An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The distance from the center of a galaxy.

→ galactocentric; → distance.

1) Generally, a large body of → gas, → dust, and → stars held together by their mutual → gravitational attraction and ranging in mass from about 10⁶ to 10¹³ Msun. If a galaxy also contains → dark matter its mass will be much larger. Galaxies are grouped into three main categories: → spiral galaxy, → elliptical galaxy, and → irregular galaxy (→ Hubble classification).
2) With capital G, the galaxy to which our Sun belongs; → Milky Way galaxy.
See also:
→ active galaxy, → Andromeda galaxy, → barred spiral galaxy, → biased galaxy formation, → binary galaxy, → blue compact dwarf galaxy, → broad-line radio galaxy, → bulge of a galaxy, → Cartwheel Galaxy, → compact galaxy, → core-halo galaxy, → disk galaxy, → dwarf elliptical galaxy, → dwarf galaxy, → dwarf irregular galaxy, → dwarf spheroidal galaxy, → early-type galaxy, → edge-on galaxy, → face-on galaxy, → field galaxy, → flocculent spiral galaxy, → galaxy bimodality, → galaxy cluster, → galaxy formation, → galaxy harassment, → galaxy main sequence, → gas-poor galaxy, → gas-rich galaxy, → grand design spiral galaxy, → green pea galaxy, → halo of galaxy, → halo of the Galaxy, → Haro galaxy, → host galaxy, → hypergalaxy, → infrared galaxy, → Irr I galaxy, → Irr II galaxy, → isolated galaxy, → late-type galaxy, → lensing galaxy, → lenticular galaxy, → low surface brightness galaxy, → luminous infrared galaxy, → Lyman break galaxy, → Markarian galaxy, → metagalaxy, → metal-deficient galaxy, → metal-poor galaxy, → parent galaxy, → passive galaxy, → passively evolving galaxy, → peculiar galaxy, → primordial galaxy, → progenitor galaxy, → protogalaxy, → radio galaxy, → receding galaxy, → retired galaxy, → ring galaxy, → Sagittarius Dwarf Elliptical Galaxy, → Sagittarius Dwarf Irregular Galaxy, → satellite galaxy, → Sculptor Dwarf Elliptical Galaxy, → Seyfert galaxy, → shell galaxy, → Sombrero galaxy, → starburst galaxy, → strong arm spiral galaxy, → submillimeter galaxy, → superthin galaxy, → superwind galaxy, → tidal dwarf galaxy, → Triangulum galaxy, → ultraluminous infrared galaxy, → violent galaxy, → weak arm spiral galaxy, → Whirlpool galaxy, → Wolf-Rayet galaxy.

From L.L. galaxias "Milky Way," from Gk. galaxis (adj.), from gala (genitive galaktos) "milk."
In Gk. mythology, Jupiter, hoping to immortalize his infant son Hercules (who was born to a mortal woman), placed the baby on Hera's breast. Her milk spilled up, forming the Milky Way. A painting by Italian artist Jacopo Tintoretto (c. 1518-1594), called "The Origin of the Milky Way," depicts the legend describing how the Milky Way was formed.

Kahkešân, short for (râh-e) kahkešân literally "the (path of the) chaff-draggers" or "trail of chaff," from kah, kâh "chaff, straw, hay" (Mid.Pers. kâh "chaff, straw;" cf. Pali kattha- "a piece of wood;" Skt. kastha- "stick;" Gk. klados "twig;" O.Ir. caill "wood;" Ger. Holz "wood;" E. holt; PIE *kldo-) + kešân pr.p. of kešidan/kašidan "to carry, draw, protract, trail, drag" (Mid.Pers. kešidan "to draw, pull;" Av. karš- "to draw; to plow," karša- "furrow;" cf. Skt. kars-, kársati "to pull, drag, plow;" Gk. pelo, pelomai "to move, to bustle;" PIE base k^wels- "to plow"). The term (râh-e) kahkešân may be a popular corruption of Mid. Pers. (râh-i) Kâwôsân "the path of Kâwos" referring to the Persian mythological king Kay Kâwôs, who built an eagle-propelled throne to fly to China, as recounted in the Dênkard and the Shâhnâmé.

The division of galaxies into a "red sequence" and a "blue sequence" in the → color-magnitude diagrams of galaxies involving large statistical surveys. In both sequences, redder galaxies tend to be brighter. The blue sequence is truncated at the red magnitude ~ -22, while the red sequence extends to brighter magnitudes. The division between the two classes of galaxies is associated with a critical stellar mass ~ 3 × 10¹⁰ Msun. Galaxies below the critical mass are typically blue, star forming spirals and reside in the field. Galaxies above the critical mass are dominated by red spheroids of old stars and live in dense environments (Kauffmann et al, 2003, MNRAS 341, 33 & 54).

→ galaxy; → bimodality.

A → spheroidal region at the center of a → spiral galaxy which mostly contains → old stars. Galactic bulges are generally classified into two types: → classical bulges and → pseudo-bulges.

→ galaxy; → bulge.

An aggregation of galaxies, made up of a few to a few thousand members, which may or may not be held together by its own gravity. Same as → cluster of galaxies.

→ galaxy; → cluster.

The study dealing with the processes that gave rise to galaxies in a remarkably → early Universe. See also → structure formation, → protogalaxy

→ galaxy; → formation.

Frequent, high speed galaxy → encounters within → galaxy clusters. Harassment can disturb the morphologies of the galaxies involved, often inducing a new → burst of star formation. Asymmetrical galaxies, → warps, → bars, and → tidal tails can all be produced through galaxy harassment.

