DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

ight point A transient H-alpha brightening
of flare intensity, less than 20 millionths of the
solar hemisphere in area. Such a brightening
when covering a larger area becomes classified
as a solar flare.
brittle behavior (brittle fracture) A phenomenologicaltermdescribingthenatureofmaterial
failure. According to the simplest or “classical”
view, brittle behavior (brittle fracture) of
a solid material is a discrete event in which the
failure of the solid occurs, without significant
prior deformation and without warning, at a particular
stress. In general, material is described
as “brittle” when fracture occurs with no preceding
appreciable permanent deformation. Sometimes
brittle behavior refers to failure following
limited amounts of inelastic strain that are small
compared with the elastic strain.
brittle-ductile transition A temperature
range over which the principal failure mechanism
of a rock body (such as the continental
crust) under deviatoric stress changes from brittle
to ductile. Brittle failure takes place at low
temperatures by developing fractures or slipping
along pre-existing fractures. Ductile failure
takes place at higher temperatures and, observed
at the macroscopic scale, by continuous
deformation. The transition occurs due to the
activation of diffusion or dislocation creep at elevated
temperatures and is affected by the rate of
ductile deformation and the pore fluid pressure
in the brittle region. In the Earth’s continental
crust, this transition is marked by the absence
of earthquakes below a certain depth where the
temperature is higher than about 300 to 400 ◦ C.
In the oceanic crust and mantle, the transition
occurs at higher temperatures.
brittle-plastic transition See brittle-ductile
transition.
broad line radio galaxies Radio galaxies
showing optical spectra very similar to, and
in several cases almost indistinguishable from,
those of Seyfert-1 galaxies. Broad line radio
galaxies are type 1, low-luminosity, radio-loud
active galactic nuclei; the radio-loud counterpart
of Seyfert-1 galaxies. Differences between
Seyfert-1 and Broad Line Radio galaxies en-
© 2001 by CRC Press LLC
brown dwarf
compass the morphology of the host galaxy
(Seyfert-1 are mostly, albeit not exclusively, spirals,
while broad line radio galaxies are hosted
by ellipticals) and some features of the optical
spectrum, like weaker singly ionized iron emission
and larger internal absorption due to dust in
broad line radio galaxies. See Seyfert galaxies.
broad line region The region where the
broad lines of active galactic nuclei are produced.
The strongest lines observed in the optical
and UV spectrum are the Balmer lines of
hydrogen, the hydrogen Lyman α line, the line
from the three times ionized carbon at 154.9 nm,
and some recombination lines from singly ionized
and neutral helium. Since no forbidden
lines are observed, the broad line region is most
likely a relatively high density region with particle
density in the range 10 9 to 10 13 ions per cubic
centimeter. The broad line region is believed to
be very close to the central source of radiating
energy of the active galactic nucleus. Observations
of variation of broad line profiles and fluxes
suggest that the broad line regions line emitting
gas is confined within 1 pc in Seyfert-1 galaxies.
Models of the broad line regions invoke a large
number of dense emitting clouds, rapidly rotating
around a central illuminating source. Alternatively,
it has been suggested that at least part
of the emission of the broad line regions could
come from the middle and outer region of the
accretion disk suspected to be a universal constituent
of the central engine of active galactic
nuclei.
brown dwarf A sphere of gas with the composition
of a star (that is, roughly 3 4 hydrogen,
1
4 helium, and at most a few percent of heavier
elements) but with a mass low enough that the
center never, as the object contracts out of interstellar
gas, gets hot enough for hydrogen burning
to balance the energy being lost from the surface
of the sphere. Very young brown dwarfs can be
as bright as low-mass stars (10−4 to 10−5 of the
solar luminosity), but they fade with time until
they are too cool and faint to see. The number
of brown dwarfs in our galaxy is not very well
known. They are probably not common enough
(at least in the galactic disk) to contribute much
to the dark matter, but small numbers have been
observed (a) in young clusters of stars, (b) as