An Etymological Dictionary of Astronomy and Astrophysics
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A system for the classification of → S-type stars. The system involves the designations of a C/O index and a temperature type. Moreover, when possible, it uses intensity estimates for → ZrO bands, the → TiO bands, the → Na I D-lines, the YO bands, and the Li I 6708 line.

Philip C. Keenan & Patricia C. Boeshaar, 1980, ApJS, 43, 379; → classification.

General: A set, collection or group formed of members with certain attributes or traits in common.

From Fr. classe, from L. classis "summons, division of citizens for military draft, hence army, fleet, also class in general."

Radé "a line, series, row," from Mid.Pers. ratak "series, row," O.Pers. râd-, Av. raz- "to direct, put in line, set," Av. razan- "order."

A low-mass → protostar deeply embedded in a → circumstellar dusty envelope and resulting from the → gravitational collapse of a dense → pre-stellar core. This stage in the process of star formation occurs typically a few 10⁴ years after the onset of the collapse. Class 0 protostars represent the earliest stage of → young stellar objects. The → spectral energy distribution (SED) of a Class 0 object resembles a → blackbody spectrum at a temperature below ~ 15-30 K, peaking at → submillimeter wavelengths beyond 100 μm. The central protostar has not yet acquired its final mass, since → accretion is still going on, and the envelope (detected in submillimeter wavelengths) is more massive than the central protostellar mass. Moreover, these objects show powerful → bipolar ejections of material in the form of collimated → carbon monoxide (CO)→ outflows which distinguish them from the pre-stellar phase of star formation. The subsequent evolution of a Class 0 is a → Class I.

→ class; → zero.

A protostellar phase resulting from the evolution of a → Class 0 object typically a few 10⁵ years after the beginning of the → gravitational collapse. The protostar grows in mass due to → accretion from the envelope, which becomes less massive than the protostar. An → accretion disk forms around the protostar through which mass is transferred to the central object. The → spectral energy distribution (SED) changes with respect to that of a Class 0. The peak of the SED shifts to → far infrared wavelengths (below 100 μm) as the temperature of the dust rises. Emission from both the envelope (about 100 K) and the thick disk (a few 100 K) are observed. The SED has a positive → spectral index (α_IR > 0), so that the bulk of the → luminosity (still due to accretion) emerges at the longer infrared wavelengths. Moreover, → bipolar outflows and → jets are observed which are generally less powerful than those in Class 0 objects. Class I objects evolve into → Class II.

→ class; → one.

A stage in the evolution of low-mass → protostars resulting from a → Class I object about 10⁶ years after the initial → gravitational collapse. Most of the envelope has been removed and the embedded object becomes visible at infrared and optical wavelengths. At this stage, the bulk of the material has → accreted onto the central object. A flattened → circumstellar disk or → protoplanetary disk is present in which material moves inward at a decreasing rate. The disk contributes only about 1% of the total mass of the system. Material from a remaining envelope may still accrete onto the outer parts of the disk. The → spectral energy distribution (SED) at → near infrared wavelengths is dominated by the emission of the central protostar and typically peaks around 2 μm, corresponding to temperatures around 1000 to 2000 K. At longer wavelengths an → infrared excess is observed, originating from the disk. The SED has a negative → spectral index (-1.5 < α_IR < 0). Estimated disk masses and → accretion rates are 10^-3 to 10^-1 → solar masses and 10^-8 solar masses per year, respectively. This stage initiates the → pre-main sequence stage of a star. The object is referred to as a → classical T Tauri star. The stellar → photosphere is revealed at optical wavelengths accompanied by strong → emission lines and photometric variability, but the infrared luminosity is far larger than can be explained by the photometric temperature and radius.

→ class; → two.

An evolutionary stage in the formation of low-mass → protostars resulting from a → Class II object between 1 to 10 million years after the initial → gravitational collapse. At this stage → accretion has ceased completely and what remains from the → circumstellar disk is a → debris disk. The temperature and density of the → pre-main sequence star keep increasing as the object slowly contracts to its final size. Most of the → luminosity derives from protostellar contraction. The → spectral energy distribution (SED) resembles a stellar → blackbody, peaking at optical and infrared wavelengths. Minor → infrared excess is still observed. The SED has a negative → spectral index (α_IR < -1.5). Class III objects are sometimes called → weak-line T Tauri stars.

→ class; → three.

1) Considered as the typical, traditional, or usual form of something. → classical T Tauri star.
2) → classical physics.

From classic (+ → -al), from Fr. classique, from L. classicus "belonging to a class, relating to the first or highest class of the Roman people," from classis perhaps akin to calare "to call."

Loan from Fr. classique, as above.

A → galaxy bulge that appears protruding from the disk plane when seen at an appropriate → inclination. Classical bulges are somewhat → spheroidal, featureless (no → spiral arms, → bars, → rings, etc.), contain mostly → old stars (not much dust or star-forming regions), and are kinematically hot, i.e. dynamically supported by the → velocity dispersion of their stars. Their → surface brightness profile follows the → de Vaucouleurs law. Currently, they are thought to form through → gravitational collapse or → mergers in violent events, inducing a fast → burst of star formation if gas is available. An example is the → Sombrero galaxy bulge (D. A. Gadotti, 2012, astro-ph/1208.2295).

→ classical; → bulge.

