An Etymological Dictionary of Astronomy and Astrophysics
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فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

M. Heydari-Malayeri    -    Paris Observatory

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Number of Results: 12948 Search : far
Lyman break galaxy
  کهکشان با بُره‌ی ِ لایمن   
kahkešân bâ bore-ye Lyman

Fr.: galaxie de la coupure de Lyman   

A star-forming galaxy at → high redshift affected by the → Lyman break. Such a galaxy is detected in the red (R, → photometric band) but not in the blue (U and B bands). At those high redshfits (above 2.5), the → Lyman limit at 912 Å is shifted between the U and B bands.

Lyman; → break; → galaxy.

Lyman continuum
  پیوستار ِ لایمن   
peyvastâr-e Lyman (#)

Fr.: continuum de Lyman   

A continuous range of wavelengths in the spectrum of hydrogen at wavelengths less than the → Lyman limit. The Lyman continuum results from transitions between the → ground state of hydrogen and → excited states in which the single electron is freed from the atom by photons having an energy of 13.6 eV or higher.

Lyman; → continuum.

Lyman continuum escape
  گریز ِ پیوستار ِ لایمن   
goriz-e peyvastâr-e Lyman

Fr.: échappement du continuum de Lyman   

The process whereby → Lyman continuum photons produced by → massive stars escape from a galaxy without being absorbed by interstellar material. Some observations indicate that the Lyman continuum escape fraction evolves with → redshift.

Lyman; → continuum; → escape.

Lyman ghost
  پرهیب ِ لایمن   
parhib-e Lyman (#)

Fr.: image fantôme de Lyman   

In spectroscopy, a false image of a spectral line formed by irregularities in the ruling of diffraction gratings.

Lyman, → ghost.

Lyman limit
  حد ِ لایمن   
hadd-e Lyman

Fr.: limite de Lyman   

The short-wavelength end of the hydrogen Lyman series, at 912 Å. Also called → Lyman continuum. It corresponds to the energy (13.6 eV) required for an electron in the hydrogen ground state to jump completely out of the atom, leaving the atom ionized.

Lyman; → limit.

Lyman series
  سری ِ لایمن   
seri-ye Lyman (#)

Fr.: séries de Lyman   

A series of lines in the spectrum of hydrogen, emitted when electrons jump from outer orbits to the first orbit. The Lyman series lies entirely within the ultraviolet region. The brightest lines are Lyman-alpha at 1216 Å, Lyman-beta at 1026 Å, and Lyman-gamma at 972 Å.

Lyman; → series.

Lyman-Werner photon
  فوتون ِ لایمن-ورنر   
foton-e Lyman-Werner

Fr.: photon de Lyman-Werner   

An → ultraviolet photon with an energy between 11.2 and 13.6 eV, corresponding to the energy range in which the Lyman and Werner absorption bands of → molecular hydrogen (H2) are found (→ Lyman band, → Werner band). The first generation of stars produces a background of Lyman-Werner radiation which can → photodissociate molecular hydrogen, the key → cooling agent in metal free gas below 104 K. In doing so, the Lyman-Werner radiation field delays the collapse of gaseous clouds, and thus star formation. After more massive → dark matter clouds are assembled, atomic line cooling becomes effective and H2 can begin to shield itself from Lyman-Werner radiation.

Lyman; → Werner band; → photon.

Lynx
  سیاهگوش   
Siyâhguš (#)

Fr.: Lynx   

The Lynx. A faint → constellation in the northern hemisphere that lies between → Auriga to the west and → Ursa Major to the east, at about 8h right ascension, 45° north declination. Abbreviation: Lyn; genitive: Lyncis.

From L. lynx, from Gk. lynx, probably from PIE *leuk-, → light, in reference to its gleaming eyes or its ability to see in the dark (cf. Lith. luzzis, O.H.G. luhs, Ger. Luchs, O.E. lox, Du. los, Swed. lo "lynx").

Siyâhguš "lynx," literally "black ear," from siyâh "black," from Mid.Pers. siyâ, siyâk, siyâvah "black," Av. sâma-, sayâva- "black, dark," cf. Skt. syama-, syava- "black, brown," Gk. skia "shadow" + guš, → ear.

