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

A → spiral galaxy, of which the → solar system is a small part. It is the second largest in our → Local Group of galaxies. The Milky Way is a disk-shaped system, with a diameter of between 80,000 and 100,000 → light-years and a thickness of about 2,000 light-years, containing more than 10¹¹ stars. The stars are divided into two main categories, → Population II stars and → Population I stars.
The core, or nucleus, of the Galaxy is surrounded by an ellipsoidal central → bulge that measures some 15,000 light-years in diameter and about 6,000 light-years in the direction perpendicular to the plane of the disk. Surrounding the bulge and extending in a near spherical distribution above and below the → Galactic plane is the → Galactic halo. The halo contains about 200 → globular clusters and an extremely thinly scattered population of individual stars.
The Sun is located just over half way out from the center to the edge of the disk at a distance of about 25,000 light-years. In common with other stars, the Sun revolves around the → Galactic Center. Its → orbital velocity is about 220 km s^-1 and its → orbital period is about 225 million years. Overall, the Galaxy exhibits → differential rotation, that is stars and gas clouds closer to the center have shorter orbital periods than those that are located further out.
The → spiral arms of the Milky Way lie within its disk, where bright → young stars, → H II regions, and → molecular clouds of gas and dust are concentrated into curved "arms" that appear to radiate from the central bulge in a spiral pattern. The Galaxy's spiral pattern consists of several major arms and a number of shorter segments, one of which, the → Orion arm, contains the Sun and the Orion star-forming region.
Near-infrared observations have shown that the stars in the central bulge are arranged in an elongated → galactic bar, about twice as long as it is wide, that is seen nearly end on from the present location of the solar system. The exact center, or nucleus, of the Galaxy coincides with a strong source of radio emission, called → Sagittarius A, that is less than 15 astronomical units in diameter. Observations of the speeds at which clouds of ionized gas are revolving round the → Galactic center imply that several million solar masses of material are concentrated within a region of about one light-year in radius. Since only about half of this mass can be accounted for by stars, it seems likely that the balance (about 2.5 million solar masses) is contained in a central black hole and that accretion onto this black hole is the underlying source of the energy radiated by Sagittarius A.
The Milky Way also has a → dark matter component. The Galactic → rotation curve indicates that there is a large amount of invisible → non-baryonic surrounding the whole Galaxy.

→ Milky Way; → galaxy.

The huge star system of which the Sun is a member. Same as the Galaxy or the Milky Way galaxy.

→ Milky Way; → system.

A chemical experiment conducted in 1953 that aimed at checking Alexander Oparin's and J. B. S. Haldane's hypothesis that under putative conditions present in the atmosphere of the early Earth inorganic molecules would spontaneously form organic molecules. Miller and Urey filled a sterile flask with a mixture of water, ammonia, methane, and hydrogen. The mixture was heated to evaporate water to produce water vapor. High-voltage electric sparks were passed through the mixture to simulate lightning. After a week, contents were analyzed. Amino acids, the building blocks for proteins, were found.

Named after Stanley L. Miller (1930-2007) and Harold C. Urey (1893-1981); → experiment.

Prefix meaning one thousandth (10^-3).

From Fr., from L. mille "thiusand."

Mili-, loan from Fr.

A unit of angle equal to one thousandth of an → arcsecond, or 1/3 600 000 degree.

→ milli-; → arcsecond.

One thousandth of a bar; a unit of atmospheric pressure. The average atmospheric pressure at sea level is 1.01325 bars or 1013.25 mb.

→ milli-; → bar.

A precision experiment for measuring the → electron charge. By studying the falling speed of small charged droplets in the gravitational field of the Earth subjected to an adjustable electric field, Millikan (1909) was able to demonstrate conclusively the discrete nature of electric charge, and moreover measure the charge of an individual electron.

Robert Andrews Millikan (1868-1953); → experiment.

Microwaves with wavelengths between 1 and 10 millimeter, corresponding to frequencies between 300 GHz to 30 GHz. → millimeter-wave astronomy.

→ milli-; → meter; → wave.

That part of radio astronomy which uses electromagnetic waves in the range 1-10 millimeter to study various components of the Universe, in particular the chemistry of interstellar matter.

→ millimeter wave; → astronomy.

A thousand thousand (10⁶).

O.Fr. million, from It. millione, literally "a great thousand," augmentative of mille "thousand," from L. mille.

Milyon, Loan from Fr.

