An Etymological Dictionary of Astronomy and Astrophysics

A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays
a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.

See also: BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the “star” was identified with a bright, variable → radio source and a very faint galaxy; → object.

A member of a family of → quasars, or extragalactic → Active Galactic Nuclei, which displays
a high radio emission and/or important optical variability over a short period of time. BL Lac objects appear star-like but their spectrum is flat, and partially polarized. Also called → blazars.

See also: BL Lac, from object BL in the constellation → Lacerta (BL Lacertae). The reason for this terminology is that it was originally thought to be an irregular variable star in our Galaxy; hence its variable star designation. In the 1970s the “star” was identified with a bright, variable → radio source and a very faint galaxy; → object.

A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.

Etymology (EN): Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.

A mechanism aimed at explaining the → disruption of a → binary system. As one component loses mass dramatically, the resulting loss of → gravitational attraction changes the orbit of, or ejects completely, the → companion star.

Etymology (EN): Adriaan Blaauw (1914-2010), 1961, Bull. Astron. Inst. Netherlands 15, 265; → mechanism.

Very dark in color;
absorbing light without reflecting any of its various rays.

Etymology (EN): Black, from O.E. blæc “black,” from P.Gmc. *blak-, from PIE *bhelg- “to shine, flash, burn” (cf. Gk. phlegein “to burn, scorch,” L. flagrare “to blaze, glow, burn,” fulgur “lightning”).

Etymology (PE): Siyâh or siyah, from Mid.Pers. siyâ, siyâk, siyâvah “black,” Av. sâma-, sayâva- “black, dark,” cf. Skt. syama-, syava- “black, brown,” Gk. skia
“shadow.”

Very dark in color;
absorbing light without reflecting any of its various rays.

Etymology (EN): Black, from O.E. blæc “black,” from P.Gmc. *blak-, from PIE *bhelg- “to shine, flash, burn” (cf. Gk. phlegein “to burn, scorch,” L. flagrare “to blaze, glow, burn,” fulgur “lightning”).

Etymology (PE): Siyâh or siyah, from Mid.Pers. siyâ, siyâk, siyâvah “black,” Av. sâma-, sayâva- “black, dark,” cf. Skt. syama-, syava- “black, brown,” Gk. skia
“shadow.”

The appearance of a band linking the solar limb to the disk of a transiting planet (Venus or Mercury) near the point of internal tangency. This effect increases the uncertainty in measuring the period from when the planet fully enters the solar disk to when it begins to depart. Historically, the black drop phenomenon limited the accuracy of the determination of the Astronomical Unit and the scale of the Solar System in the 18th and 19th centuries. While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury in 1999, which was observed by the NASA’s TRACE satellite. Two effects could fully explain the black drop: the inherent blurriness of the image caused by the finite size of the telescope (→ point spread function), and an extreme dimming of the Sun’s surface just inside the apparent outer edge (→ limb darkening). See Schneider et al. 2004, Icarus 168, 249.

See also: → black; → drop.

The appearance of a band linking the solar limb to the disk of a transiting planet (Venus or Mercury) near the point of internal tangency. This effect increases the uncertainty in measuring the period from when the planet fully enters the solar disk to when it begins to depart. Historically, the black drop phenomenon limited the accuracy of the determination of the Astronomical Unit and the scale of the Solar System in the 18th and 19th centuries. While there have been many proposed theories over the years, the true cause of the effect was revealed during a transit of Mercury in 1999, which was observed by the NASA’s TRACE satellite. Two effects could fully explain the black drop: the inherent blurriness of the image caused by the finite size of the telescope (→ point spread function), and an extreme dimming of the Sun’s surface just inside the apparent outer edge (→ limb darkening). See Schneider et al. 2004, Icarus 168, 249.

See also: → black; → drop.

A fantastically → compact object, predicted by the theory of → general relativity, whose → gravity is so powerful that not even light can escape from it. A black hole forms when matter → collapses to → infinite  → density, producing a → singularity of infinite → curvature in the fabric of → space-time. Each black hole is surrounded by an → event horizon, at which the → escape velocity is the → speed of light. The → Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: → stellar black hole, → intermediate-mass black hole, → primordial black hole, → mini black hole, → supermassive black hole, → Schwarzschild black hole, → Kerr black hole.

See also: Historically, the Newtonian concept of such a celestial body appeared at the end of the 18th century when light was shown to have particle characteristics. In fact the English geologist John Mitchell (1724-1793) and French mathematician and astronomer Pierre Simon Laplace (1749-1827), independently, suggested that regions of space, where gravitational attraction was so strong that not even light could escape, may exist in the Universe. However, the term black hole was coined in 1967 by the Princeton physicist John A. Wheeler (1911-2008); → black; → hole.

A fantastically → compact object, predicted by the theory of → general relativity, whose → gravity is so powerful that not even light can escape from it. A black hole forms when matter → collapses to → infinite  → density, producing a → singularity of infinite → curvature in the fabric of → space-time. Each black hole is surrounded by an → event horizon, at which the → escape velocity is the → speed of light. The → Schwarzschild radius for the Sun is about 3 km and for the Earth about 1 cm. There is observational evidence for black holes on a remarkable range of scales in the Universe: → stellar black hole, → intermediate-mass black hole, → primordial black hole, → mini black hole, → supermassive black hole, → Schwarzschild black hole, → Kerr black hole.

