejection of gas from the dying star and diverting it into polar outflows.
The last sources of stellar jets are compact objects (stellar black holes and neutron stars) in binary systems. The compact object is surrounded by an accretion disk formed through Roche-lobe overflow from a companion giant star. As is the case with T Tauri jets, the jets form as material falls onto the compact object and is ejected from its magnetic poles.
STELLAR WIND - Fast continuous outflow of material (p+, e–, and atoms of heavier metals) ejected from stars. Stellar winds are characterized by speeds of 20–2,000 km/sec. The causes, ejection rates and speeds of stellar winds vary with the mass of the star. In relatively cool, low-mass stars, such as the Sun, the wind is caused by the extremely high temperature (106s K) of the corona. This high temperature probably results from interactions between magnetic fields at the star's surface, and gives the coronal gas sufficient energy to escape the gravitational attraction of the star. These stars eject only a tiny fraction of their mass per year as a stellar wind but this still represents losses of millions of tonnes of material each second. Over their entire lifetime, stars like our Sun lose only a tiny fraction of 1% of their mass through stellar winds.
In contrast, hot, massive stars can produce stellar winds ~109 times stronger than those of low-mass stars and over their short lifetimes, they can eject up to 50% of their initial mass as 2,000 km/sec winds. These stellar winds are driven directly by radiation pressure from photons escaping the star. In some cases, high-mass stars can eject virtually all of their outer envelopes in winds. The result is a Wolf-Rayet star. Stellar winds play an important part in the chemical evolution of the Universe, as they carry dust and metals back into the interstellar medium where they will be incorporated into the next generation of stars.
STERADIAN (sr) - Unit used to express the dimensionless quantity of solid angle. A sphere subtends a solid angle of 4p (»12.5663706) for an observer at the centre of the sphere. This factor appears in the formula for the surface area, SA, of a sphere with radius, r:
STISHOVITE - Dense, high-pressure phase of quartz; so far identified only in shock-metamorphosed, quartz-bearing rocks from meteorite impact craters. Stishovite was synthesized in 1961 before it was discovered at Meteor Crater, Arizona. Its structure consists of parallel chains of single SiO6 octahedra. The octahedra are on their sides, sharing opposing edges.
STOICHIOMETERY - Quantitative relationships (proportions) among elements in compounds.
STONY-IRON METEORITE - Meteorite composed of roughly equal amounts by weight of silicate minerals and Ni-Fe metal. The stony-irons consist of two groups: mesosiderites and pallasites. However, there is gradual shading into metal-rich stony meteorites such as the lodranites (once considered stony-irons) and silicate-rich iron meteorites. Stony-iron meteorites are less abundant than their stony and iron cousins, comprising a total known mass of ~10 tons, which is ~1.8% of the entire mass of all known meteorites.
STONY METEORITE - Meteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely heterogeneous as a group, ranging from samples of primordial matter that have remained more or less unchanged for the last 4.56 Ga (chondrites) to highly evolved younger rocks from differentiated worlds, such as the Moon or Mars (achondrites). They account for 95% of all known falls, the majority (86% of all falls) being chondrites.
STREWN FIELD - Area on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's flight direction can be roughly determined from the size pattern in the strewnfield. The Australasian strewnfield, which covers ~10% of Earth's surface, is the largest and the youngest of tektite strewnfields. This ~800,000 year-old field includes most of Southeast Asia, and extends across the ocean to include the Philippines, Indonesia, Malaya and Java and reaches far out into the Indian Ocean and south to the western side of Australia. The impact crater may have been between 32 and 114 km in diameter.
STRÖMGREN SPHERE - Spherical region of ionized hydrogen (HII) around a single or tight cluser of young stars of spectral classes O or B. THe region is ionized by the strong UV radiation produced by such stars. The size of the sphere is controlled by two main factors: (1) radiation output of the central star(s), with hotter more radiant star(s) producing larger spheres, and (2) gas density of the surrounding cloud, with denser gas yielding smaller spheres.
