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nuclear star cluster | Space and Astronomy | 1 | A compact and dense concentration of stars located at the center of a galaxy. |
nsc | Space and Astronomy | 1 | A compact and dense concentration of stars located at the center of a galaxy. |
number density | Space and Astronomy | 1 | The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically in cm3 or m3. With stars, cubic parsecs (pc3) are often used. |
nutation | Space and Astronomy | 1 | A continuous, gravity-induced change in the orientation of an astronomical body's axis of rotation which results from the combined effects of small, short-term variations. Nutation is distinguished from precession, which is a similar but longer-term change in axial orientation. |
o–c diagram | Space and Astronomy | 1 | A diagram of observed minus calculated values over time, showing how observed data differ from theoretical values which have been calculated according to a particular scientific model. It is often used as a diagnostic tool to determine the accuracy of the model. With a variable star, it is typically used to compare phase differences over time. |
ob association | Space and Astronomy | 1 | A group of massive stars which are not gravitationally bound to each other, but move together through space in a loose association. The OB in the name is a reference to stars of stellar classifications O and B. |
obliquity | Space and Astronomy | 1 | See axial tilt. |
observation arc | Space and Astronomy | 1 | The duration of time between the earliest and latest observations made by astronomers of an object within the Solar System, which defines the length of the path traced by the object between these same observations. The term is primarily used in the discovery and tracking of asteroids and comets, which can be difficult to continuously track because of their size and great distance from Earth. Very short observation arcs, e.g. where the time between the initial observation and the most recent observation is less than 30 days, are of limited descriptive power because they represent only a very small fraction of the total path traced by the object in its orbit around the Sun (or other primary), and therefore result in a high degree of uncertainty when estimating the shape and characteristics of the object's orbit. |
observational astronomy | Space and Astronomy | 1 | The practice and study of directly observing astronomical objects with the use of telescopes and other astronomical instruments. It is concerned with recording data about the observable universe, as opposed to theoretical astronomy, which is concerned with calculating the measurable implications of astronomical models. |
occultation | Space and Astronomy | 1 | A celestial event that occurs when a distant astronomical body or object is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view. Solar and lunar eclipses are specific types of occultations. |
oort cloud | Space and Astronomy | 1 | A vast theoretical cloud of predominantly icy planetesimals hypothesized to surround the Sun at distances ranging from 2,000 to 200,000 AU. It is thought to be divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of the Solar System. |
opacity | Space and Astronomy | 1 | A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whether convection occurs. |
open cluster | Space and Astronomy | 1 | A gravitationally bound group of up to one thousand stars that formed together in the same molecular cloud. |
opposition | Space and Astronomy | 1 | The positioning of two celestial objects on opposite sides of the sky, from the perspective of an observer. This occurs, for example, when a planet makes its closest approach to the Earth, placing it in opposition to the Sun. |
orbit | Space and Astronomy | 1 | The gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately elliptical orbits around a common center of mass positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory. |
orbit plot | Space and Astronomy | 1 | A schematic diagram of a complete orbit. For a binary system, it is typically presented from the primary's frame of reference. |
orbital eccentricity | Space and Astronomy | 1 | A parameter that determines how much an orbit deviates from a perfect circle. For an elliptical orbit, the eccentricity ranges from greater than zero to less than one. |
orbital elements | Space and Astronomy | 1 | The set of parameters that uniquely define an orbit. |
orbital inclination | Space and Astronomy | 1 | The tilt of an object's orbit around an astronomical body, expressed as the angle between the orbital plane or axis of direction of the orbiting object and a plane of reference. |
orbital node | Space and Astronomy | 1 | One of two points at which the plane of an orbit intersects a specified plane of reference to which it is inclined; in some contexts, the two nodes may be distinguished as the ascending node and the descending node. A non-inclined orbit, which is coplanar with the reference plane, has no nodes. |
orbital period | Space and Astronomy | 1 | The time a given astronomical object takes to complete one orbit around another object. For objects in the Solar System, the orbital period is often referred to as the sidereal period. |
orbital plane | Space and Astronomy | 1 | The imaginary geometric plane defined by the orbit of an astronomical body around its primary. The Earth's orbital plane, which defines the ecliptic, is commonly used as a plane of reference for the orbits of other objects in the Solar System. |
orbital resonance | Space and Astronomy | 1 | The situation that occurs when two or more orbiting bodies exert regular, periodic gravitational influences on each other such that one or more of their orbital parameters (e.g. eccentricity, semi-major axis, inclination, etc., or any combination thereof) exist in some definite mathematical relationship with each other. Most commonly, the term refers to mean-motion orbital resonance, in which the bodies' orbital periods are related by a ratio of small integers. For example, the dwarf planet Pluto exists in a stable 2:3 resonance with Neptune, such that Pluto completes two orbits around the Sun in the same time it takes Neptune to complete three. Resonance may act on any time scale, from short-term to secular, and often leads to either long-term stabilization of the orbits or their eventual destabilization. |
orbital speed | Space and Astronomy | 1 | The speed at which an astronomical body or object orbits around a barycenter, or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entire orbital period, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs at periapsis. |
