Fr.: noyau de galaxie
A concentration of stars and gas in the innermost region of a galaxy, sometimes extending over thousands of light-years from the center of the galaxy.
Fr.: flot galactique
Fr.: plan galactique
Fr.: pôle galactique
The point on the sky, north or south, at which the galaxy's rotation axis would meet the celestial sphere.
Galactic radio noise
nufe-ye râdioi-ye kahkešân
Fr.: bruit radio de la Galaxie
A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.
Fr.: rotation galactique
The revolving of the gaseous and stellar content of a galaxy around its central nucleus. The rotation is not uniform, but differential. One revolution of the Sun within our own Galaxy takes about 220 million years, or one cosmic year.
galactic rotation problem
parâse-ye carxeš-e kahkešâni
Fr.: problème de la rotation galactique
The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.
Fr.: structure galactique
The global shape and the arrangement of the various parts or constituents of a galaxy.
Fr.: système galactique
Same as → galactic coordinates.
Fr.: vent galactique
An outflow of hot gas, analogous to the → solar wind, from a galaxy that has recently undergone a high → burst of star formation or has an → active galactic nucleus. Galactic winds are streams of high speed charged particles blowing out of galaxies with speeds of 300 to 3,000 km s-1. In the case of starbursts, galactic winds are powered by → stellar winds driven by → massive stars and → supernova explosions. Galactic winds contain a mixture of extremely hot metal-enriched supernova ejecta and cooler entrained gas and dust. Outflowing material has been observed at great distances from galaxies (10 to 100 kpc). In some cases they escape the galaxy potential well and pollute the → intergalactic medium with → heavy elements. A prominent example is the → superwind of the starburst galaxy M82.
Fr.: fenêtres galactiques
The regions near the Galactic plane where there is low absorption of light by interstellar clouds so that some external galaxies may be seen through them.
sâl-e kahkešâni (#)
Fr.: année galactique
The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.
ostacân bâ marpel-e kahkešâni
Fr.: flot à l'échelle galactique
The enormous amounts of → mass and → energy released from active galaxies into the → intergalactic medium. → Supermassive black holes, believed to exist at the centres of active galaxies (→ active galaxy), → accrete matter and liberate huge quantities of energy. The energy output is often observed as → active galactic nuclei (AGN) outflows in a wide variety of forms, e.g. → collimated → relativistic jets and/or huge overpressured cocoons in → radio, → blueshifted broad → absorption lines in the → ultraviolet and → optical, → warm absorbers and ultrafast outflows in → X-rays, and → molecular gas in → far infrared. Moreover, the processes of → star formation and → supernova explosions release mass/energy into the surroundings. This → stellar feedback heats up, ionizes and drives gas outward, often generating large-scale outflows/→ winds. Galactic outflows are observed at low redshifts reaching a velocity as large as 1000 km s-1 and at high-z up to z ~ 5, sometimes extending over distances of 60-130 kpc. Galactic-scale outflows may be a primary driver of galaxy evolution through the removal of cool gas from star-forming regions to a galaxy's → halo or beyond.
Of or relative to the center of a galaxy.
Fr.: distance galactocentrique
The distance from the center of a galaxy.
Fr.: facteur de Gaunt
In the atomic theory of spectral line formation, a quantum mechanical correction factor applied to the absorption coefficient in the transition of an electron from a bound or free state to a free state.
Gaunt, after John Arthur Gaunt (1904-1944), English physicist born in China, who significantly contributed to the calculation of continuous absorption using quantum mechanics; → factor
Fr.: réfraction géodésique
The limiting case of → astronomical refraction when the light path is entirely within the Earth's atmosphere.
žirandegi-ye zamin-meqnâtisi, ~ zamin-meqnâti
Fr.: activité géomagnétique
giant impact hypothesis
engâre-ye barxord-e qulâsâ
Fr.: hypothèse de l'impact géant
A model for → Moon formation (initially put forward by
Hartmann and Davis, 1975,
Icarus 24, 504), according to which the → proto-Earth
suffered a collision with another → protoplanet
near the end of the → accretion process
that ejected material into a → circumterrestrial
disk, out of which the Moon formed. Also called
→ canonical model.
The giant impact hypothesis is the leading theory for lunar formation.
There are, however, some key observations that cannot be explained using this
model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is
difficult to get enough mass into orbit to form such a massive Moon.
Second, the Moon has a similar bulk composition to the Earth, but
it is missing large amounts of more
→ volatile elements. The model does not properly
explain Moon's distinctive composition.
Finally, Earth and the Moon share virtually the same
→ isotopic ratios.
It is therefore expected that the body that hit the
Earth, often called → Theia,
would have had a different isotopic
ratio than the proto-Earth. In the canonical model, most of
the mass of the Moon comes from Theia and so the Moon should have a
different isotopic fingerprint than Earth, but it does not.
The type of impact that formed the Moon in the canonical model is
dictated by a very strong constraint, the → angular momentum
of the Earth-Moon system. It is
assumed that the angular momentum of the Earth-Moon system
immediately after the Moon formed was the same as it is today. This
assumption limits the velocity of the impact, the mass of the
impacting bodies, and the angle at which the two bodies
collided. It was found that only a grazing impact with a Mars-mass
impactor at near the escape velocity can put enough mass into orbit
to potentially form a lunar-mass Moon. This is why the canonical
model is such a specific type of impact.
However, the angular momentum of the Earth-Moon system could
have been reduced over time by competition between the
gravitational pull of Earth, the Moon and the Sun. Therefore,
the Moon-forming collision could have been much more energetic than
the canonical impact.
Fr.: attraction gravitationnelle
The force that pulls material bodies toward one another because of → gravitation.