STRONG LENSING - Effect due to gravitational bending of light by mass concentrations which results in (1) the strong distortion of background galaxy images (arcs) behind some massive foreground galaxy clusters, and (2) multiple images of the same background quasar behind a foreground galaxy. Over 50 lensed quasi-stellar objects are known.
STRONG NUCLEAR FORCE - Fundamental force binding nucleons within an atomic nucleus and preventing like-charged protons from flying apart. Particles that are acted on by the strong nuclear force (including protons and neutrons) are known collectively as hadrons. The strong nuclear interaction between individual hadrons is believed to be a remnant of a more powerful "color force" (carried by gluons) that acts between quarks inside hadrons.
STRUCTURE FORMATION - Development of organized structures in the universe, ranging from galaxies to clusters to superclusters, and possibly beyond to huge filaments and voids.
SUBLIMATION - Process in which a material changes from a frozen solid to a gas without passing through the liquid state. Whether a material will sublimate, melt, or vaporize depends on the temperature and pressure of its environment. For example, if the pressures are low enough, water ice will turn directly into water vapor as the temperature increases, bypassing the liquid water stage.
An example of a material that sublimates here on Earth is frozen carbon dioxide, CO2, 'dry ice'. At room temperature and 1 atmosphere of pressure, frozen CO2 turns directly into a gas. Sublimation occurs in many places in the Solar System. Two examples are the sublimation of water from cometary nuclei as the comet approaches the Sun and the sublimation of the polar ice caps on Mars during the Martian summer.
SUBSTITUTION - Replacement of one ion or ionic group for another in the same structural site in a mineral yielding a solid solution. Most substitution in minerals is of cations which are smaller and essentially sit in a lattice of oxygen anions. Anionic substitution does occur in halides. Substitutions are classified based on the exact nature of the processes, which include: simple substitution, coupled substitution, and interstitial substitution.
Four factors control whether substitution will occur: (1) ionic radii; (2) ionic charges; (3) temperature; and (4) natures of the bonds formed. Substitution is more likely if the two ions or ionic groups are similar in size. We can calculate the size difference as a percentage:
Substitution is common if sizes are within 15 %, limited if sizes are between 15–30 % and unlikely if sizes differ by >30 %. Similarly, substitution is more likely if the charge difference is 0 or 1; larger differences make substitution unlikely. At higher temperatures, the sites into which the ions must fit are larger (bonds lengthen: thermal expansion) and substitution is easier. Increasing temperature effectively makes the size constraint less rigid, providing an additional ~10% leeway. Lastly, substitution is unlikely if the types of bonds formed by the two ions are very different (essentially a function of electronegativity). The chart below applies to terrestrial rocks.
SUEVITE - Terrestrial breccia composed of angular fragments of different rock types and glass inclusions. Glass can make up more than half of the volume of suevite. Mineral grains in the rock fragments commonly display shock-metamorphic effects. Suevite was named after a rock found at the Ries Crater in southern Germany (photograph).
SUN - Our parent star. The structure of Sun's interior is the result of the hydrostatic equilibrium between gravity and the pressure of the gas. The interior consists of three shells: the core, radiative region, and convective region.
The core is the hot, dense central region in which the nuclear reactions that power the Sun take place, temperatures range from 8-15 x 106 K with densities from 10-160 g/cm3. It comprises about 25% of the interior radius. The core is the region where the energy of the Sun is produced. Density and temperature are high enough to cause nuclear fusion reactions. These reactions release energy both in the form of γ-rays and particles (in particular neutrinos).
The radiative zone extends from ~25% to 85% of the solar radius. Here, and in the core, the primary transport of energy is by the movement of photons; temperatures range from 0.5-8 x 106 K with densities from 0.01-10 g/cm3. The radiative zone is not transparent. On average, photons only ~2 cm before being scattered in a random direction by an encounter with an electron. The resulting "drunkard's walk" is very inefficient and it typically takes ~170,000 years for energy generated in the core to escape to the surface.
