An Etymological Dictionary of Astronomy and Astrophysics
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A rise in a planet's surface → reflectivity between red → absorbance and → near-infrared reflection due to → vegetation. The red-edge is one of the possible signs of life on distant → habitable  → exoplanets. Its presence is attributed to the chlorophyll molecule and leaf structure. The leaves of land plants reflect sunlight much more efficiently long-ward of this edge than they do in the visible. Although the red-edge position for Earth's vegetation is fixed at around 700-760 nm, that for exoplanets may not necessarily be the same (Takizawa et al., 2017, Nature Scientific Reports 7, Article number: 7561).

→ red; → edge.

1) (v.intr.) Of a spectral line, to reduce in intensity due to absorption by interstellar dust grains.
2) (v.tr.) Of interstellar dust, to absorb the light that passes through it.

Infinitives from → red.

A star whose light has undergone → reddening.

Reddened, p.p. of → redden; → star.

The process by which light from an astronomical object grows red as it travels through interstellar dust. Dust scatters blue light more than red, thus leaving predominantly red light transmitted.

Verbal noun of → redden.

A dimensionless quantity determined from the comparison of the observed → Balmer decrements with respect to the theoretical values for given physical conditions of electron temperature and density. The reddening coefficient at Hβ is defined as c(Hβ) = log (I(Hβ)/F(Hβ)), where I(Hβ) and F(Hβ) are → de-reddened and reddened Hβ fluxes respectively. Also called logarithmic extinction.

→ reddening; → coefficient.

The normalized interstellar extinction at a given wavelength. It is defined by f(λ) = A(λ)/A(Hβ) - 1, where A(λ) is the extinction at the given wavelength and A(Hβ) the extinction at Hβ, with f(Hβ) = 0. It is used to → de-redden observed fluxes: I(λ)/I(Hβ) = F(λ)/F(Hβ).10^c(Hβ).f(λ), where I represents the flux in the absence of extinction and F the observed flux affected by extinction, c(Hβ) being the → reddening coefficient.

→ reddening; → function.

A dimensionless quantity characterizing the → interstellar extinction, defined by the total-to-selective extinction ratio: R_V = A_V/E(B-V). The typical value found for the reddening parameter in the Milky Way is R_V ~ 3.1, but it is known to vary from one line of sight to another, from values as 2 to as large as 6. Very large → dust grains would produce extinction with R_V → ∞.

→ reddening; → parameter.

A vector indicating the direction in which interstellar reddening moves the position of a star in a multi-dimensional space of color indices.

→ reddening; → vector.

A shift in the lines of an object's spectrum toward longer wavelengths. Redshift indicates that an object is moving away from the observer. The larger the redshift, the faster the object is moving. Redshift is expressed by z = Δλ/λ = v/c, where λ is the wavelength, Δλ the wavelength shift, v the velocity of the source relative to the observer, and c the → speed of light. When v approaches c, redshift is expressed by the → relativistic formula z = ((1 +v/c)/(1 - v/c))^½ - 1.

→ red; → shift.

The space corresponding to → redshift measurements, as contrasted with real space. See also → redshift space distortion

→ redshift; → space.

The distortion observed in → redshift space of → galaxy clusters caused by peculiar velocities of the members (→ peculiar velocity). In a perfectly homogeneous → Friedmann-Lemaitre Universe the redshifts would accurately measure radial distances from the observer, and the mapping from real space to redshift space would simply be an identity. In an inhomogeneous Universe the peculiar velocities associated with any inhomogeneous structure will introduce a distortion in this mapping (N. Kaiser, 1987, MNRAS 227, 1). See also: → fingers of God, → Kaiser effect.

→ redshift; → space; → distortion;.

A survey of a large region of the sky to measure the redshifts of all the galaxies down to a certain limiting magnitude.

→ redshift; → survey.

The correlation, first established by E. Hubble, between the cosmological recession velocities of galaxies and their distances.

→ redshift; → distance; → relation.

A spectral line whose wavelength does not coincide with its theoretical value and is shifted toward longer wavelengths.

→ red; → shift; → line.

1) To bring to a certain state, condition, arrangement, etc.
Math.: To change the denomination or form, but not the value, of (a fraction, polynomial, etc.).
Chem.: To add an electron to.
2) To bring down to a smaller extent, size, amount, number, etc.; synonymous with → decrease.

M.E. reducen "to lead back," from O.Fr. reducer, from L. reducere, from → re- "back" + ducere "to bring, to lead."

From bâz-, → re- + Mid.Pers. hâxtan, hâzidan "to lead, guide, persuade," Av. hak-, hacaiti "to attach oneself to, to join," cf. Skt. sacate "accompanies, follows," Gk. hepesthai "to follow," L. sequi "to follow;" PIE *sekw- "to follow."
Kâstan, → decrease.

The "effective" → inertial mass appearing in the → two-body problem of → Newtonian mechanics. The reduced mass is a quantity which allows the two-body problem to be solved as if it were a one-body problem. For the masses m₁ and m₂, it is given by the ratio μ = m₁m₂ / (m₁ + m₂). The value of μ is generally smaller than m₁ and m₂. The larger the difference between the two masses, the closer μ will be to the smaller mass. If the particles are of equal mass, μ is half the mass of either.

→ reduce; → mass.

The → Planck constant divided by 2π and denoted ħ, pronounced h-bar. Also called the → Dirac constant.

Reduced, p.p. of → reduce; → Planck's constant.

→ reduced Planck constant.

→ reduced Planck constant.

A substance which removes → oxygen from, or adds → hydrogen, to another substance. In the more general sense, one which loses electrons. Also called → reductant.

→ reduce; → -ing; → agent.

1) An atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidating gasses or vapours. Usually nitrogen or hydrogen gas is used in order to produce specific effects, e.g. on ceramic wares being fired.
2) An atmosphere of a planet or moon which has a high hydrogen content, either in the form of free hydrogen or hydrogen-containing compounds, such as methane or ammonia. The early atmosphere of Earth is thought to be reducing, dominated by carbon dioxide.

Reducing verbal adj. of → reduce; → atmosphere.