Lyot division
  شکاف ِ لی‌یو   
šekâf-e Liyot (#)

Fr.: division de Lyot   

In Saturn's rings, the gap between rings B and C.

Named after Bernard Lyot (1897-1952), French astronomer who discovered the division. He was also a distinguished solar observer and invented (1930) the → coronagraph; → division.

Lyot filter
  پالایه‌ی ِ لی‌یو   
pâlâye-ye Lyot (#)

Fr.: filtre de Lyot   

A type of narrow-band filter consisting of a series of birefringent crystals and polarizers invented by the French astronomer Bernard Lyot (1897-1952) for isolating and observing significant wavelengths of solar light.

Lyot division; → filter.

Lyra
  چنگ   
Cang (#)

Fr.: Lyre   

The Lyre. A small, bright constellation in the northern hemisphere at about 19h right ascension, 40° declination. The brightest star in Lyra is → Vega.

L. lyra, from Gk. lyra, a foreign word of uncertain origin.

Cang "harp," frpm Mid.Pers. cang "harp."

Lyrids
  چنگیان   
Cangiyân (#)

Fr.: Lyrides   

A meteor shower that occurs between 18 and 24 April. Its radiant is in the constellation → Lyra.

Lyra + → -ids.

Lysithea (Jupiter X)
  لوسیته‌آ   
Lusitea (#)

Fr.: Lysithéa   

The eleventh of Jupiter's known satellites; it is 36 km across and orbits Jupiter at a distance of about 11,720,000 km with a period of 259 days. It was discovered by S. Nicholson in 1938.

Lysithea was a daughter of Oceanus and one of Zeus' lovers.

M dwarf
  کوتوله‌ی ِ M   
kutule-ye M

Fr.: naine M   

A star whose spectrum is dominated by the absorption bands of → titanium oxide (TiO) and → vanadium oxide (VO) and has many neutral metal lines. The → effective temperature of M dwarfs ranges from about 3850 to 2600 K. They are low mass stars with masses ranging from 0.6 times that of the Sun at spectral type M0 to less than 0.1 → solar masses. M dwarfs are very abundant, they account for about 70-80% of stars in the → Galactic disk. The nearest star to the Sun, → Proxima Centauri, is an M dwarf.

M, letter of alphabet in the → Harvard classification; → dwarf.

M star
  ستاره‌ی ِ M   
setâre-ye M (#)

Fr.: étoile de type M   

A cool, red star of spectral type M with a surface temperatures of less than 3600 K. The spectra of M stars are dominated by molecular bands, especially those of TiO. Naked-eye examples are Betelgeuse and Antares.

M, letter of alphabet, → star.

M-type asteroid
  سیارک ِ گونه‌ی ِ M   
sayârak-e gune-ye M

Fr.: astéroïde de type M   

A type of → asteroid that consists of metallic → iron and displays high → albedo (0.10-0.18). M-type asteroids inhabit the middle part of the → asteroid belt.

M for → metallic; → type; → asteroid.

M16
     
M16

Fr.: M16   

An → open cluster in the constellation → Serpens associated with the → Eagle Nebula. It is usually mistakenly stated as being that nebula. Also called NGC 6611. First discovered by Philippe Loys de Chéseaux in 1745-6.

Object bearing the number 16 in the → Messier catalog.

Mach angle
  زاویه‌ی ِ ماخ   
zâviye-ye Mach

Fr.: angle de Mach   

Half of the vertex angle of the → Mach cone generated by a body in → supersonic flight. It is given by sin μ = (a/v), where a is the speed of the sound waves and v the velocity of the object. In terms of Mach number: μ = asin (1/M). The Mach angle diminishes with the object velocity. For M = 1 it is 90°, for M = 2, it is 30°, and for M = 5, its is 11.5°.

Mach number; → angle.

Mach cone
  مخروط ِ ماخ   
maxrut-e Mach

Fr.: cône de Mach   

The cone that confines the pressure disturbance created by a → supersonic object moving in a → compressible medium.

Mach number; → cone.

Mach disk
  گرده‌ی ِ ماخ، دیسک ِ ~   
gerde-ye Mach, disk-e ~

Fr.: disque de Mach   

Same as → shock diamond.

So named because Ernst Mach (1838-1916) was the first to record its existence.

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