The convective zone starts at ~85% of the solar radius and extends to just below its surface; its density is <0.01 g/cm3. In this region, the change in temperature with increasing radius is so rapid (0.1-5 x 105 K) that the interior becomes unstable and undergoes convection (rapid up and down motion of large packets of gas). Convective movement is responsible for the granulation pattern seen on the surface.
Basic data for the Sun: RSun = 7.0 x 105 km; MSun = 2.0 x 1030 kg; ρSun = 1.4 g/cm3; Composition: 74% H, 25% He, 1% other elements; Tsurface = 5800 K; LSun = 3.9 x 1026 W.
SUNSPOT - Regions on the Sun’s surface that appear dark because they are cooler than the surrounding photosphere, typically by ~1500-1800 K. Sunspots develop and persist for periods ranging from hours to months, and are carried around the surface of the Sun by its rotation. Sunspots travel in pairs (north and south magnetic poles). The pairs are due to magnetic flux tubes on the surface of the Sun that carry energy away causing the surface to be cooler than the surrounding material.
SUNSPOT CYCLE - Eleven-year periodicity in the number of sunspots observed on the Sun. Sunspot maxima are associated with times of high solar activity (many flares and solar storms). During sunspot minima, there may be no spots visible on the Sun for several days. The number of sunspots present during the maxima varies with each cycle. The 1958 maxima showed the highest sunspot activity on record. There were very few sunspots from 1645-1715, even during the maxima. This interval is called the “Maunder Minimum,” and represented a period of solar inactivity that probably caused the “Little Ice Age” on Earth.
SUPERCLUSTER - Cluster of galactic clusters.
SUPERCONDUCTIVITY - Phenomena by which, at sufficiently low temperatures, a conductor can conduct charge with zero resistance. The current theory for explaining superconductivity is the BCS theory.
SUPERFLUIDITY - Phenomena by which, at sufficiently low temperatures, a fluid can flow with zero viscosity. Its causes are associated with superconductivity.
SUPERMASSIVE BLACK HOLE - Black holes that contain between a million and a billion times more mass than a typical stellar black hole. There are only a handful of confirmed supermassive black holes (most are too far away to be observed), but they probably exist at the centers of most large galaxies, including the center of our Milky Way. For many years there was only indirect evidence of supermassive black holes: the existence of quasars. Observations of the energy output and variability timescales revealed that quasars radiate >1012 times more energy as our Sun from a region about the size of the Solar System. The only mechanism capable of producing such enormous amounts of energy is the conversion of gravitational energy into light by a massive black hole. More recently, direct evidence for supermassive black holes has come from observations of material orbiting galactic centers. High orbital velocities of stars and gas are best explained if they are being accelerated by a massive object with a strong gravitational field contained within a small region of space – a supermassive black hole (below).
There is still debate about how supermassive black holes form. Supermassive black holes could form from the collapse of massive clouds of gas during the early stages of the formation of the galaxy. Alternatively, a stellar black hole might consume sufficient of material over millions of years to grow into a supermassive black hole proportions. Yet another idea is that a cluster of stellar black holes might merge to form a supermassive black hole. Regardless of the mechanism, most astronomers agree that accretion of material onto a supermassive black hole drives both active galactic nuclei and galactic jets (below).
SUPERNOVA - Stellar explosion that expels much or all of the stellar material with great force, driving a blast wave into the surrounding space, and leaving a supernova remnant. Supernovae are classified based on the presence or absence of features in their optical spectra taken near maximum light. They were first categorized in 1941 by Rudolph Minkowski, who divided them into those that showed H in their spectra (Type II), and those that did not (Type I). Type I supernovae were further sub-divided in the 1980s based on the presence or absence of Si and He in their spectra. Type Ia supernovae have obvious Si absorption at 6150 Å. Type Ib have no Si but show He emission lines, and Type Ic display neither Si nor He.
Type Ia supernovae can be found anywhere and in any type of galaxy, but Type Ib and Type Ic supernovae occur primarily in populations of massive stars, similar to Type IIs. It is now known that Type II, Type Ib and Type Ic supernovae result from the core-collapse of massive stars, while Type Ia supernovae are the thermonuclear explosions of white dwarfs.
