An Etymological Dictionary of Astronomy and Astrophysics

Theodore Lyman (1874-1954), an American physicist who was a pioneer in studying the spectroscopy of the → extreme ultraviolet region of the electromagnetic radiation.
→ Lyman alpha blob, → Lyman alpha emitting galaxy (LAEs), → Lyman alpha forest, → Lyman alpha line, → Lyman alpha nebula , → Lyman band, → Lyman break, → Lyman break galaxy, → Lyman continuum, → Lyman continuum escape, → Lyman limit, → Lyman ghost, → Lyman series, → Lyman-Werner photon.

See also: Named for Th. Lyman, as above.

A gigantic cloud of → hydrogen hydrogen gas emitting the → Lyman alpha line identified in → high redshift, → narrow band → surveys. LABs can span hundreds of thousands of → light-years that is larger than galaxies. Normally, Lyman alpha emission is in the ultraviolet part of the spectrum, but Lyman alpha blobs are so distant, their light is redshifted to (longer) optical wavelengths. The most important questions in LAB studies remain unanswered: how are they formed and what maintains their power?
One of the largest LABs known is SSA22-LAB-01 (z = 3.1). Embedded in the core of a huge → cluster of galaxies in the early stages of formation, it was the very first such object to be discovered (in 2000) and is located so far away that its light has taken about 11.5 billion years to reach us. Recent observations of SSA22-LAB-01 using → ALMA shows two galaxies at the core of this object and they are undergoing a burst of → star formation that is lighting up their surroundings. These large galaxies are in turn at the centre of a swarm of smaller ones in what appears to be an early phase in the formation of a massive cluster of galaxies (see J. E. Geach et al. 2016, arXiv:1608.02941).

See also: → Lyman; → alpha; → blob.

A galaxy belonging to an important population of low mass → star-forming galaxies at → redshift z > 2. Their number increases with redshift. A large fraction of the → dwarf starburst galaxies during the → reionization epoch may be intrinsic LAEs, but their Lyα photons can be scattered by the → neutral hydrogen (H I) in the → intergalactic medium (IGM), which makes Lyα line a powerful probe of reionization. These high-z LAEs have low → metallicity, low stellar masses, low dust → extinction, and compact sizes.
The current best nearby analogs of high-z LAEs are → Green Pea galaxies (Yang et al, 2017, arxiv/1706.02819 and references therein).

See also: → Lyman alpha line; → emit; → -ing; → galaxy.

The appearance of many differentially → redshifted→ Lyman alpha lines in → absorption in a → quasar’s → spectrum, caused by intervening → hydrogen clouds along our → line of sight to the quasar.

See also: → Lyman; → forest.

The spectral line in the → Lyman series which is associated with the → atomic transition between → energy levels n = 2 and n = 1. The corresponding wavelength is 1216 Å in the → far ultraviolet.

See also: → Lyman; → line.

A huge gaseous nebula (≥ 50 kpc) lying at high → redshifts
(z ~ 2-6) and strongly emitting radiation due to the
→ Lyman alpha line (luminosities of  ≥ 10⁴³ erg s^-1) of hydrogen gas.
Also called Lyman alpha blobs, they are thought to lie in massive (M ~ 10¹³ solar masses) → dark matter halos,
which would subsequently evolve into those typical of rich → galactic clusters.

See also: → Lyman; → nebula.

A sequence of → permitted transitions in the → ultraviolet from an → excited state (B) of the → molecular hydrogen (H₂) to the electronic → ground state, with ΔE > 11.2 eV, λ < 1108Å (first → band head).
When a hydrogen molecule absorbs such a photon, it undergoes a transition from the ground electronic state to the excited state (B). The following rapid → decay creates an → absorption band in that wavelength range.
See also → Werner band. → Lyman-Werner photon.

See also: → Lyman (Th. Lyman, 1906, Astrophys. J. 23, 181); → band.

The dividing point in a galaxy’s spectrum at wavelengths shorter than the → Lyman limit. Galaxies contain large amounts of → neutral hydrogen which is very effective at absorbing radiation shortward of 912 Å. Hence galaxies are virtually dark at these wavelengths.

See also: → Lyman; → break.

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.

See also: → Lyman; → break; → galaxy.

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.

See also: → Lyman; → continuum.

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.

See also: → Lyman; → continuum; → escape.

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

See also: → Lyman, → ghost.

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.

See also: → Lyman; → limit.

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.

See also: → Lyman; → series.

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.

See also: → Lyman; → Werner band; → photon.

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.

Etymology (EN): 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”).

Etymology (PE): 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.

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

See also: 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.

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.

See also: → Lyot division; → filter.

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

Etymology (EN): L. lyra, from Gk. lyra, a foreign word of uncertain origin.

Etymology (PE): Cang “harp,” frpm Mid.Pers. cang “harp.”

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

See also: → Lyra + → -ids.

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.

See also: Lysithea was a daughter of Oceanus and one of Zeus’ lovers.