→ galaxy; → harassment.

The dominant member of the → Virgo cluster of galaxies, which contains some 2,000 galaxies. Also known as NGC 4486, it has an → apparent visual magnitude 9.6. Discovered in 1781 by Charles Messier, this → elliptical galaxy is located 55 million → light-years away from Earth in the constellation → Virgo. M87 is the home of several thousand billion stars, a → supermassive black hole (SMBH) and a family of roughly 15,000 → globular clusters. For comparison, our → Milky Way galaxy contains only a few hundred billion stars and about 150 globular clusters. M87 is characterized by a prominent kiloparsec scale → relativistic jet emitted by the central SMBH. As gaseous material from the center of the galaxy → accretes onto the black hole, the energy released produces a stream of subatomic particles that are accelerated to velocities near the → speed of light.

→ galaxy; → Messier catalog.

A scaling relation between the → star formation rate (SFR) in galaxies and the total stellar mass (M_*) of the galaxies. This relation, colloquially called the "galaxy main sequence," extends over several orders of magnitudes in M_* and out to → high redshifts, with a modest scatter of ~ 0.3 dex which includes both intrinsic scatter and measurement uncertainties. The existence of such tight scatter at all observed epochs suggests that most galaxies assembled their stellar mass fairly steadily rather than predominantly in → starburst episodes, implying that → mergers have a sub-dominant contribution to the global star formation history (Wuyts et al., 2011 ApJ 742, 96).

→ galaxy; → main; → sequence.

An unusually strong wind.

Gale, from gaile "wind," origin uncertain, perhaps from O.N. gol "breeze," or O.Dan. gal "bad, furious."

Tondbâb "gale," from tond "swift, rapid, brisk; fierce, severe," Mid.Pers. tund "sharp, violent;" Sogdian tund "violent;" cf. Skt. tod- "to thrust, give a push," tudáti "he thrusts;" L. tundere "to thrust, to hit" (Fr. percer, E. pierce, ultimately from L. pertusus, from p.p. of pertundere "to thrust or bore through," from per- + tundere, as explained); PIE base *(s)teud- "to thrust, to beat" + bâd, → wind.

Of or pertaining to Galileo Galilei (1564-1642), Italian physicist and astronomer.

Same as → Galilean relativity.

→ Galilean; → invariance.

Same as → Galilean satellites.

→ Galilean; → moon.

Same as → inertial reference frame.

→ Galilean; → reference; → frame.

The principle according to which the fundamental laws of physics are the same in all frames of reference moving with constant velocity with respect to one another (→ inertial reference frames). Same as → Galilean invariance and → Newtonian relativity.
See also: → Galilean transformation, → Einsteinian relativity.

→ Galilean; → relativity.

The four largest and brightest satellites of → Jupiter, that is: → Io (Jupiter I), → Europa, → Ganymede, and → Callisto.

Galileo, who had discovered them, called them Sidera Medicæa "Medicean Stars" in honor of the Medici family. → Galilean Moons; → satellite.

The method of relating a measurement in one → reference frame to another moving with a constant velocity with respect to the first within the → Newtonian mechanics. The Galilean transformation between the coordinate systems (x,y,z,t) and (x',y',z',t') is expressed by the relations: x' = x - vt, y' = y, z' = z. Galilean transformations break down at high velocities and for electromagnetic phenomena and is superseded by the → Lorentz transformations.

→ Galilean; → transformation.

A space mission whose main goal was to explore → Jupiter and its moons and rings. The spacecraft was launched on October 19, 1989, arrived at Jupiter in December 1995. It disappeared on September 21, 2003, after eight years orbiting Jupiter, when mission controllers crashed it into → Jupiter's atmosphere. On December 7, 1995, Galileo's probe dived into Jupiter's atmosphere, and measured atmospheric pressure, density, and composition, and explored the planet's → radiation belts. Galileo had two parts: an orbiter and a descent probe that parachuted into Jupiter's atmosphere. The orbiter sent back hundreds of pictures of the four large → Galilean satellites of Jupiter (→ Io, → Europa, → Ganymede, and → Callisto). It made many discoveries during its eight years looping around Jupiter. It found evidence for layers of salt water below the surface on Europa, Ganymede, and Callisto, and measured high levels of volcanic activity on Io. When → Shoemaker-Levy slammed into Jupiter in 1994, Galileo had the only direct view of the → comet striking Jupiter's atmosphere. Galileo determined that → Jupiter's rings are formed from dust hurled up by → meteorite impacts on planet's inner moons. Measurements by the orbiter's → magnetometer revealed that Io, Europa, and Ganymede have metallic cores, while Callisto does not. Also, Galileo discovered that Ganymede possesses its own → magnetic field; it is the first moon known to do so. The orbiter also found that the Galilean satellites all have thin atmospheres. During it's trip from Earth to Jupiter, Galileo passed by and studied two asteroids: → Gaspra in 1991 and → Ida in 1993, around which it discovered → Dactyl, the first moon orbiting an asteroid (windows2universe.org).

→ Galileo; → mission.

In the absence of air resistance, any two bodies that are dropped from rest at the same moment will reach the ground at the same time regardless of their mass.

Galileo (1564-1642) was the first to determine, at the start of the seventeenth century, the law of constant acceleration of free-falling bodies. → law; → fall; → body.

An electrolytic cell capable of producing electric energy by electrochemical reaction.

→ galvanism; → cell.