Type Ia Supernova (SNIa) - Result of the explosion of a carbon-oxygen white dwarf in a binary system. They are the brightest of all supernovae with an absolute magnitude of -19.5 at maximum light, occur in all galaxy types, and are characterized by a silicon absorption feature (rest wavelength = 6355 angstroms) in their maximum light spectra. They can eject material at speeds of the order of 10,000 km/s and outshine an entire galaxy at their peak brightness.
Both the nature of Type Ia progenitors and the manner in which the star explodes are still uncertain. It is generall accepted that as a white dwarf gains mass from its companion, it contracts and increases its temperature and density. As the mass approaches the Chandrasekhar limit of 1.4 Msun, temperature and pressure in the interior of the star increase until a burning front forms, where C is fused into Fe and Ni almost instantaneously. However, iIf the accretion model is right, the white dwarf ought to emit an abundance of X-rays for ~10 million years before it explodes. Recent (2010) measurmeents reveal that X-ray emissions from five nearby elliptical galaxies and the central region of the Andromeda spiral galaxy are one-thirtieth to one-fiftieth the amount expected in the accretion model. An alternative model is that type Ia supernovae form by merger of white dwarf stars.
The most popular theory is that the white dwarf undergoes a delayed detonation, where the burning front is initially subsonic (deflagration = “burning”) but later becomes supersonic (detonation). Observed explosion energies and observed amounts of unburnt C and O rule out a pure deflagration scenario. The fact that the whole star is not burnt to Fe and Ni rules out a pure detonation scenario. However, variations in explosion energies and the distributions of elements observed in Type Ia supernovae can be modeled by altering when the transition from deflagration to detonation occurs.
The B-band light curves of all SNIa look the same. There is an initial very rapid increase in luminosity, where the brightness of the supernova can change by up to 3 magnitudes in 15 days, that ends at maximum light. After reaching a maximum, brightness declines fairly rapidly (~0.087 mag/day) for the next 3–4 weeks. About a month after maximum light, the decline rate changes to a steady ~0.015 mag/day, dominated by the radioactive decay of 56Co.
Originally it was thought that every SNIa had the same peak brightness; although close to the truth, this is not quite correct. SNIa exhibit maximum brightnesses that range from about +1.5 to -1.5 magnitudes around a typical SNIa. The decline rate of the brightness after maximum light is correlated with the width of the maximum and the peak brightness of the supernova. Brightness can be standardized by applying the luminosity-decline rate relation. This is done using one of three light curve fitting techniques: the Δm15 method (shown below), the Multicoloured Light Curve Shapes method, or the Stretch method.
Each uses a standard or series of standard light curves to determine how under- or over-luminous a supernova compared to a “typical” SNIa. Astronomers then correct for the luminosity difference before using the supernova as a distance indicator. Consequently, SNIa are very precise distance indicators that can be observed over large distances. They have been instrumental in narrowing down the value of the Hubble Constant, and were the objects used to discover the accelerating universe.
Type Ib Supernova (SNIb) - Type originally lumped with Type Ia (SNIa) and Type Ic (SNIc) supernovae due to the similar appearance of their light curves. Type Ib supernovae were recognized as a separate class based on the absence of the Si absorption feature typical of Type Ia spectra, and the presence of He absorption which is absent in Type Ic spectra at maximum light. In addition, late-time spectra of SNIb spectra are dominated by intermediate mass elements whereas SNIa spectra are dominated by heavy elements.
SNIb have much more in common with SNII and SNIc than SNIa. They are primarily found in arms of spiral galaxies close to HII regions, and emit radiation at radio wavelengths indicating that circumstellar material surrounded the progenitor star. Some SNII have been observed to transition to SNIb at late times. These observations lead to a model where SNIb result from the core-collapse of a massive star; the difference between SNII and SNIb is that the Type Ib progenitor lost its outer H layer before the collapse.