See also: Historically, the Newtonian concept of such a celestial body appeared at the end of the 18th century when light was shown to have particle characteristics. In fact the English geologist John Mitchell (1724-1793) and French mathematician and astronomer Pierre Simon Laplace (1749-1827), independently, suggested that regions of space, where gravitational attraction was so strong that not even light could escape, may exist in the Universe. However, the term black hole was coined in 1967 by the Princeton physicist John A. Wheeler (1911-2008); → black; → hole.

A → binary system in which each component is a → black hole. The binary’s evolution
can be divided into three stages: → inspiral, → merger, and → ringdown.

See also: → black; → hole; → binary.

A → binary system in which each component is a → black hole. The binary’s evolution
can be divided into three stages: → inspiral, → merger, and → ringdown.

See also: → black; → hole; → binary.

An object that seems likely to be a → black hole, but waits for more observational confirmations.

See also: → black; → hole; → candidate.

An object that seems likely to be a → black hole, but waits for more observational confirmations.

See also: → black; → hole; → candidate.

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).

See also: → black; → hole; → corona.

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).

See also: → 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.

See also: → black; → hole; → merger.

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.

See also: → 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).

See also: → black; → hole; → surface; → gravity.

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).

See also: → 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.

See also: → black; → hole; → shadow.

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.

See also: → 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.

See also: → black; → string.

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.

See also: → 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.

Etymology (EN): 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.

Etymology (PE): Tapâr , → pulsar; siyâh-bivé “black widow,” from siyâh, → black + bivé, akin to E. widow, as explained above.

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.

Etymology (EN): 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.

Etymology (PE): 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.

See also: → black; → body.

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.

See also: → 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).

See also: → blackbody; → curve.

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).

See also: → blackbody; → curve.

The → blackbody surface of the → Universe defined at a → redshift of about z ≥ 2 &times 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.

See also: → blackbody; → atmosphere.

The → blackbody surface of the → Universe defined at a → redshift of about z ≥ 2 &times 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.

See also: → 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.

See also: → blackbody; → radiation.

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.

See also: → 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.

See also: → blackbody; → spectrum.

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.

See also: → 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.

See also: → blackbody; → temperature.

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.

See also: → 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.

Etymology (EN): → black; → out.

Etymology (PE): 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.”

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.

Etymology (EN): → black; → out.

Etymology (PE): 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).

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

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).

See also: 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.

Etymology (EN): From M.E., from O.Fr. blanchet, diminutive of blanc “white; white cloth.”

Etymology (PE): Patu “blanket; a kind of woolen cloth,” Kermâni dialect poto “wollen; woolly;” cf. Skt. patta- “cloth, colored or fine cloth.”

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.

Etymology (EN): From M.E., from O.Fr. blanchet, diminutive of blanc “white; white cloth.”

Etymology (PE): 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.

→ 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.

Etymology (EN): Blazar, a combination of BL Lac and quasar.

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

Etymology (EN): 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.

Etymology (EN): O.E. blæse “a torch, flame,” from P.Gmc. *blason, from PIE *bhel- “to shine.”

Etymology (PE): 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.”

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.

Etymology (EN): O.E. blæse “a torch, flame,” from P.Gmc. *blason, from PIE *bhel- “to shine.”

Etymology (PE): 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.”

The angle between the operating facet of the grooves and the overall plane of a diffraction grating.

Etymology (EN): → blaze; → angle.

Etymology (PE): Angâl, zâviyée, → angle; beliz→ blaze.

The angle between the operating facet of the grooves and the overall plane of a diffraction grating.

Etymology (EN): → blaze; → angle.

Etymology (PE): Angâl, zâviyée, → angle; beliz→ blaze.

The wavelength in a given diffraction order for which the efficiency curve reaches its maximum.

Etymology (EN): → blaze; → wavelength.

Etymology (PE): Mowjtul→ wavelength; beliz→ blaze.

The wavelength in a given diffraction order for which the efficiency curve reaches its maximum.

Etymology (EN): → blaze; → wavelength.

Etymology (PE): Mowjtul→ wavelength; beliz→ blaze.

A → diffraction grating ruled appropriately so that a large proportion of the diffracted light is concentrated into a few, or even a single → order of interference.

Etymology (EN): Blazed, adj. of → blaze; → grating.

Etymology (PE): Turi, noun from tur “a net, a fishing net;” belizi adj. from beliz, → blaze.

A → diffraction grating ruled appropriately so that a large proportion of the diffracted light is concentrated into a few, or even a single → order of interference.

Etymology (EN): Blazed, adj. of → blaze; → grating.

Etymology (PE): Turi, noun from tur “a net, a fishing net;” belizi adj. from beliz, → blaze.

A long term, generally irregular modulation of → light curves of a large subclass of → RR Lyrae stars. Most of the modulations occur on the time scale of some 60 periods, although the range extends from some tens to some hundreds of periods. Since its discovery
over a hundred years ago, a number of explanations have been proposed for this
effect, but its nature is still a matter of investigation. The explanations include: closely spaced pulsation modes, a modal 1 : 2 resonance, an oblique rotator model, a non-radial modal interaction, convective cycles, and nonlinear resonant mode coupling between the 9th overtone and the fundamental mode (see, e.g., R. Buchler and Z. Kolláth 2011, astro-ph/1101.1502).

