An Etymological Dictionary of Astronomy and Astrophysics
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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.

A magnifying optical instrument for inspecting objects too small to be seen or too small to be seen distinctly and in detail by the unaided eye.

From Mod.L. microscopium "an instrument for viewing what is small," from Gk. → micro- + -skopion "means of viewing," from skopein "look at."

Rizbin, from riz→ micro- + bin "to see; seer" (present stem of didan; Mid.Pers. wyn-; O.Pers. vain- "to see;" Av. vaēn- "to see;" Skt. veda "I know;" Gk. oida "I know," idein "to see;" L. videre "to see;" PIE base *weid- "to know, to see").

Being or characterized as exceedingly small; not large enough to be seen with the naked eye. Compare → macroscopic. → macroscopic state.

→ microscope; → -ic.

Same as → microstate.

→ microscopic; → state.

The Microscope. A minor constellation in autumn southern sky lying just south of → Capricornus at 21h right ascension, 37° south declination. The constellation contains only 4th magnitude or fainter stars. Abbreviation: Mic; genitive: Microscopii.

Microscopium was named by Abbé Nicolas Louis de Lacaille (1713-1762); → microscope.

Statistical physics: For a system made up of a large number of components, a state of the system which is specified by describing the current dynamical variables of each constituting component. For example, for a gas system composed of a large number of molecules, the microstate is defined by the set of quantities which defines the state of each molecule in the system (position, velocity, vibration, etc.). In practice, it is impossible to know perfectly the microstate of a system. The aim of → statistical physics is to relate the macroscopic (average ) observables (→ pressure, → temperature, → internal energy) to the microstate of the system. Also called → microscopic state. See also → macrostate and → multiplicity.

→ micro-; → state.

The → turbulence phenomenon involving relatively smaller physical volumes compared to → macroturbulence. In stellar atmospheres, it is a bulk gas motion with a characteristic size less than the local photon → mean free path. Microturbulence is one of the most significant mechanisms that can cause → line broadening in the stellar spectrum. The presence of microturbulence de-saturates strong lines and increases their → equivalent widths. Microturbulence in → hot stars brings about gas motions with velocities 0-20 km s^-1. A physical connection may exist between microturbulence in hot star atmospheres and a subsurface → iron convection zone. Microturbulence may also be at the origin of → wind clumping in hot stars.

→ micro-; → turbulence.

Electromagnetic radiation having wavelengths in the 1 to 300 mm range.

→ micro-; → wave.

Thermal radiation with a temperature of 2.73 K that is apparently uniformly distributed in the Universe. It is believed to be a redshifted remnant of the hot radiation that was in thermal equilibrium with matter during the first hundred thousand years after the Big Bang. Same as → cosmic microwave background (CMB) radiation.

→ microwave; → background; → radiation.

A type of electrically operated oven that cooks food very quickly using microwaves instead of heat.

→ microwave; → oven.