DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ed line
primary energy source is hydrogen burning (by
the CNO cycle), in a thin shell around an inert
helium core (more massive analogs are called
red supergiants). The star is about 10 times as
bright as it was on the main sequence (see HR diagram),
and as the core grows, the star leaves the
main sequence track of the Hertzsprung–Russell
diagram and ascends the red giant branch, gaining
in luminosity (on the order of 10 3 L⊙) yet decreasing
in surface temperature (near 3800 K).
The phase lasts about 10% of the main sequence
lifetime and is terminated by the ignition of helium
burning. As the star eventually depletes
its core supply of He, hydrogen fusion begins
again in a thin shell around the core. Eventually,
the core becomes inert C and O with separate,
concentric shells where He-fusion and H fusion
continues. At this stage, the star is considered to
be in the asymptotic giant branch stage of evolution
and has achieved a luminosity of 10 4 L⊙
or greater and a surface temperature of 1800 to
3500 K. The sun will become a red giant about 5
billion years in the future, increasing the surface
temperature of the Earth to about 900 K, where
some metals melt and sulfur boils.
red line A coronal line observed at 6374 Å
resulting from a forbidden transition in highly
ionized iron atoms (Fe X). Important for study
of coronal structures at temperatures of order
1 MK.
redshift A displacement of a spectral line
toward longer wavelengths. This can occur
through the Doppler effect or, as described by
general relativity, from the effects of a star’s
gravitational field or in the light from distant
galaxies. The relative displacement toward
longer wavelength (or equivalently, of lower energy)
of light is measured from spectral features
like emission or absorption of lines and is defined
as
z = (λ − λ0) /λ0 ,
where λ0 is the rest wavelength of a spectral feature.
The displacement toward a longer wavelength
is conventionally termed a redshift even if
the photons are not in the visible spectral range.
Galaxies and quasars, with the only exceptions
of several galaxies very close to the Milky Way,
invariably show all their spectral lines shifted to
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the red, with redshift increasing with distance
according to Hubble’s Law. Redshifts up to
z ≈ 5 have been (in 1998) observed for quasars.
Red Spot A feature in Jupiter’s atmosphere,
some 10,000 by 50,000 km in extant. It was
probably first observed by Hooke in 1664, but
a continuous record of observations only goes
back to the end of 1831. Its position is roughly
fixed, with some drift relative to the average position.
Although a number of suggestions have
been forwarded to explain this feature, the most
likely is an atmospheric disturbance, similar to a
hurricane. Scaling arguments indicate that such
a disturbance would be much longer lived on
Jupiter than on Earth.
reduced gravity Reduced gravity g ′ is defined
by g ′ = g(ρ2 − ρ1)/ρ2 where ρ1 and ρ2
are the density of the upper and lower layer.
reduced gravity model A model treating the
ocean thermocline as an infinitesimally thin interface
separating a warm upper layer and an
infinitely deep abyssal. In such a model, the
motion in the abyssal is negligible, and the upper
layer motion is described by a set of shallow
water equations with the gravitational acceleration
g replaced by a reduced gravity g ′ =
g(ρ1−ρ2)/ρ2, where ρ1 and ρ2 are water density
in the upper layer and abyssal, respectively. The
reduced gravity model is particularly useful in
modeling the longest wavelength internal mode
in the tropical oceans where the thermocline is
tight and shallow.
reef A ridge of coral or rock which lies at or
near the surface of the sea.
reference frame A standard of reference
used for physical systems, which is in principle
equivalent to observations by a real observer, via
measurements using the observer’s clock and a
local set of Euclidean measurement axes with
a length standard. The reference frame is expressed
in terms of a set of basis vectors e(a). In
principle another set e ′ (b ′ ) of basis vectors can be
defined by a nonsingular (not necessarily constant)
linear combination
e ′ (b ′ ) = Ab ′ a e(a) .