See also: Named after Sergei N. Blazhko (1870-1956), a Russian astronomer who discovered the effect for the star EW Dra (1907, Astron. Nachr. 175, 325); → effect.

A long term, generally irregular modulation of → light curves of a large subclass of → RR Lyrae stars. Most of the modulations occur on the time scale of some 60 periods, although the range extends from some tens to some hundreds of periods. Since its discovery
over a hundred years ago, a number of explanations have been proposed for this
effect, but its nature is still a matter of investigation. The explanations include: closely spaced pulsation modes, a modal 1 : 2 resonance, an oblique rotator model, a non-radial modal interaction, convective cycles, and nonlinear resonant mode coupling between the 9th overtone and the fundamental mode (see, e.g., R. Buchler and Z. Kolláth 2011, astro-ph/1101.1502).

See also: Named after Sergei N. Blazhko (1870-1956), a Russian astronomer who discovered the effect for the star EW Dra (1907, Astron. Nachr. 175, 325); → effect.

A star showing the → Blazhko effect.

See also: → Blazhko effect; → star.

A star showing the → Blazhko effect.

See also: → Blazhko effect; → star.

The capcity of a diffraction grating, in certain configurations, to concentrate a large percentage of the incident light
into a specific diffraction order.

Etymology (EN): Blazing, noun from → blaze.

Etymology (PE): Belizeš, noun from beliz “blaze.”

The capcity of a diffraction grating, in certain configurations, to concentrate a large percentage of the incident light
into a specific diffraction order.

Etymology (EN): Blazing, noun from → blaze.

Etymology (PE): Belizeš, noun from beliz “blaze.”

General: A flaw or defect.
Detectors: Latent imperfections in the photodiodes (pixels) of a solid-state detector.

Etymology (EN): From O.Fr. blemiss “to turn pale,” extended stem of blemir, blesmir “to injure, make pale.”

Etymology (PE): Âk “defect, blemish,” Mid.Pers. ak, âk “evil, harm,” Av. aka- “bad, wicked;” cf. Skt. aka- “pain , trouble.”

General: A flaw or defect.
Detectors: Latent imperfections in the photodiodes (pixels) of a solid-state detector.

Etymology (EN): From O.Fr. blemiss “to turn pale,” extended stem of blemir, blesmir “to injure, make pale.”

Etymology (PE): Âk “defect, blemish,” Mid.Pers. ak, âk “evil, harm,” Av. aka- “bad, wicked;” cf. Skt. aka- “pain , trouble.”

1a) Description of two or more adjacent → spectral lines which are mixed due to insufficient → resolving power of the → spectrograph.

1b) Linguistics: A word which is coined by extracting and combining arbitrary pieces of two or more existing words. Examples include → smog, motel (motor + hotel), brunch (breakfast + lunch), → pulsar, and → shellular. Same as portmanteau.

2. Of two or more → spectral lines, to become merged into one line as a consequence of insufficient instrumental → resolution.

Etymology (EN): M.E., from O.N. blanda; akin to O.E. blandan “to mix,” Lith. blandus “impure, cloudy.”

Etymology (PE): Tuham, from tu “inside” + ham “together,” → com-.

1a) Description of two or more adjacent → spectral lines which are mixed due to insufficient → resolving power of the → spectrograph.

1b) Linguistics: A word which is coined by extracting and combining arbitrary pieces of two or more existing words. Examples include → smog, motel (motor + hotel), brunch (breakfast + lunch), → pulsar, and → shellular. Same as portmanteau.

2. Of two or more → spectral lines, to become merged into one line as a consequence of insufficient instrumental → resolution.

Etymology (EN): M.E., from O.N. blanda; akin to O.E. blandan “to mix,” Lith. blandus “impure, cloudy.”

Etymology (PE): Tuham, from tu “inside” + ham “together,” → com-.

→ Spectral lines intermingled.

See also: → blend; → line.

→ Spectral lines intermingled.

See also: → blend; → line.

Unable to see; sightless.

Etymology (EN): M.E., from O.E. blind “blind,” akin to Du., Ger. blind, O.N. blindr, Goth. blinds “blind.”

Etymology (PE): Kur “blind,” variants kul “squint-eyed,” kolok, kalek, kelek, kalâž, kâž, kâj, kâc “squint-eyed,” Lori, Laki, Kurd. xêl “cross-eyed, squinting;” Mid.Pers. kôr “blind;” akin to O.Irish coll “one-eyed;” M.Irish goll “blind;”
Gk. (Hes.) kellas “one-eyed;”
Skt. kāná- “blind of one eye;” PIE *kolnos “one-eyed.” The Pers. luc “crossed-eyed” may be related to a separate
group containing L. lusca, luscus “one-eyed” (Fr. louche “squinting”).

Unable to see; sightless.

Etymology (EN): M.E., from O.E. blind “blind,” akin to Du., Ger. blind, O.N. blindr, Goth. blinds “blind.”

Etymology (PE): Kur “blind,” variants kul “squint-eyed,” kolok, kalek, kelek, kalâž, kâž, kâj, kâc “squint-eyed,” Lori, Laki, Kurd. xêl “cross-eyed, squinting;” Mid.Pers. kôr “blind;” akin to O.Irish coll “one-eyed;” M.Irish goll “blind;”
Gk. (Hes.) kellas “one-eyed;”
Skt. kāná- “blind of one eye;” PIE *kolnos “one-eyed.” The Pers. luc “crossed-eyed” may be related to a separate
group containing L. lusca, luscus “one-eyed” (Fr. louche “squinting”).

