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

→ ideal magnetohydrodynamics.

→ magnetohydrodynamics (MHD); → condition.

An older name for the unit of electrical → conductance, which is defined to be the reciprocal of the → ohm. It is now replaced by the → siemens.

Ohm spelt backward.

A shiny silicate mineral composed of varying amounts of → aluminum, → potassium, → magnesium, and → iron. Mica appears as thin, flexible layers in → granite and other rocks, or as → crystals. It is used as a thermal or electrical → insulator.

From L. mica "crumb, grain."

An apparatus that produces interference fringes by splitting a beam of monochromatic light so that one beam strikes a fixed mirror and the other a movable mirror. When the reflected beams are brought back together, an interference pattern results. It is used to measure very precise lengths, such as the wavelength of light, and for high-resolution spectroscopy.

Named after Albert Abraham Michelson (1852-1931), German-American physicist, who built the interferometer for the → Michelson-Morley experiment of 1887; → interferometer.

Andarzanešsanj, → interferometer.

An experiment performed in 1887 to establish the presence or absence of an → ether, a medium through which light was supposed to travel. The experiment aimed to measure the speed of light coming from different directions. However no → ether drift was found. The null results obtained showed that the ether hypothesis was incorrect. Consequently, the theory of → special relativity, with its hypothesis that the speed of light is the same in all → inertial frames, reconciled the results of the Michelson-Morley experiment with the rest of physics.

→ Michelson interferometer; Michelson received the Nobel Prize in 1907 for his work, the first American to receive the Prize in science. Edward Williams Morley (1838-1923), an American chemist; → experiment.

Same as → apogee full Moon.

→ micro-; → Moon.

1) A combining form for "small."
2) A prefix meaning one millionth (10^-6).

From Gk. mikros "small."

Riz-, from riz "very small."

The area of computer science dealing with the use and development of microcomputers, and related peripheral devices and softwares. Also microinformatics.

→ micro-; → compute.

Riz-, → micro-; azdâyik, → informatics.

Anything that is regarded as a world in miniature.
A unity that is an epitome of a larger unity; opposed to macrocosm.

→ micro-; → cosmos.

The state or condition where the force of → gravity is very weak, e.g. the → weightlessness experienced inside an orbiting spacecraft.

→ micro-; → gravity.

A type of → gravitational lens, where the foreground → lensing object is of low mass, and the multiple images produced are too close together on the sky to be observed as separate images. Gravitational microlensing occurs when a foreground star happens to lie very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two or more images, which are typically unresolved. However, these images of the source are magnified, by an amount that depends on the angular separation between the lens and source. If with the passage of time the lens moves across the Earth-source, the amount of brightening changes. Typically the source will appear to brighten, reach a maximum and then fade symmetrically back to normal over the course of a few weeks or months; this is called a → microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet. Microlensing is used in the search for → dark matter in the → Milky Way galaxy and its nearest neighbours, as well as for → extrasolar planets (e.g. B. S. Gaudi, 2010, arXiv:1002.0332).

→ micro-; → lensing.

Determining the three various parameters of a microlensing event (the lens-source relative parallax and proper motion, and the mass of the lens) from only one physical parameter (the event time scale). Currently the microlensing degeneracy affects the vast majority of events and makes any individual event impossible to interpret with certainty.

→ microlensing; → degeneracy.

The effect arising whenever a source star and lens star pass each other at an angular separation involving the → Einstein radius (R_E) of the lens. The time-scale for such an event is defined as t_E = R_E/v, where v is the magnitude of the relative transverse velocity between source and lens projected onto the lens plane.

→ microlensing; → event.

A small grain sized meteorite which can only be positively identified under the microscope.

→ micro-; → meteorite.

1) A screw thread device used to make accurate physical linear measurements.
2) Same as → micron.

→ micro-; → -meter.

A unit of length in the → metric system equal to one millionth of a → meter (10^-6 m); symbol μm. Also called → micrometer. The average thickness of a human hair is about 50 μm (30-100 μm). The human eye cannot see anything smaller than 40 μm in size. Other small sizes: white blood cells = 15 μm; red blood cells = 8 μm; bacteria 2 μm.

Coined 1880 in Fr. from Gk. mikron, neutral of mikros "small."

A localized → thermonuclear burst on the surface layers of an → accreting white dwarf. In comparison, classical → nova explosions are caused by global → thermonuclear runaways on the surface of such white dwarfs. Micronovae are less powerful than novae; they have been observed to release up to 10³⁹ ergs of energy, that is approximately 10⁶ times less than the energies released in classical novae (thus the term micronova describing these events). They are also much short-lived, lasting only several hours, while nova outbursts last for weeks. The micronova phenomenon is provoked by the accumulation of accreted matter on the poles of → white dwarfs under the confining effect of strong → magnetic fields (S. Scaringi et al., 2022, arXiv:2204.09073).

→ micro-; → nova.

Any organism too small to be seen by the naked eye, e.g. bacteria, viruses, and protozoa.

→ micro-; → organism.

Controlling unit of a microcomputer; laid out on a tiny silicon chip and containing the logical elements for handling data, performing calculations, carrying out stored instructions, etc.

→ micro-; → processor.

A → binary system where an ordinary star orbits around a → neutron star or a → stellar-mass black hole that accretes the outer layers of the star's atmosphere. The accreted material falling on the → compact object warms up drastically and emits huge amounts of energy as → X-rays. The → accretion disk that emits this radiation also produces → relativistic jets of → plasma along the rotation axis of the compact object. The jets of material exhibit superluminal motion and resemble those emitted from → quasars, but on scales millions of times smaller. The first microquasar, 1E1740.7-2942, was discovered by F. Mirabel et al. 1992, Nature, 358, 215.

→ micro- + → quasar.