DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Bogomol’nyi bound The mass of a (magnetic)
monopole depends on the constants that
couple the Higgs field with itself and with the
gauge fields forming the monopole configuration.
The energy of a monopole configuration
can be calculated, albeit numerically, for any
value of these constants. It can, however, be
seen that the mass always exceeds a critical
value, first computed by E.B. Bogomol’nyi in
1976, which is
m M ≥η/e ,
whereη is the mass scale at which the monopole
is formed, and e is the coupling of the Higgs
field to the gauge vectors Aa µ (a being a gauge
index). The bound is saturated, i.e., equality
holds, when the ratio of the coupling constants
vanishes, a limit known as the Prasad–
Sommerfield limit. Since magnetic monopoles
may be produced in the early universe, this limit
is of relevance in cosmology. See cosmic topological
defect, monopole, Prasad–Sommerfield
limit, t’Hooft–Polyakov monopole.
Bohr’s theory of atomic structure Theory
of the atomic structure of hydrogen postulated
by Neils Bohr (1885–1962). The theory is based
upon three postulates: 1. The electrons rotate
about the nucleus without radiating energy in the
form of electromagnetic radiation. 2. The electron
orbits are such that the angular momentum
of the electron about the nucleus is an integer
of h/2π, where h is Planck’s constant. 3. A
jump of the electron from one orbit to another
generates emission or absorption of a photon,
whose energy is equal to the difference of energy
between the two orbits. This theory has
nowbeensupersededbywavemechanics, which
has shown that for the hydrogen atom spectrum,
Bohr’s theory is a good approximation.
bolide A meteoroid that explodes or breaks
up during its passage through the atmosphere as
a meteor.
bolometric correction (B.C.) The correction
to an observed magnitude (luminosity) in a
particular wavelength region to obtain the bolometric
magnitude (luminosity). This correction
is to account for the stellar flux that falls beyond
the limits of the observed wavelength region.
© 2001 by CRC Press LLC
borrow site
bolometric magnitude/luminosity The luminosityofastarintegratedoverallwavelengths
the star radiates. This is a measure of the total
energy output of a star.
Boltzmann’s constant ( k) Constant of proportionality
between the entropy of a system
S and the natural logarithm of the number of
all possible microstates of the system , i.e.,
S=k ln(). When one applies statistical mechanics
to the ideal or perfect gas, one finds
that Boltzmann’s constant is also the constant
of proportionality of the equation of state of
an ideal gas when the amount of gas is expressed
in the number of molecules N rather
than the number of moles, i.e., PV =NkT.
k= 1.3806503(24) × 10 −23 J K −1 . Named
after Ludwig Boltzmann (1844–1906).
Bondi mass In general relativity the function
m(r,t) equal to the total energy contained inside
the sphere of radius r at the time t. If there is
no loss or gain of energy at r →+∞, and the
space is asymptotically flat, then
lim
r→+∞ m(r,t)=MADM ,
where MADM is the ADM mass. See ADM
mass, asymptotic flatness.
Bonnor symmetry A stationary (i.e., rotating)
axisymmetric vacuum space-time can be
mapped to a static axisymmetric electrovacuum
space-time. In this process, the twist potential
must be extended in the complex plane. When
the twist is proportional to a real parameter, the
complex continuation of this parameter can be
used to make the resulting twist potential real.
See electrovacuum.
bore A steep wave that moves up narrowing
channels, produced either by regular tidal
events, or as the result of a tsunami. A bore
of this type, generated by an earthquake off the
coast of Chile, destroyed Hilo on the island of
Hawaii on May 22, 1960.
borrow site A source of sediment for construction
use. May be on land or offshore.