The state or condition of being sightless.

See also: → blind; → -ness.

The state or condition of being sightless.

See also: → blind; → -ness.

1. To close and open the eyes rapidly.
   

2. To shine intermittently; flash on and off.

Etymology (EN): M.E. blinken, variant of blenken “to blench;” cf. Du. and Ger. blinken.

Etymology (PE): Možidan, from možé “eyelash,” Mid.Pers. mec “eyelash,” mecitan “to blink,” cf. Skt. mes “to open the eyes,” O.C.S. po-mežiti “to close the eyes.” → twinkling.

1. To close and open the eyes rapidly.
   

2. To shine intermittently; flash on and off.

Etymology (EN): M.E. blinken, variant of blenken “to blench;” cf. Du. and Ger. blinken.

Etymology (PE): Možidan, from možé “eyelash,” Mid.Pers. mec “eyelash,” mecitan “to blink,” cf. Skt. mes “to open the eyes,” O.C.S. po-mežiti “to close the eyes.” → twinkling.

An instrument for comparing two photographs of the same stellar field, taken at different times, by quickly alternating from one to the other.
The purpose of the comparison is to detect subtle changes in the position or brightness of the stars.

Etymology (EN): → blink; → comparator.

Etymology (PE): Hamsanjgar, → comparator; možeš noun from možidan, → blink.

An instrument for comparing two photographs of the same stellar field, taken at different times, by quickly alternating from one to the other.
The purpose of the comparison is to detect subtle changes in the position or brightness of the stars.

Etymology (EN): → blink; → comparator.

Etymology (PE): Hamsanjgar, → comparator; možeš noun from možidan, → blink.

An intermittent appearance of a group of characters on the display terminal, usually used to convey a message to the user.

Etymology (EN): Blinking, noun from to blink.

Etymology (PE): Možeš, noun from možidan, → blink (v).

An intermittent appearance of a group of characters on the display terminal, usually used to convey a message to the user.

Etymology (EN): Blinking, noun from to blink.

Etymology (PE): Možeš, noun from možidan, → blink (v).

A small cyst on the skin, containing watery liquid, as from a burn or other injury.

Etymology (EN): M.E. blister, blester, from O.Fr. blestre, of Germanic origin.

Etymology (PE): Tâval “blister” (variants Torbet-Heydariye-yi toval, Guqari tavol), from suffixed (-al) tâv- tav, taf- “to heat, burn, shine,” variant of tâb-, tâbidan “to shine,” → luminous.

A small cyst on the skin, containing watery liquid, as from a burn or other injury.

Etymology (EN): M.E. blister, blester, from O.Fr. blestre, of Germanic origin.

Etymology (PE): Tâval “blister” (variants Torbet-Heydariye-yi toval, Guqari tavol), from suffixed (-al) tâv- tav, taf- “to heat, burn, shine,” variant of tâb-, tâbidan “to shine,” → luminous.

A model according to which an → H II region is a hot mass of ionized gas located on the surface of a → molecular cloud, like a blister on the body skin.

See also: → blister; → model.

A model according to which an → H II region is a hot mass of ionized gas located on the surface of a → molecular cloud, like a blister on the body skin.

See also: → blister; → model.

A new type of astronomical object, appearing as an intense → burst of → radio emission, proposed to explain → fast radio bursts. In some models, blitzars result from the sudden → collapse of a hypothetical → supermassive neutron star.

See also: From Ger. Blitz, “→ flash, lightening,”

• -âr ending component, as in → pulsar.

A new type of astronomical object, appearing as an intense → burst of → radio emission, proposed to explain → fast radio bursts. In some models, blitzars result from the sudden → collapse of a hypothetical → supermassive neutron star.

See also: From Ger. Blitz, “→ flash, lightening,”

• -âr ending component, as in → pulsar.

A severe weather condition characterized by high winds (at least 55 km/h) and reduced visibility due to violent snowstorm.

Etymology (EN): Blizzard, of unknown origin.

Etymology (PE): Damé “wind and snow storm.”

A severe weather condition characterized by high winds (at least 55 km/h) and reduced visibility due to violent snowstorm.

Etymology (EN): Blizzard, of unknown origin.

Etymology (PE): Damé “wind and snow storm.”

1. General: Drop of liquid; small round mass (e.g. wax); spot of color.
   

2. Astro.: A relatively → small, → unresolved source lying toward a much larger structure. → high-excitation blob.

Etymology (EN): From M.E. bubelen “to bubble.”

Etymology (PE): Žig “drop,” probably from žohidan “to drop,” variant of cakidan “to drop.”

1. General: Drop of liquid; small round mass (e.g. wax); spot of color.
   

2. Astro.: A relatively → small, → unresolved source lying toward a much larger structure. → high-excitation blob.

Etymology (EN): From M.E. bubelen “to bubble.”

Etymology (PE): Žig “drop,” probably from žohidan “to drop,” variant of cakidan “to drop.”

The red liquid that circulates in the arteries and veins of humans and other vertebrate animals, carrying oxygen to and carbon dioxide from the tissues of the body (OxfordDictionaries.com).

