An Etymological Dictionary of Astronomy and Astrophysics
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A spherical volume of hot plasma over a broader → accretion disk around a → black hole. The observation of energetic X-ray emission from black holes, which is inconsistent with → thermal emission from an accretion disk, is attributed to the presence of a putative hot corona. It has been widely postulated that the → hard X-rays are the product of → inverse Compton scattering of seed photons from accretion disks by hot ccoronae (See, e.g., F.L. Vieyro et al., 2010, arXiv:1005.5398 and R. C. Reis & J. M. Miller, 2013, arXiv:1304.4947).

→ black; → hole; → corona.

The collision of two → black holes in a → binary black hole system once they come so close that they cannot escape each other's gravity. They will merge in an extremely violent event to become one more massive black hole. The merger would produce tremendous energy and send massive ripples, called → gravitational waves, through the → space-time fabric of the Universe. Such an event (called GW150914) was first detected by the → Laser Interferometer Gravitational-Wave Observatory (LIGO) on September 14, 2015. The initial black hole masses were 36 and 29 Msun which gave a final black hole mass of 62 Msun, with 3 Msun radiated in gravitational waves. The event happened at a distance of 1.3 billion → light-years from Earth (Abbott et al., 2016, Phys. Rev. Lett. 116, 061102). Black hole merger is preceded by → inspiral and followed by → ringdown.

→ black; → hole; → merger.

The acceleration of gravity at the → event horizon of a → black hole. For a → Schwarzschild back hole it is given by κ = GM/R_Sch² = c⁴/(4GM).

→ black; → hole; → surface; → gravity.

A gravitationally lensed image of a → black hole as seen by a distant observer if the black hole is in front of a bright background. According to → general relativity, photons circling the black hole slightly inside the boundary of the → photon sphere will fall down into the → event horizon, while photons circling just outside will escape to infinity. The shadow appears therefore as a rather sharp boundary between bright and dark regions and arises from a deficit of those photons that are captured by the event horizon. Because of this, the diameter of the shadow does not depend on the photons energy, but uniquely on the → angular momentum of the black hole. In a pioneering study, Bardeen (1973) calculated the shape of a dark area of a → Kerr black hole, that is, its "shadow" over a bright background appearing, for instance, in the image of a bright star behind the black hole.

→ black; → hole; → shadow.

The extension of the → black hole concept in a → space-time with → dimensions higher than 4. Theoretically, it is possible to extend the 4D black hole with S² horizon into the fifth dimension producing a hypercylindrical black hole S²× R. Black strings are unstable; it is not yet well understood whether they end up as black holes or different objects.

→ black; → string.

A class of binary millisecond pulsars in which the pulsar is eclipsed by its stellar companion, and the companion is being gradually ablated by the relativistic wind of the pulsar. The first system discovered in 1988 was PSR 1957+20, a 1.6074 millisecond in a near circular 9 hr orbit around a low-mass companion star.

Black widow, a venomous spider (Latrodectus mactans), shiny, coal black in color, that lives in North and South America. The female averages 8-10 mm in length and has long slender legs and a round abdomen. → black; widow, from O..E. widewe, widuwe, from P.Gmc. *widewo (cf. Du. weduwe, weeuw, Ger. Witwe), from PIE *widhewo (cf. Av. viδavâ-, Mid.Pers. wêwag, Mod.Pers. bivé, Skt. vidhava-, L. vidua, Rus. vdova,); → pulsar.

Tapâr , → pulsar; siyâh-bivé "black widow," from siyâh, → black + bivé, akin to E. widow, as explained above.

A theoretical object that is simultaneously a perfect → absorber (it does not reflect any radiation) and a perfect → emitter of → radiation in all → wavelengths and whose radiation is governed solely by its → temperature. Blackbody radiation cannot be explained by → classical physics. The study of its characteristics has, therefore, played an important role in the development of → quantum mechanics. A blackbody can be realized in the form of a cavity with highly absorbing internal walls and a small aperture. Any ray entering through the aperture can leave the cavity only after repeated reflection from the walls. When the aperture is sufficiently small, therefore, the cavity will absorb practically all the radiation incident on the aperture, and so the surface of the aperture will be a black body. The light within the cavity will always interact and exchange energy with the material particles of the walls and any other material particles present. This interaction will eventually → thermalize the radiation within the cavity, producing a → blackbody spectrum, represented by a → blackbody curve.
See also → blackbody photosphere; → blackbody radiation; → Planck's blackbody formula; → Planck's radiation law; → Rayleigh-Jeans law; → Stefan-Boltzmann law; → thermalization; → Wien's displacement law.

→ black; → body.