Type Ic Supernova (SNIc) - Supernova subtype recognized in 1987. Whereas, Type Ia supernova result from the explosion of a white dwarf, Types Ib and Ic supernovae result from core-collapse of a massive star and are similar to Type II supernovae (SNII). Some astronomers label both Type Ib supernova and SNIc supernova as SNIb/c, because they have similar light curves, spectral evolution and radio properties. The only observable difference between the two types is the lack of He in the spectra of SNIc.
Progenitor stars of SNIc are probably similar to those of SNII supernovae. However, whereas the progenitors of SNII retain H and He envelopes prior to explosion, and SNIb retain He envelopes, SNIc appear to have lost both, resulting in an almost featureless spectrum. The light curves of SNIc are very similar to those of SNIb and SNIa. Like SNIb, the light curves tend to be 1.0–1.5 magnitudes fainter than those of typical SNIa supernova. However, it is generally not possible to distinguish between SNIa, SNIb and SNIc based on light curve shape alone. Spectra (preferably at maximum light) are required to correctly classify new supernovae.
Type II Supernova (SNII) - Explosive death of a >4 Msun star. Type II supernovae (SNII) were recognized as a distinct type of supernova in the early 1940s. They are distinguished by H emission lines in their spectra, and light curve shapes significantly different than those of Type I supernovae. SNII are sub-classified depending on whether their light curves show linear decline after the maximum (SNII-L) or a plateau phase where the brightness remains constant for an extended period of time (SNII-P). The peak brightnesses of all SNII are several magnitudes fainter than that of Type Ia supernovae. SNII-P show large variability in maximum brightness; peak brightnesses of SNII-L are nearly uniform at 2.5 magnitudes fainter than SNIa supernovae.
SNII are only found in regions of star formation, indicating that they result from core-collapse of massive stars. The difference between the three types of core-collapse supernovae is whether they have retained their outer envelopes of H and He before the explosion. The progenitors of SNII retain both H and He layers (progenitors of Type Ib supernovae have lost their H envelope but retain the He envelope; progenitors of Type Ic supernovae have lost both H and He envelopes before the core-collapse). Unlike Type Ia supernova, SNII tend to form shells of ejected stellar material around either a neutron star (core mass <3 Msun), or a black hole. The ejected material is rich in heavy elements synthesized during the explosion, making SNII one of the principal sources for heavy elements in the Universe.
The first evidence that a core-collapse supernova has occurred is a massive burst of neutrinos. A shock wave emerges from the star a few hours later, releasing electromagnetic radiation initially as a UV flash. The supernova becomes visible at optical wavelengths as it expands. Brightness increases as the surface area increases combined with a relatively slow temperature decrease.
The peak in brightness occurs as temperature of the outer layers starts to decrease more rapidly. At this point, Type II supernovae may be divided into two classes based on the shape of their light curves. SNII-L (Linear) supernovae show a fairly rapid, linear decay after maximum light. SNII-P (Plateau) supernovae remain bright for an extended time after maximum. The lack of plateaus in SNII-L probably arises because SNII-L have much smaller hydrogen envelopes than SNII-P. The peak brightness of SNII-L are nearly uniform at ~2.5 magnitudes fainter than a Type Ia supernova. However, the peak brightnesses of SNII-P show large variations due to differences in the radii of the progenitor stars. The end of the light curve is a radioactive tail, powered by the conversion of 56Co into 56Fe; it has the same shape for all core-collapse supernovae.
SUPERNOVA REMNANT (SNR) - Diffuse, expanding nebula that results from a supernova explosion. A SNR consists of material ejected in the supernova explosion and interstellar material swept up by the passage of the shock wave from the exploded star. SNRs tend to be powerful X-ray and radio emitters due to interactions with the surrounding ISM. They typically last several hundred thousand years before dispersing into the ISM.