Etymology (EN): M.E. blo(o)d, O.E. blôd; akin to O.Frisian, O.Saxon blôd, O.H.G. bluot (Ger. Blut), Gothic bloth.

Etymology (PE): Xun, from Mid.Pers. xûn; cf. Sogd. xurn, Khotanese hûna, Yaghnobi waxin, Av. vohunī,

The red liquid that circulates in the arteries and veins of humans and other vertebrate animals, carrying oxygen to and carbon dioxide from the tissues of the body (OxfordDictionaries.com).

Etymology (EN): M.E. blo(o)d, O.E. blôd; akin to O.Frisian, O.Saxon blôd, O.H.G. bluot (Ger. Blut), Gothic bloth.

Etymology (PE): Xun, from Mid.Pers. xûn; cf. Sogd. xurn, Khotanese hûna, Yaghnobi waxin, Av. vohunī,

In a → CCD detector, the spill of charge to adjacent → pixels due to over-illumination by a too bright source. Same as charge bleeding.

Etymology (EN): Blooming “glare,” from to bloom “to glare, glow.”

Etymology (PE): Sar-riz “spill-out, overflowing,” from sar “top,” → head, + riz “pouring,” from rixtan “to pour,” → overflow.

In a → CCD detector, the spill of charge to adjacent → pixels due to over-illumination by a too bright source. Same as charge bleeding.

Etymology (EN): Blooming “glare,” from to bloom “to glare, glow.”

Etymology (PE): Sar-riz “spill-out, overflowing,” from sar “top,” → head, + riz “pouring,” from rixtan “to pour,” → overflow.

1. To move along, carried by or as by the wind.
   

2. To produce or emit a current of air, as with the mouth or a bellows.
   

3. Of a horn, trumpet, etc.) to give out sound.
   

4. To make a blowing sound; whistle (Dictionary.com).

Etymology (EN): M.E., from O.E. blawan “blow, breathe, make an air current; kindle; inflate; sound a wind instrument;” cf. O.H.G. blaen, Ger. blähen; from PIE *bhle- “to swell, blow up.”

Etymology (PE): Damidan, from Mid.Pers. damidan “to blow, breathe;” dam “breath, breath of an owen; bellows; smoke; air,” also “moment, time;” Av. dāδmainya- “blowing up;” cf.
Skt. dahm- “to blow,” dhámati “blows;” Gk. themeros “austere, dark-looking;” Lith. dumti “to blow;” PIE dhem-/dhemə- “to smoke, to blow.”

1. To move along, carried by or as by the wind.
   

2. To produce or emit a current of air, as with the mouth or a bellows.
   

3. Of a horn, trumpet, etc.) to give out sound.
   

4. To make a blowing sound; whistle (Dictionary.com).

Etymology (EN): M.E., from O.E. blawan “blow, breathe, make an air current; kindle; inflate; sound a wind instrument;” cf. O.H.G. blaen, Ger. blähen; from PIE *bhle- “to swell, blow up.”

Etymology (PE): Damidan, from Mid.Pers. damidan “to blow, breathe;” dam “breath, breath of an owen; bellows; smoke; air,” also “moment, time;” Av. dāδmainya- “blowing up;” cf.
Skt. dahm- “to blow,” dhámati “blows;” Gk. themeros “austere, dark-looking;” Lith. dumti “to blow;” PIE dhem-/dhemə- “to smoke, to blow.”

The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers.

Etymology (EN): From O.Fr. bleu, P.Gmc. *blæwaz, from PIE base *bhle-was “light-colored, blue, blond, yellow.”

Etymology (PE): Âbi “color of water,” from âb “water,” Mid.Pers. âb, O.Pers./Av. âp-, Skt. âp-, PIE *âp-; → Aquarius.

The hue of that portion of the visible spectrum lying between green and indigo, evoked in the human observer by radiant energy with wavelengths of approximately 420 to 490 nanometers.

Etymology (EN): From O.Fr. bleu, P.Gmc. *blæwaz, from PIE base *bhle-was “light-colored, blue, blond, yellow.”

Etymology (PE): Âbi “color of water,” from âb “water,” Mid.Pers. âb, O.Pers./Av. âp-, Skt. âp-, PIE *âp-; → Aquarius.

An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars
which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.

See also: → blue; → compact; → dwarf; → galaxy.

An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars
which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.

See also: → blue; → compact; → dwarf; → galaxy.

The → continuum emission of an astronomical source with wavelengths between about 492 and 455 nm.

See also: → blue; → continuum.

The → continuum emission of an astronomical source with wavelengths between about 492 and 455 nm.

See also: → blue; → continuum.

A giant star with spectral type O or B.

Etymology (EN): → blue; → giant.

Etymology (PE): Qul, → giant; âbi, → blue.

A giant star with spectral type O or B.

Etymology (EN): → blue; → giant.

Etymology (PE): Qul, → giant; âbi, → blue.

A star belonging to a variety of stars located above the → horizontal branch and blueward of the → red giant branch in the → Hertzsprung-Russell diagram of the → halo population.

Etymology (EN): → blue; → halo; → star.

Etymology (PE): Setâregân plural of setâré, → star, âbi, → blue, hâlé, → halo.

A star belonging to a variety of stars located above the → horizontal branch and blueward of the → red giant branch in the → Hertzsprung-Russell diagram of the → halo population.