The characteristic way in which the → intensity of → radiation emitted by a → blackbody varies with its → frequency (or → wavelength), as described by → Planck's radiation law. Also referred to as the → Planck curve. The exact form of the curve depends only on the object's → temperature. The wavelength at which the emitted intensity is highest is an indication of the temperature of the radiating object. As the temperature of the blackbody increases, the peak wavelength decreases (→ Wien's displacement law) and the total energy being radiated (the area under the curve) increases rapidly (→ Stefan-Boltzmann law).

→ blackbody; → curve.

The → blackbody surface of the → Universe defined at a → redshift of about z ≥ 2 × 10⁶. This is distinct from the → last scattering surface, in other words the → cosmic microwave background radiation (CMBR), which refers to z = 1100. Prior to the epoch of the blackbody photosphere the distortions from the → Big Bang are exponentially suppressed.

→ blackbody; → atmosphere.

The radiation emitted by a blackbody at a given → temperature. The → distribution of radiation with → wavelength is given by → Planck's blackbody formula or → Planck's radiation law.

→ blackbody; → radiation.

A curve displaying → blackbody radiation intensity versus the wavelength for a given temperature, according to → Planck's blackbody formula. It is an asymmetrical curve with a sharp rise on the short wavelength side and a much more gradually sloping long-wavelength tale. Same as → Planck spectrum.

→ blackbody; → spectrum.

The temperature at which a blackbody would emit the same radiation per unit area as that emitted by a given body at a given temperature.

→ blackbody; → temperature.

1) A period of darkness caused by a complete loss of electrical power in a particular area.
2) The extinguishing of all artificial light enforced as a precaution against air raids.

→ black; → out.

Xâmušzâr, târikzâr from xâmuš "extinguished," → extinction, târik, → dark, + -zâr suffix denoting profusion and abundance, sometimes with negative nuance, such as in šurezâr "unfertile, salty ground; nitrous earth," xoškzâr "arid land far from water," lajanzâr "field of black mud, marsh," kârzâr "a field of battle; conflict; engagement."

A mechanism for the extraction of energy from a rotating → Kerr black hole. It relies on the assumption that the material → accreted by a → black hole would probably be → magnetized and increasingly so as the material gets closer to the → event horizon. Since all black holes of current astrophysical interest are probably accreting from magnetized disks, this has led to suggestions that the Blandford-Znajek process plays a vital role in → active galactic nuclei (AGN) and other accreting black hole systems. The power, P, generated is given by: P = (4π/μ₀) B²R_S²c, where B is the → magnetic field of the → accretion disk, and R_S is the → Schwarzschild radius of the black hole. As an example, for a 10⁸ solar mass black hole with a 1 T magnetic field, the power generated is approximately 2.7 × 10³⁸ W. In perspective, the annual energy consumption of the world is estimated around 5 × 10²⁰ J. The example case presented produces more energy in a single second than the entire globe consumes in a year. While this is a bold claim to make, it is only an example case where not all the energy produced is extractable as useable energy. However, at that point, even a system which is less that < 10-15 % efficient would be sufficient to supply enough energy to power the world for a full year. Of course, the system itself is limited in its lifetime due to the extraction of energy by slowing down the rotation of the black hole. Hence, the system can only exist as long as the black hole has angular momentum, continuing to rotate. At some point, the rotation will cease and the energy source will be unusable (D. Nagasawa, PH240, Stanford University, Fall 2011).

Blandford, R. D., & Znajek, R. L., 1977, MNRAS 179, 433; → process.

1) A large piece of thick cloth for use as a bed covering, animal covering, etc, enabling a person or animal to retain natural body heat.
2) Any extended covering or layer (Dictionary.com).
→ blanketed model, → blanketing, → blanketing effect, → line blanketing, → line-blanketed model, → unblanketed model, → wind blanketing.

From M.E., from O.Fr. blanchet, diminutive of blanc "white; white cloth."

Patu "blanket; a kind of woolen cloth," Kermâni dialect poto "wollen; woolly;" cf. Skt. patta- "cloth, colored or fine cloth."

→ line blanketing.

→ line blanketing.

→ line blanketing.

A term specifying → BL Lac objects or → quasars when the → continuum radiation emitted from the active nucleus is highly polarized and very variable.

Blazar, a combination of BL Lac and quasar.

1) General: A bright → burst of fire, a flame; a bright or steady light or glare.
2) Optics: The concentration of a limited region of the spectrum into any → order other than the zero order.

O.E. blæse "a torch, flame," from P.Gmc. *blason, from PIE *bhel- "to shine."

Beliz, from Lori beleyz "flame, blaze," Kordi belise "flame, blaze," Mid.Pers. brâh, Av. braz- "to shine, gleam, flash, radiate," cf. Skt. bhâ- "to shine," bhrajate "shines, glitters," O.H.G. beraht "bright," O.E. beorht "bright;" PIE *bhereg- "to shine."