The theory that studies distributions of → energy, → matter, and other physical quantities under circumstances where their discrete nature is unimportant. Classical field theory traditionally includes → Newtonian mechanics, Maxwell's → electromagnetic theory, and Einstein's theory of → general relativity. The main scope of classical field theory is to construct the mathematical description of → dynamical systems with an infinite number of degrees of freedom. The word "classical" is used in contrast to those field theories that incorporate → quantum mechanics (→ quantum field theory). Classical field theories are usually categorized as → non-relativistic and → relativistic.

→ classical; → field; → theory.

The traditional logic in which → sets are sharply defined (→ crisp set) for example, the number of students registered for a course, or the names beginning with P in a given telephone directory. Classical logic also defines relations between sets of → propositions. Consider for example two sets: elephants and mammals, a simple proposition would be the assertion that all elephants are mammals, that is E ⊂ M, where E is the elephant set and M is the mammal set. The classical logic proposition is either true or false. Compare with → fuzzy logic.

→ classical; → logic.

The branch of physical science which deals with the motions of bodies travelling at velocities that are very much less than that of light in a vacuum. Same as → Newtonian mechanics.

→ classical; → mechanics.

Physics not taking into account → quantum mechanics or Einstein's → relativity theory. Classical physics includes the branches developed before the beginning of the 20th cantury: Mechanics, Acoustics, Optics, Thermodynamics, and Electricity and Magnetism. Most of classical physics is concerned with matter and energy on the normal scale of observation.

→ classical; → physics.

A → T Tauri star in which → accretion from a → circumstellar disk is responsible for ultraviolet and infrared excess emission and for a moderate to strong emission line spectrum superimposed on the photospheric spectrum. Classical T Tauri stars probably evolve into → weak-line T Tauri stars when their disks are fully accreted by the stars.

→ classical; → T Tauri star.

The systematic grouping of astronomical objects into categories on the basis of physical, morphological, or evolutionary characteristics.

Classification, from O.Fr., from classifier, from → class + -fier, from L. -ficare, root of facere "to make, do;" PIE base *dhe- "to put, to do" (cf. Skt. dadhati "puts, places;" Av. dadaiti "he puts," O.Pers. ada "he made," Gk. tithenai "to put, set, place."

Radebandi, from radé, → class, + bandi, verbal noun of bastan "to bind, shut; to get, acquire, incur," from Mid.Pers. bastan/vastan "to bind, shut;" Av./O.Pers. band- "to bind, fetter," banda- "band, tie;" cf. Skt. bandh- "to bind, tie, fasten;" Ger. binden, E. bind, → band; PIE base *bhendh- "to bind."

A star near the beginning of the → spectral classification sequence. A star of → spectral type O, B, A, or F0 to F5. Same as → early-type star.

→ early; → spectral; → class; → star.

In the → Fanaroff-Riley classification, sources with R_FR < 0.5. Fanaroff and Riley (1974) found that nearly all sources with luminosity L(178MHz) ≤ 2 × 10²⁵h₁₀₀^-2 W Hz^-1 sr^-1 were of class I. FR-I → radio jets are thought to be → subsonic, possibly due to mass entrainment, which makes them amenable to distortions in the interaction with the ambient medium.

→ Fanaroff-Riley classification; → class.

In the → Fanaroff-Riley classification, → radio sources with hotspots in their lobes at distances from the center which are such that R_FR > 0.5. The → radio jets in FR-II sources are expected to be highly → supersonic, allowing them to travel large distances.

→ Fanaroff-Riley classification; → class.

A classification scheme for distinguishing a → radio galaxy from an → active galaxy based on their → radio frequency  and → luminosity and their kpc-scale appearance. Analyzing a sample of 57 radio galaxies from the → 3CR catalogue, which were clearly resolved at 1.4 GHz or 5 GHz, Fanaroff & Riley (1974) discovered that the relative positions of regions of high and low → surface brightness in the → lobes of extragalactic → radio sources are correlated with their radio luminosity. They divided the sample into two classes using the ratio R_FR of the distance between the regions of highest surface brightness on opposite sides of the central galaxy or quasar, to the total extent of the source up to the lowest brightness contour in the map. → Fanaroff-Riley Class I (FR-I) , → Fanaroff-Riley Class II (FR-II). The boundary between the two classes is not very sharp, and there is some overlap in the luminosities of sources classified as FR-I or FR-II on the basis of their structures. The physical cause of the FR-I/II dichotomy probably lies in the type of flow in the → radio jets.

Bernard L. Fanaroff and Julia M. Riley, 1974, MNRAS 167, 31P; → classification.

A → geochemical classification scheme in which → chemical elements on the → periodic table are divided into groups based on their → affinity to form various types of compounds: → lithophile, → chalcophile, → siderophile, and → atmophile. The classification takes into account the positions of the elements in the periodic table, the types of electronic structures of atoms and ions, the specifics of the appearance of an affinity for a particular → anion, and the position of a particular element on the → atomic volume curve.

Developed by Victor Goldschmidt (1888-1947); → classification.

A classification of stellar spectra published in the Henry Draper catalogue, which was prepared in the early twentieth century by E. C. Pickering and Miss Annie Canon. It is based on the characteristic lines and bands of the chemical elements. The most important classes in order of decreasing temperatures are as follows: O, B, A, F, G, K, M.

Harvard, named for John Harvard (1607-1638), the English colonist, principal benefactor of Harvard College, now Harvard University. → classification