Etymology (EN): → blue; → halo; → star.

Etymology (PE): Setâregân plural of setâré, → star, âbi, → blue, hâlé, → halo.

Same as → blue horizontal branch star.

See also: → blue; → horizontal; → branch; → star.

Same as → blue horizontal branch star.

See also: → blue; → horizontal; → branch; → star.

A rare class of → horizontal branch (HB)
stars that so far have been found in only very few Galactic → globular clusters.

These stars are such called because they form a blue hook at the hot end of the HB in → far ultraviolet (FUV) → color-magnitude diagrams. The physical mechanism that produces blue hook populations is still uncertain. At least two scenarios have been proposed.

In the first scenario these stars are explained as a consequence of extreme → mass loss during the → red giant branch phase and late helium flashing while descending the → white dwarf cooling track. Due to the thin residual hydrogen envelope, helium is mixed into the envelope and hydrogen is mixed into the core during the late → helium flash. As a result, the stars are hotter and UV-fainter than canonical → extreme horizontal branch stars (EHB).

By contrast, in the He self-enrichment scenario the EHB and blue hook stars are produced via the normal evolution of He-enriched sub-populations in globular clusters.

These sub-populations might have formed from the ejecta of intermediate-mass → asymptotic giant branch (AGB) stars of the first generation of stars. For the same age and → metallicity, He-enriched HB stars have smaller masses than normal HB stars, resulting in bluer → zero age horizontal branch star (ZAHB) locations. They are also brighter in the FUV, but this effect is reversed for very hot He-enriched HB stars with → effective temperatures larger than 19000 K.
See Dieball A., et al., 2013, arXiv:0901.1309v1, and for blue hook stars in ω Cen cluster, M. Tailo et al., 2015, Nature 523, 318.

See also: → blue; → hook; → star.

A rare class of → horizontal branch (HB)
stars that so far have been found in only very few Galactic → globular clusters.

These stars are such called because they form a blue hook at the hot end of the HB in → far ultraviolet (FUV) → color-magnitude diagrams. The physical mechanism that produces blue hook populations is still uncertain. At least two scenarios have been proposed.

In the first scenario these stars are explained as a consequence of extreme → mass loss during the → red giant branch phase and late helium flashing while descending the → white dwarf cooling track. Due to the thin residual hydrogen envelope, helium is mixed into the envelope and hydrogen is mixed into the core during the late → helium flash. As a result, the stars are hotter and UV-fainter than canonical → extreme horizontal branch stars (EHB).

By contrast, in the He self-enrichment scenario the EHB and blue hook stars are produced via the normal evolution of He-enriched sub-populations in globular clusters.

These sub-populations might have formed from the ejecta of intermediate-mass → asymptotic giant branch (AGB) stars of the first generation of stars. For the same age and → metallicity, He-enriched HB stars have smaller masses than normal HB stars, resulting in bluer → zero age horizontal branch star (ZAHB) locations. They are also brighter in the FUV, but this effect is reversed for very hot He-enriched HB stars with → effective temperatures larger than 19000 K.
See Dieball A., et al., 2013, arXiv:0901.1309v1, and for blue hook stars in ω Cen cluster, M. Tailo et al., 2015, Nature 523, 318.

See also: → blue; → hook; → star.

A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0,
they have evolved past the → red giant stage and are burning helium in their core.

See also: → blue; → horizontal; → branch, → star.

A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0,
they have evolved past the → red giant stage and are burning helium in their core.

See also: → blue; → horizontal; → branch, → star.

A transient optical phenomenon in the → stratosphere that emerges from the tops of → thunderstorm clouds at tremendous speeds. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence, they typically propagate upward in narrow cones at vertical speeds of roughly 100 km/s, fanning out and disappearing at heights of about 40-50 km. See also → sprite; → elve.

See also: → blue; → jet.

A transient optical phenomenon in the → stratosphere that emerges from the tops of → thunderstorm clouds at tremendous speeds. As their name implies, blue jets are optical ejections from the top of the electrically active core regions of thunderstorms. Following their emergence, they typically propagate upward in narrow cones at vertical speeds of roughly 100 km/s, fanning out and disappearing at heights of about 40-50 km. See also → sprite; → elve.

See also: → blue; → jet.

Leakage phenomenon in a filter, causing an unwanted response to the blue or green light.

Etymology (EN): → blue; leak, from M.Du. leken “to drip, to leak,” or from O.N. leka, cognate of O.E. leccan “to moisten,” from P.Gmc. *lek- “deficiency” (cf. O.H.G. lecchen “to become dry,” Ger. lechzen “to be parched with thirst”).

Etymology (PE): Našt “leak,” origin unknown; âbi, → blue.

Leakage phenomenon in a filter, causing an unwanted response to the blue or green light.

Etymology (EN): → blue; leak, from M.Du. leken “to drip, to leak,” or from O.N. leka, cognate of O.E. leccan “to moisten,” from P.Gmc. *lek- “deficiency” (cf. O.H.G. lecchen “to become dry,” Ger. lechzen “to be parched with thirst”).

Etymology (PE): Našt “leak,” origin unknown; âbi, → blue.

An evolutionary behavior of certain stars, particularly massive stars, which return to the blue stage after becoming a red supergiant. The phenomenon appears as a blueward loop on the theoretical evolutionary tracks.

