An Etymological Dictionary of Astronomy and Astrophysics
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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.

A roughly spherical aggregation of → globular clusters, as well as the oldest stars and unseen mass that surrounds the Galaxy.

→ galactic, → halo.

In the → Galactic coordinate system, the angle between the line of sight to an object and the → Galactic equator. Galactic latitude, usually represented by the symbol b^II, ranges from +90 degrees to -90 degrees.

→ galactic; → latitude.

In the → Galactic coordinate system, the angle between the → Galactic Center and the projection of the object on the → Galactic plane. Galactic longitude, usually represented by the symbol l^II, ranges from 0 degrees to 360 degrees.

→ galactic; → longitude.

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.

→ galactic; → nucleus.

→ galactic-scale outflow.

→ galactic; → outflow.

The plane in which the → disk of a → spiral galaxy, such as our → Milky Way, lies.

→ galactic; → plane.

The point on the sky, north or south, at which the galaxy's rotation axis would meet the celestial sphere.

→ galactic; → pole.

A diffuse radio signal that originates outside the solar system. It is strongest in the direction of the Galactic plane.

→ galactic; → radio; → noise.

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.

The discrepancy between observed galaxy → rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material.

→ galactic; → rotation; → problem.

The global shape and the arrangement of the various parts or constituents of a galaxy.

→ galactic; → structure.

Same as → galactic coordinates.

→ galactic; → system.