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

Same as → relative density.

→ specific; → density.

The ratio of the density of a substance at the temperature under consideration to the density of water at the temperature of its maximum density (4 °C).

→ specific; → gravity.

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.