Etymology (EN): → blue; → loop.

Etymology (PE): Gerdâl, → loop, âbi, → blue.

An evolutionary behavior of certain stars, particularly massive stars, which return to the blue stage after becoming a red supergiant. The phenomenon appears as a blueward loop on the theoretical evolutionary tracks.

Etymology (EN): → blue; → loop.

Etymology (PE): Gerdâl, → loop, âbi, → blue.

The second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average.

See also: The folkloric term blue Moon for the calendrical event is new, and apparently goes back to the Maine Farmers’ Almanac for 1937. But its original meaning in that work was the third full Moon in a season when there were four full Moons in that season. Some have related the term to the much older English expression moon is blue, which goes back to a couplet from 1528, interpreting it as “something that occurs rarely.”
However in that poem the expression had a meaning of “something that was absurd.” Alternatively, the term blue Moon may have been borrowed from the Chinese lunar calendar particularly in its usage among American Chinese community. In fact in that calendar when there are two full Moons in a month they use the term “blue Moon” and add a thirteenth intercalary month. → blue; → moon.

The second full moon in a calendar month. For a blue moon to occur, the first of the full moons must appear at or near the beginning of the month so that the second will fall within the same month. Full moons are separated by 29 days, while most months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two and a half years, on average.

See also: The folkloric term blue Moon for the calendrical event is new, and apparently goes back to the Maine Farmers’ Almanac for 1937. But its original meaning in that work was the third full Moon in a season when there were four full Moons in that season. Some have related the term to the much older English expression moon is blue, which goes back to a couplet from 1528, interpreting it as “something that occurs rarely.”
However in that poem the expression had a meaning of “something that was absurd.” Alternatively, the term blue Moon may have been borrowed from the Chinese lunar calendar particularly in its usage among American Chinese community. In fact in that calendar when there are two full Moons in a month they use the term “blue Moon” and add a thirteenth intercalary month. → blue; → moon.

The portion of the → visible spectrum lying between 455 and 492 nm.

See also: → blue; → region.

The portion of the → visible spectrum lying between 455 and 492 nm.

See also: → blue; → region.

A phenomenon which results from → Rayleigh scattering of sunlight by → atmospheric molecules. → Nitrogen and → oxygen molecules that compose about 78% and 21% of the air, respectively, are small compared to the light → wavelengths, and thus more effective at scattering shorter wavelengths of light (blue and violet). The → selective scattering by these → molecules is responsible for producing the blue skies on a clear sunny day. The sky over the horizon appears much paler in color, because the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Hence, less blue light reaches the observer’s eyes.

See also: → blue; → sky.

A phenomenon which results from → Rayleigh scattering of sunlight by → atmospheric molecules. → Nitrogen and → oxygen molecules that compose about 78% and 21% of the air, respectively, are small compared to the light → wavelengths, and thus more effective at scattering shorter wavelengths of light (blue and violet). The → selective scattering by these → molecules is responsible for producing the blue skies on a clear sunny day. The sky over the horizon appears much paler in color, because the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Hence, less blue light reaches the observer’s eyes.

See also: → blue; → sky.

Any of stars, often found in → globular clusters and old → open clusters, that lie on the blueward extension of the → main sequence beyond the → turnoff point. Blue stragglers have an anomalously blue color and high luminosity in comparison with other cluster members. The most probable ways in which they could form are: → mass transfer or → coalescence in → close binary systems, encounters or collisions in overcrowded cores of globular clusters.

See also: → blue; → straggler.

Any of stars, often found in → globular clusters and old → open clusters, that lie on the blueward extension of the → main sequence beyond the → turnoff point. Blue stragglers have an anomalously blue color and high luminosity in comparison with other cluster members. The most probable ways in which they could form are: → mass transfer or → coalescence in → close binary systems, encounters or collisions in overcrowded cores of globular clusters.

See also: → blue; → straggler.

An evolved star of spectral type O, B, or A; e.g. → Rigel, → Deneb.

See also: → blue; → supergiant.

An evolved star of spectral type O, B, or A; e.g. → Rigel, → Deneb.

See also: → blue; → supergiant.

The → line wing with wavelengths shorter than that of the emission or absorption peak.

See also: → blue; → wing.

The → line wing with wavelengths shorter than that of the emission or absorption peak.

See also: → blue; → wing.

The edible, usually blueish berry of various shrubs belonging to the genus Vaccinium, of the heath family (TheFreeDictionary).

See also: → blue; → berry.

The edible, usually blueish berry of various shrubs belonging to the genus Vaccinium, of the heath family (TheFreeDictionary).

See also: → blue; → berry.

A galaxy having a very small size (< 1 kpc), very low stellar mass
(typically 10^6.5 to 10^7.5  Msun),
very low gas → metallicity (3 to 10% solar → metallicity, and very high → ionization.

Blueberry galaxies, compared to star forming
→ <i><a class="linkVoir" href="/terms/dwarf-galax/">dwarf galax</a></i>ies, 
have similar stellar mass and luminosity,
  but much stronger → <i><a class="linkVoir" href="/terms/o-iii-doublet/">[O III] doublet</a></i>
 (λλ4959+5007) line   
  strength and gas ionization. Because Blueberry galaxies
  are selected by the strong [O III] → <i><a class="linkVoir" href="/terms/emission-line/">emission line</a></i>s, 
  they represent
  the star-forming → <i><a class="linkVoir" href="/terms/dwarf-galax/">dwarf galax</a></i>ies 
  with the highest

emission line strength and gas ionization.

