An Etymological Dictionary of Astronomy and Astrophysics
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A star on the → horizontal branch which results from the evolution of a → red giant with an initial mass of ~ 1 Msun.

→ red; → clump; → star.

A small, cool, very faint, main sequence star whose surface temperature is under about 3500 K. Red dwarfs generally have masses of less than one-third that of the Sun. In the neighbourhood of the Sun the majority of stars are red dwarfs.

→ red; → dwarf.

A certain star of spectral type K or later that occupies the upper right portion of the → H-R diagram. Red giants are evolved stars that have exhausted their hydrogen fuel in the core. They may have a → luminosity up to 1000 times greater than → main sequence stars of the same → spectral type. Red giants belong to the → luminosity class III or II (bright giants). They are luminous because of their great size, but have a relatively low surface temperature. All normal stars are expected to pass eventually through a red-giant phase as a consequence of stellar evolution. When a main sequence star has converted approximately 10% of its hydrogen to helium, nuclear reactions in the core stop (→ Schönberg-Chandrasekhar limit). The → hydrostatic equilibrium is no longer maintained, and the core contracts while the outer layers expand and cool. This process produces the low surface temperature and large size (from 10 to 100 times that of the Sun) that characterize the red giant. In the core the temperature continues to rise. When it approaches 100,000,000 K helium will begin to fuse into carbon. → helium flash. Prominent bright red giants in the night sky include → Aldebaran and → Arcturus.

→ red; → giant.

The evolutionary path of a star that has exhausted its available hydrogen content in the core, between the → main-sequence turnoff and the → helium flash.

→ red giant; → branch.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

→ red; → horizontal; → branch; → star.

Unwanted secondary window in a filter band pass, on the red side of the main window.

→ red; leak, from M.E leken, from O.N. leka "to drip, leak;" akin to Du. lek, Ger. lech "leaky," O.E. leccan "to moisten."

Našt "leak, leakage," of unknown origin; sorx, → red.

See → Great Red Spot, on Jupiter.

→ red; → spot.

A supergiant star with spectral type K or M. Red supergiants are the largest stars in the Universe, but not necessarily the most massive. Betelgeuse and Antares are the best known examples of a red supergiant.

→ red; → supergiant.

A member of a class of exploding stars that are more luminous than → novae but not as luminous as → supernovae. Moreover, their outburst → light curves have multiple peaks. One of the most characteristic features of red transients is that after exploding they cool down to → late-type → M star and develop circumstellar material rich in molecules and dust. Some of the members of the red transients in our Galaxy are V838V, OGLE-2002-BLG-360, V4332 Sgr, and V1309 Sco.

→ red; → transient.

Of a spectral line profile, the → line wing with wavelengths longer than that of the emission or absorption peak.

→ red; → wing.

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.