During supernova explosion, a shock wave is sent out through the star, passing through the stellar material and into the surrounding ISM creating a shock wave in the interstellar gas in the forward direction, and a shock in the reverse direction, back into the supernova ejecta. The shocked material is heated to 106s K resulting in the emission of thermal X-rays. The shock wave also pushes the ISM into an expanding shell which emits huge amounts of synchrotron radiation. The supernova ejecta expands freely into the surrounding volume of relatively low density with typical velocities of ~10,000 km/s. This free expansion phase lasts 100–200 years until the mass of the ISM swept up by the shock wave exceeds the mass of the ejected material.
When free expansion stops and Rayleigh-Taylor instabilities arise. These instabilities mix the shocked ISM with the supernova ejecta and enhance the magnetic field inside the SNR shell. This adiabatic (Sedov-Taylor) phase lasts 10,000–20,000 years. The shock wave continues to cool, and once temperatures drop below about 20,000 K, electrons start recombining to form heavier elements and radiating energy. During this radiative phase, the recombination process radiates energy much more efficiently than by thermal X-rays and synchrotron emission and the shock wave cools and ultimately disperses into the surrounding ISM.
SNR are categorized into three main types based on their appearance, with the differences arising due to variations in initial progenitor and explosion conditions, density variations in the interstellar medium (ISM) and Rayleigh-Taylor instabilities. Shell-type remnants emit most of their radiation from a shell of shocked material. This appears as a bright ring, due to limb brightening. Crab-type remnants (also named plerions), which are named for the prototype – the Crab Nebula – are powered by a pulsar located at their center. In contrast to shell-type remnants, this type emits most of its radiation from within the expanding shell. Consequently, they appear as a filled region of emission rather than a ring of emission. Composite remnants are a cross between the other two remnant types, and appear either shell-like or Crab-like, depending on the wavelength of the observations. In general, thermal composites appear shell-like at radio wavelengths and Crab-like in X-rays, while plerionic composites appear Crab-like at both radio and X-ray wavelengths, but also show shell structures.
Supernova remnants disperse the heavy elements made in supernova explosions into the ISM. Additionally, they provide much of the energy to heat the ISM and are probably responsible for the acceleration of galactic cosmic rays.
SYMMETRY - Property of an object if some spatial manipulation of it results in an indistinguishable object. A symmetric object can be superimposed on itself by some operation.
SYMMETRY AXIS - Element of rotational symmetry. Imaginary axis is placed through a perfect crystal so that during a single rotation about this axis the outline of the crystal form appears identically more than once; 2, 3, 4 or 6 times.
SYMMETRY ELEMENT - A geometrical entity such as a point, line, or plane about which a symmetry operation is performed.
SYMMETRY OPERATORS - Four operations which lead to superimposition of an object on itself. These are rotation, translation, reflection and inversion. The symmetry of any object can be described by some combination of symmetry operations.
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.
SYNCHROTRON EMISSION - Type of non-thermal radiation generated by charged particles (usually electrons) spiraling around magnetic field lines at close to the speed of light. The electrons are always changing direction and are, in effect, accelerating and emitting photons with frequencies determined by the speed of the electron at that instant. The resulting radiation is confined to a narrow cone that points in the direction of the motion of the particle, in a process called beaming. It is also polarized in the plane perpendicular to the magnetic field. The degree and orientation of the polarization provide information about the magnetic field of the source.
The spectrum of synchrotron emission results from summing the emission spectra of individual electrons. As a electron spirals around the magnetic field, it emits radiation over a range of frequencies peaking at a critical frequency,v0. The longer the electron spins around the magnetic field, the more energy it loses, the wider the spiral it makes, and the longer the wavelength of the critical frequency. Synchrotron emission has a characteristic spectrum, where the flux steadily declines with frequency according to the relation:
Where, α, known as the spectral index for the object, has an observed range between -3 and +2.5 (which is also the theoretical upper limit). Although particularly important to radio astronomers, depending on electron energy and the strength of the magnetic field, synchrotron emission can also occur at visible, ultraviolet and X-ray wavelengths.
SYSTEM -Definable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with its surroundings; it has walls through which heat cannot pass.