  On the other hand, compared to → <i><a class="linkVoir" href="/terms/green-pea-galax/">Green Pea galax</a></i>ies 
 at → <i><a class="linkVoir" href="/terms/redshift/">redshift</a></i>s 
  <i>z</i> ~ 0.2-0.3 and typical high-<i>z</i>→ <i><a class="linkVoir" href="/terms/lyman-alpha-emitting/">Lyman alpha emitting</a></i> galaxies (LAEs)  
  found in the current

narrow-band surveys, Blueberry galaxies have similarly strong emission lines but about 10-100 times smaller stellar mass, → star formation rate, and luminosity. So Blueberry galaxies represent the faint-end of → Green Pea galaxies and → Lyman alpha emitting galaxies (Yang et al, 2017, arxiv/1706.02819, and references therein).

See also: → blueberry; → galaxy.

A galaxy having a very small size (< 1 kpc), very low stellar mass
(typically 10^6.5 to 10^7.5  Msun),
very low gas → metallicity (3 to 10% solar → metallicity, and very high → ionization.

Blueberry galaxies, compared to star forming
→ <i><a class="linkVoir" href="/terms/dwarf-galax/">dwarf galax</a></i>ies, 
have similar stellar mass and luminosity,
  but much stronger → <i><a class="linkVoir" href="/terms/o-iii-doublet/">[O III] doublet</a></i>
 (λλ4959+5007) line   
  strength and gas ionization. Because Blueberry galaxies
  are selected by the strong [O III] → <i><a class="linkVoir" href="/terms/emission-line/">emission line</a></i>s, 
  they represent
  the star-forming → <i><a class="linkVoir" href="/terms/dwarf-galax/">dwarf galax</a></i>ies 
  with the highest

emission line strength and gas ionization.

  On the other hand, compared to → <i><a class="linkVoir" href="/terms/green-pea-galax/">Green Pea galax</a></i>ies 
 at → <i><a class="linkVoir" href="/terms/redshift/">redshift</a></i>s 
  <i>z</i> ~ 0.2-0.3 and typical high-<i>z</i>→ <i><a class="linkVoir" href="/terms/lyman-alpha-emitting/">Lyman alpha emitting</a></i> galaxies (LAEs)  
  found in the current

narrow-band surveys, Blueberry galaxies have similarly strong emission lines but about 10-100 times smaller stellar mass, → star formation rate, and luminosity. So Blueberry galaxies represent the faint-end of → Green Pea galaxies and → Lyman alpha emitting galaxies (Yang et al, 2017, arxiv/1706.02819, and references therein).

See also: → blueberry; → galaxy.

The apparent shift of the wavelength towards the shorter wavelength region of the radiation spectrum of an approaching object due to the Doppler effect.

See also: → blue; → shift.

The apparent shift of the wavelength towards the shorter wavelength region of the radiation spectrum of an approaching object due to the Doppler effect.

See also: → blue; → shift.

A constituent of a composite astronomical object which has a motion directed towards the observer, as revealed by its spectrum.

See also: → blueshift; → component.

A constituent of a composite astronomical object which has a motion directed towards the observer, as revealed by its spectrum.

See also: → blueshift; → component.

1. (v.tr.) To make indistinct and hazy in outline or appearance.
   
2. (v.intr.) To become indistinct.

Etymology (EN): Probably akin to M.E. bleren “to blear.”

Etymology (PE): Târ “dark, obscure, cloudy” Mid.Pers. târ, from Mid./Mod.Pers. târ “dark, obscure, cloudy.”

1. (v.tr.) To make indistinct and hazy in outline or appearance.
   
2. (v.intr.) To become indistinct.

Etymology (EN): Probably akin to M.E. bleren “to blear.”

Etymology (PE): Târ “dark, obscure, cloudy” Mid.Pers. târ, from Mid./Mod.Pers. târ “dark, obscure, cloudy.”

An image which is dim, indistinct, or vague in appearance, for instance when the optics is not well-focused or when the seeing is poor. The same as → unsharp image, contrary to
→ sharp image.

Etymology (EN): Blurred, p.p. of → blur;
→ image.

An image which is dim, indistinct, or vague in appearance, for instance when the optics is not well-focused or when the seeing is poor. The same as → unsharp image, contrary to
→ sharp image.

Etymology (EN): Blurred, p.p. of → blur;
→ image.

In → galactic dynamics models, the
→ scattering of stars
at radii substantially away from → corotation resonance, especially at the → Lindblad resonances, leading to a higher → eccentricity. The → spiral wave response of a → galactic disk to a co-orbiting mass → clump blurs the distinction between scattering by → spiral arms and by mass clumps. See also → churning
(J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).

See also: Verbal noun of → blur.

In → galactic dynamics models, the
→ scattering of stars
at radii substantially away from → corotation resonance, especially at the → Lindblad resonances, leading to a higher → eccentricity. The → spiral wave response of a → galactic disk to a co-orbiting mass → clump blurs the distinction between scattering by → spiral arms and by mass clumps. See also → churning
(J. A. Sellwood & J. J. Binney, 2002, astro-ph/0203510 and references therein).

See also: Verbal noun of → blur.