A type of pulsar that spins around its axis every few thousands of a second. The prototype 1937+21, discovered in 1982, has a period of 1.56 milliseconds; i.e. it spins more than 600 times per second, the fastest so far found (Backer et al. 1982, Nature 300, 615). These pulsars are distinguished from typical pulsars by the extreme stability of their rotation period. It is thought that they have been rejuvenated by a "spin-up process" involving the accumulation of matter from a companion star. → recycled pulsar.

→ milli-; → second; → pulsar.

One thousands of a → sievert.

→ milli- + → sievert.

A design of → radio interferometer made of two lines of → antennae at right angles to one another.

Named after the Australian engineer and astronomer Bernard Yarnton Mills (1920-2011; see R.H. Frater et al. 2013, arXiv:1306.6371); → cross.

Same as → Milne Universe.

→ Milne Universe; → cosmological; → model.

A model of the → Universe which is devoid of matter and where the → space-time is → open (Ω_M = 0, Ω_R = 0, Ω_Λ = 0, k = -1). The Universe will expand at a constant rate for ever. See also → empty Universe, → de Sitter Universe.

Put forward by Edward Arthur Milne (1896-1950), a British astrophysicist, who introduced the → cosmological principle; → cosmological; → model.

The approximation of a stellar atmosphere with a plane parallel grey atmosphere in radiative equilibrium. It is assumed that a spectral is formed in such a way that the ratio of the line absorption coefficient to the continuous absorption coefficient is constant with depth.

→ Milne Universe; Arthur Stanley Eddington (1882-1944), prominent British astrophysicist; → approximation.

The seventh of Saturn's known satellites. It is 392 km in diameter and orbits Saturn at a mean distance of 185,520 km. Mimas' low density (1.17) indicates that it is composed mostly of water ice with only a small amount of rock. The surface is saturated with impact craters, dominated by the largest one measuring 130 km across, known as Herschel. Mimas was discovered in 1789 by Herschel.

In Gk. mythology, Mimas was one of the Gigantes slain by Hephaestus, the god of fire, volcanism, smiths and craftsmen, with barrage of red-hot metal.

1) The human faculty to which are ascribed thought, feeling, etc; often regarded as an immaterial part of a person (Dictionary.com).
2) Psychology: The totality of conscious and unconscious → mental processes and activities (Dictionary.com).

M.E. mynd(e), from O.E. gemynd "memory, remembrance; thought, purpose" (cf. Gothic muns "thought," munan "to think;" ON minni "mind;" Ger. Minne (archaic) "love," originally "memory"), from PIE root *men- "think, remember;" cf. Pers. mân, man "mind, thought;" Av. man- "to think;" Skt. matih "thought," Gk. mania "madness," mentio "remembrance;" Lith. mintis "thought, idea," O.C.S. mineti "to believe, think," Russ. pamjat "memory."

Ment, from Mid.Pers. mênitan "to think," Av. mainyeite "he thinks;" O.Pers. man- "to think," maniyaiy "I think," Ardumaniš- (proper noun) "upright-minded," Haxāmaniš- (proper noun, Hellenized Achaemenes, founder of the Achaemenian dynasty) "having the mind of a friend;" Av. mân- "to think," manah- "mind, thinking, thought; purpose, intention," mainyu- "mind, mentality, mental force, inspiration," cf. Sogdian mân "mind;" Skt. man- "to think," mánye "I think," manyate "he thinks," mánas- "intelligence, understanding, conscience;" Gk. mainomai "to be angry," mania "madness," mantis "one who divines, prophet;" L. mens "mind, understanding, reason," memini "I remember," mentio "remembrance;" Lith. mintis "thought, idea;" Goth. muns "thought," munan "to think;" Ger. Minne "love," originally "loving memory;" O.E. gemynd "memory, thinking, intention;" PIE base *men- "to think, mind; spiritual activity."

A naturally occurring inorganic solid. The internal crystalline structure of a mineral is controlled by its elemental composition.

From M.L. minerale "something mined," from neuter of mineralis "pertaining to mines," from minera "mine."

Kâni "mineral," from kân "mine," from kandan "to dig" (Mid.Pers. kandan "to dig;" O.Pers. kan- "to dig," akaniya- "it was dug;" Av. kan- "to dig," uskən- "to dig out" (→ ex- for prefix us-); cf. Skt. khan- "to dig," khanati "he digs").

A black hole of mass as low as 10^-6 gram supposed to have formed in the early Universe following the Big Bang event. Same as primordial black hole.

Mini a shortening of → minimum; → black hole.