An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

1) The quantity of heat required to raise the temperature of 1 gm of a substance through 1 °C. More generally, the → heat capacity of a unit mass of a substance. For a homogeneous body it is expressed as: C = dQ/M dT, where dQ is the quantity of heat transferred to a mass of M to raise the temperature by dT. It is often convenient to use the gram-mole as a unit of mass, → molar heat capacity.
2) For a gas there are two principal specific heats depending on the way in which the temperature is increased: i) that measured at constant pressure, C_P, and ii) that measured at constant volume, C_V. The specific heat C_P is greater than C_V, because a gas heated at constant pressure expands, and heat energy must be supplied equivalent to the work done in the expansion. The ratio γ = C_P/C_V is called the → adiabatic index. It varies from 1.66 for mono-atomic gases to a little over 1 for gases with complex molecules.

→ specific; → heat.

The dimensionless ratio of the mass of water vapor to the total mass in a particular volume. → humidity

→ specific; → humidity.

A measure of the amount of radiation received per unit solid angle per unit time per unit area normally from an element of surface.

→ specific; → intensity.

Star formation rate per unit → mass. More specifically, the → star formation rate in a galaxy divided by the → stellar mass of the galaxy. Observations of galaxies over a wide range of → redshifts suggest that the slope of the SFR-M_* relation is about unity, which implies that their sSFR does not depend strongly on stellar mass. Specific star formation rates increase out to z ~ 2 and are constant, or perhaps slowly increasing, from z = 2 out to z = 6, though with a large scatter, sSFR ~ 2-10 Gyr^-1 (Lehnert et al., 2015, A&A 577, A112, and references therein).

→ specific; → star; → formation; → rate.

The volume occupied by unit mass of a substance. Specific volume is the reciprocal of density.

→ specific; → volume.

1) The act of specifying.
2) A particular item, aspect, calculation, etc., in such a description.
3) Something specified, as in a bill of particulars; a specified particular, item, or article (Dictionary.com).

Verbal noun of → specify.

The state or character of being → specific.

→ specific; → -ity.

1) To mention or name specifically or definitely; state in detail.
2) To give a specific character to; to set forth as a specification (Dictionary.com).

→ specific; → -fy.

A part or an individual taken as exemplifying a whole mass or number; a typical animal, plant, mineral, part, etc. → sample.

From L. specimen "mark, example, indication, sign, evidence," from speci- stem of specere "to look at," → -scope, + -men noun suffix denoting result or means.

Nemuné, from nemudan "to show;" Mid.Pers. nimūdan, nimây- "to show," from O.Pers./Av. ni- "down; in, into," → ni- (PIE), + māy- "to measure;" cf. Skt. mati "measures," matra- "measure;" Gk. metron "measure;" L. metrum; PIE base *me- "to measure."

1) Optics: An image defect, one of a large number of bright and dark spots, that appears when an object is illuminated by monochromatic, highly → coherent light. This phenomenon results from the → interference of a number of randomly phased complex contributions of electromagnetic → wavefronts scattered from an object with rough structure, such as a piece of paper, a display screen, or a metallic surface. In particular, whenever the object is rough on the scale of an optical wavelength, the image has a grainy appearance. Also called speckle noise.
2) Astro.: The pattern produced by a short-exposure image of a → point source, such as a star, when the → wavefront is torn apart under the effect of the → atmospheric turbulence. Speckles change very rapidly with time as a function of the atmospheric turbulence. → speckle lifetime. Long exposure images of these changing speckle patterns result in a blurred image of the star, called a → seeing disk. → Fried parameter.

Speckle "a speck or small spot, as a natural dot of color on skin, plumage, or foliage," from M.E.speck (from O.E. specca "small spot, stain," of unknown origin; probably related to Du. speckel "speck, speckle") + -le a noun suffix having originally a diminutive meaning.

Pakâl, from pak "spot" (Lâri, Gerâši), pašy "mingled, confused" (Tâleši), probably related to pisé "dappled, variegated," pis, pisi "leprosy," neveštan "to write," pišé "profession," → professional astronomer; Mid.Pers. parš "speckled, spotted," pēsīdan "to color, adorn," pēsit "adorned;" O.Pers. pais- "to adorn, cut, engrave;" Av. paēs- "to paint, adorn," paēsa- "adornment;" cf. Skt. peś- "to adorn, hew out, decorate," piśáti "adorns; cuts;" Gk. poikilos "multicolored;" L. pingit "embroiders, paints;" O.C.S. pisati "to write;" O.H.G. fēh "multicolored;" Lith. piēšti "to draw, adorn;" PIE base *peik- "colored, speckled."

A technique for generating a clear composite image of a celestial object blurred by → atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the → speckles in a series of images it is possible to approach the theoretical resolution of the telescope.

→ speckle; → interferometry.

The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r₀/Δv, where r₀ is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.

→ speckle; → life; → time.

Omr "life-time;" from Ar. 'umr; pakâl, → speckle.

An image defect associated with the → speckle phenomenon.

→ speckle; → noise.

Of or pertaining to a → spectrum.

→ spectrum; → -al.

A system that assigns a → spectral type to a star according to characteristics of its spectrum. The earliest attempt to divide stars on the basis of their spectra was the → Secchi classification in the 1860s. This scheme paved the way for the → Harvard classification that led to the current → Morgan-Keenan classification of spectral types. In the Harvard system stars were originally thought to follow an evolutionary sequence from the "early" O and B types to the "late" K and M types. Although this is now known to be wrong, the terms → early-type star and → late-type star are still in use. In the Morgan-Keenan system stars are classified as type O, B, A, F, G, K, or M in order of decreasing → effective temperature, and each type further subdivided into subclasses from 0 (hottest, except for → O-type stars) to 9 (coolest). They are also accompanied by a → luminosity class. In the late 1990s, spectral types L and T were added to the sequence to accommodate the coolest stars and → brown dwarfs (with class Y reserved for the coolest brown dwarfs of all, as yet unobserved).

→ spectral; → classification.

The → range of → wavelengths or frequencies (→ frequency) at which a → detector is sensitive. Same as → bandwidth.

→ spectral; → coverage.

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

→ spectral; → density.

→ dispersion.

→ spectral; → dispersion.

A plot showing the energy emitted by a source as a function of the radiation wavelength or frequency. It is used in many branches of astronomy to characterize astronomical sources, in particular mainly in → near infrared and → middle infrared to study → protostars or → young stellar objects. The SED of these objects is divided in four classes.
Class 0 in which the SED represents a very embedded protostar, where the mass of the central core is small in comparison to the mass of the → accreting envelope. The SED is characterized by the → blackbody radiation of the envelope and peaks at → submillimeter wavelengths.
Class I objects possess a SED that peaks in the → far infrared and is characterized by a weak contribution of the blackbody of the central protostar (detected in near infrared) and the emission of a thick disk and dense envelope. These objects have less mass in the envelope and more massive central cores with respect to Class 0.
Class II objects are the → classical T Tauri stars with a SED due to the emission of a thin disk and the central star. They have accumulated most of their final mass and have dispersed almost completely their circumstellar envelope.
Finally, Class III objects have pure photospheric spectra. Their SED is peaked in the optical and is well approximated by a blackbody emission with a faint → infrared excess due to the presence of a residual optically thin disk that may be the origin of → planetesimals.
This classification scheme can be made more quantitative by defining a → spectral index.

→ spectral; → energy; → distribution.

An emission or absorption mark in the spectrum of an astronomical object, of known or unknown origin, usually with complex structure.

→ spectral; → feature.