DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

a statement about why the universe is so homogeneous.
As a solution, a period of rapid expansion
(see inflation) has been advocated that
would have stretched the observed universe out
of a horizon volume at early times.
Big Bang nucleosynthesis The production
of deuterium, 3 He and 4 He (the latter to about
25% mass fraction) in the first 500 to 1000 sec
of the early universe. These light isotopes, plus
measurable amounts of 7 Li and trace amounts of
elements B, Be, are the result of non-equilibrium
nuclear reactions as the universe cooled to about
10 8 K. Heavierisotopeswereproducedinstellar
nucleosynthesis.
big blue bump A feature in the spectral energy
distribution of active galactic nuclei, dominating
the emission shortwards of 4000 Å and
in the UV. The shape, luminosity, and spectral
extension of the big blue bump are as yet uncertain
since the big blue bump lies mostly in
the unobserved far UV, with maximum emission
probably right below the Lyman limit. Only the
low and high energy tails of the big blue bump
have actually been observed. The high energy
end of the big blue bump appears to be for several
objects in the soft X-ray domain, where a
sharp rise toward lower energies, the so-called
“soft X-ray excess” is observed. The current
interpretation of the active galactic nuclei spectral
energy distribution, still highly debated, ascribes
this feature to thermal emission from a
hot accretion disk surrounding a supermassive
black hole. See active galactic nuclei, Lyman
limit.
binaries, X-ray X-ray binaries are close binaries
in which one of the objects is either a
black hole or neutron star, and the companion is
either a star with a strong wind or a star overfilling
its Roche-lobe. The material transfers from
the companion to the compact object and forms
an accretion disk. The gravitational energy releasedasthematerialaccretesontothediskpowers
the X-ray flux. Two classes of X-ray binaries
exist: low-mass (neutron star) and high-mass
(black hole) X-ray binaries. Low-mass X-ray
binaries have companion star masses less than
5M⊙, whereas high-mass X-ray binaries have
more massive companions.
© 2001 by CRC Press LLC
binary star
binary black holes Two black holes orbiting
each other, like stars in a binary system. A binary
system of two black holes may radiate away
significant orbital energy by emission of gravitational
radiation, with efficiency that increases
with the eccentricity of the orbit; orbital energy
loss may lead to orbital decay, with the two black
holes spiraling down toward each other and ultimately
coalescing to form a single black hole.
Thereisasyetnodefinitiveevidencefromobservations
of the actual occurrence of binary black
holes. Binary black holes have been invoked to
explain peculiar features of kpc-sized radio jets,
of the photometric light curve of the blazar OJ
287, and of rarely observed peculiarities in the
spectral line profiles of active galactic nuclei.
Gravitational wave detectors may provide the
first incontrovertible evidence of merging binary
black holes. See black hole, black hole binary.
binary fraction The fraction of stars in a stellar
association which are binary stars. For many
years it was believed that there were no binary
starsamongPopulationIIstars. However, itnow
seems that there may be no difference in the relative
number of binary stars between groups of
stars of the same age, regardless of whether they
are Population I or Population II.
binary pulsar The pulsar PSR 1913+16,
which is in orbit with another neutron star;
both stars have a mass of approximately 1.4
solar masses. Orbital parameters are extracted
from the almost periodic Doppler shifts of the
pulses from the pulsar. The pulse repetition period
is 59 milliseconds, and the orbital period
is 2.79 × 10 4 sec. General relativistic effects,
including periastron advance, are verified, and
they, along with a general relativistic model of
gravitational radiation from the orbiting neutron
stars (period decrease of −2.4 ×10 −12 ), give excellent
agreement with observations.
binary star Two stars gravitationally bound
together. There are numerous types of binaries
including, among others, the following classes:
Algol, RS Canus Venaticorum, W Ursa Majoris
(see contact binary), β Lyrae, cataclysmic variables.
Some binaries show eclipses as one star
passes in front of the other. In such systems,
the amount of light received as a function of