An Etymological Dictionary of Astronomy and Astrophysics
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A central region of an → active galaxy, which is a → light-year or less in diameter and has an abnormally high luminosity. The nucleus emits high energy radiation (→ gamma rays, → X-rays, → ultraviolet) and shows → variability over various time-scales, sometimes very short (hours to weeks). Emission line spectra reveal high velocity motions up to 10⁴ km s^-1. AGNs are divided into two main types. Type I refers to an AGN whose nucleus is visible (the spectra has both narrow and broad emission lines), while in type II AGN, the broad line region (BLR) is obscured and the lines are very narrow. This may be due either to the viewing angle or some intrinsic difference in structure. See also → broad-line region, → narrow-line region, → quasar.

→ active; → galactic; → nucleus.

Of or pertaining to an → apogalacticon.

→ apo-; → galactic.

The point at which a celestial body is farthest from the center of a galaxy; opposite of → perigalacticon.

From → apo- "away from, off" + galacticon, → galaxy.

From apâ, → apo-, + kahkašân, → galaxy.

The interface between a galaxy and the → intergalactic medium. The circumgalactic medium comprises gas located in the → halo of a galaxy extending out to the → virial radius.

→ circum-; → galactic.

A minor component of galactic light resulting from the diffusion of starlight by → interstellar dust near the → galactic plane.

→ diffuse; → galactic; → light.

Of, pertaining to, or dealing with the space beyond the Milky Way.

From → extra- + → galactic.

The branch of astronomy that deals with objects beyond the Milky Way, especially galaxies and quasars.

→ extragalactic; → astronomy.

The integrated intensity of all of the light emitted throughout the history of the Universe across the whole of the → electromagnetic spectrum, including those which are not individually detected. The EBL spectrum includes cosmological backgrounds associated with either primordial phenomena, such as the → cosmic microwave background radiation (CMBR), or photons emitted by stars, galaxies and → active galactic nuclei (AGN) due to → nucleosynthesis or other → radiative processes, including → dust scattering, → absorption and reradiation. The EBL may also contain signals that are diffuse and extended, including high-energy photons associated with dark matter particle decays or annihilation.

→ extragalactic; → background; → light.

1) Of or pertaining to a → galaxy.
2) Usually with capital G, pertaining to our galaxy, the → Milky Way.

Adjective of → galaxy.

The point in the → Galactic plane that lies directly opposite the → Galactic center. It lies in the constellation → Auriga at approximately R.A. 05h 46m, Dec. +28° 56'.

→ galactic; → anticenter.

An elongated bar-shaped structure composed of stars present in some spiral galaxies. About two-third of such galaxies contain bars that cross their centers. Bars, like → spiral arms, result from a → density wave in which stars take very elliptical orbits. They form when the → galactic disk dominates the → galactic bulge, → Ostriker-Peebles criterion. Bars play an extremely important role in a galaxy's evolution. The gravity from a bar is the mechanism that drives → interstellar gas from the outer parts of a → spiral galaxy inward toward the central regions, and into the galactic nucleus itself. This causes tremendous bursts of star formation. Therefore, a majority of massive stars are born in such starbursts in the nuclei of galaxies. Bars may also channel the material that falls into black holes within active galactic nuclei, releasing enormous power in radiation and particles from tiny regions at the centers of some galaxies. Bars disappear as galactic centers grow more massive (after some 2 to 8 Gyr).

→ galactic; → bar.

The central → galaxy bulge of the → Milky Way.

→ galactic; → bulge.

1) The rotational center of the → Milky Way galaxy located in the direction of the → Sagittarius constellation at a distance of 7.62 ± 0.32 kpc (2005, ApJ 628, 246). Its equatorial coordinates (J2000 epoch) are: R.A. 17h45m40.04s, Dec. -29° 00' 28.1''. The Sun orbits around the Galactic center once every 200 million years at a speed of 220 km per second. It is believed that there is a → supermassive black hole at the Galactic center.
2) The innermost region of a → spiral galaxy characterized by high number of stars per unit volume. The center may contain a → supermassive black hole.

→ galactic; → center.

One of the three massive clusters located toward the → Galactic center: → Quintuplet cluster, → Arches cluster, → Central cluster. Heavily extinguished by the presence of dust clouds and only accessible at infrared (and longer) wavelengths or in X-rays, each of these clusters has a population of more than a hundred → massive stars. The three clusters are similar in most respects, each containing about 10⁴ solar masses in stars. The Arches cluster is younger than the two others.

→ galactic; → center; → cluster.

1) Same as → open cluster.
2) same as → cluster of galaxies.

→ galactic; → cluster.

A system of astronomical coordinates using → latitude (b^II) measured north and south from the → Galactic equator and → longitude (l^II), measured from the → Galactic Center in the sense of increasing → right ascension from 0 to 360 degrees. In the old system (l^I,b^I), the Galactic center was at l^I = 327°41'. Same as → galactic system.

→ galactic; → coordinate.

The flattened component of a → spiral galaxy which is composed of stars and concentrations of dust and molecules. → Star formation takes place mainly in the disk.

→ galactic; → disk.

The study of the → motions of the → stars, → gas, and → dark matter in a → galaxy to explain the main → morphological and → kinematical features of the galaxy.

→ galactic; → dynamics.

The great circle in the sky defined by the place of the → Galactic plane or the → Milky Way. At an angle of about 62°, the Galactic equator intersects the celestial equator at two points located in the constellations → Monoceros and → Aquila.

→ galactic; → equator.

A region of the Galaxy whose boundaries are set by its calm and safe environment and access to the chemical materials necessary for building terrestrial planets similar to the Earth. → circumstellar habitable zone; → habitable zone.

→ galactic, → habitable; → zone.