osculating orbit | Space and Astronomy | 1 | The hypothetical, idealized Kepler orbit that an orbiting object would follow around its primary if all perturbations were absent, i.e. the orbit that coincides with the instantaneous orbital state vectors at a given moment in time. |
outer space | Space and Astronomy | 1 | The vast, nearly empty expanse that exists beyond the Earth and between all celestial bodies, characterized generally by extremely low densities of particles, extremely low temperatures, and minimal gravity. Most of the volume of the Universe is intergalactic space, and even galaxies and star systems consist almost entirely of empty space. |
parsec | Space and Astronomy | 1 | A unit of length defined as the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 light-years or 206,265 astronomical units. The word "parsec" is a portmanteau of the words parallax and second. |
pc | Space and Astronomy | 1 | A unit of length defined as the distance at which a star would show a parallax shift of exactly one arcsecond as observed from Earth's orbit. It is equal to 3.2616 light-years or 206,265 astronomical units. The word "parsec" is a portmanteau of the words parallax and second. |
peak magnitude | Space and Astronomy | 1 | For a variable star, this is the highest amplitude achieved during a rise in luminosity, followed by a decline. This data point can provide useful distance information for a cataclysmic variable. It can be determined from a light curve of the stellar variability. |
periapsis | Space and Astronomy | 1 | The point at which an orbiting body is closest to its primary. Contrast apoapsis. |
perigee | Space and Astronomy | 1 | The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is closest to the Earth. Contrast apogee. |
perihelion | Space and Astronomy | 1 | The point in the elliptical orbit of an object (such as a planet or comet) about the Sun at which the object is at its closest distance to the Sun. |
perturbation | Space and Astronomy | 1 | The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of its primary alone, or any force which complicates the orbital characteristics of the body such that the idealized Kepler orbit of the two-body problem is not an accurate representation of the body's actual orbit. Perturbing forces may include the gravitational forces exerted by any number of additional bodies, the off-center gravitational forces which are consequences of bodies not being perfectly spherical, and/or atmospheric resistance. |
phase angle | Space and Astronomy | 1 | The elongation or angle between an orbiting body and the Sun as viewed from a particular perspective, such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have low phase angles as seen from Earth, so they often appear as a slim crescent; superior planets such as Mars and Jupiter usually have high phase angles, so that little of the shadowed side is visible. |
plane of reference | Space and Astronomy | 1 | An arbitrarily chosen, imaginary plane from which to measure and define orbital elements such as inclination and longitude of the ascending node. The ecliptic plane, invariable plane, and equatorial plane are all commonly used as reference planes in various contexts. |
plane of the sky | Space and Astronomy | 1 | An imaginary plane that is perpendicular to the line of sight. Typically this is used as a reference plane for the inclination of an orbital plane of a distant star system. |
planet | Space and Astronomy | 1 | In our Solar System, a planet is defined as an object that orbits the Sun, is big enough for its own gravity to make it ball-shaped and keeps space around it clear of smaller objects. |
planetary | Space and Astronomy | 1 | Of or relating to a planet or planets. |
planetary body | Space and Astronomy | 1 | Any secondary body that is geologically differentiated or in hydrostatic equilibrium and therefore has a planet-like geology, such as a planet, dwarf planet, or other planetary-mass object, but excluding smaller objects such as planetesimals. |
planetary differentiation | Space and Astronomy | 1 | The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core). |
planetary nebula | Space and Astronomy | 1 | A type of emission nebula formed from a glowing shell of expanding plasma that has been ejected from a red giant star late in its life. The name derives from their resemblance to a planet. An example is the Ring Nebula. |
planetary science | Space and Astronomy | 1 | The scientific study of planets, moons, and planetary systems, with the aim of understanding their formation, composition, topography, dynamics, and interactions with other bodies. |
planetary system | Space and Astronomy | 1 | Any set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. In general, planetary systems include one or more planets, though such systems may also consist of dwarf planets, moons, asteroids, meteoroids, planetesimals, and debris discs, among other objects. |
planetesimal | Space and Astronomy | 1 | Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in protoplanetary disks and debris disks during the process of planet formation, but is also sometimes used to refer to various types of small Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and a protoplanet. |
planetoid | Space and Astronomy | 1 | Another name for a minor planet or dwarf planet. |
polar orbit | Space and Astronomy | 1 | An orbit in which the orbiting object passes directly over or nearly over both poles of the body being orbited during each revolution. It therefore has an inclination equal or nearly equal to 90 degrees to the body's equator. |
precession | Space and Astronomy | 1 | Any slow change in the orientation of an object's axis of rotation. For the Earth in particular, this phenomenon is referred to as the precession of the equinoxes. Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit of Mercury that was explained by the theory of general relativity. |
primary | Space and Astronomy | 1 | The main physical body of a gravitationally bound, multi-object system. The primary constitutes most of the system's mass and is generally located near the system's barycenter. |
prograde motion | Space and Astronomy | 1 | Orbital or rotational motion of an object in the same direction as the rotation of the object's primary. The direction of rotation is determined by an inertial frame of reference such as the fixed stars. Contrast retrograde motion. |
projected separation | Space and Astronomy | 1 | The observed physical separation between two astronomical objects, as determined from their angular separation and estimated distance. For planets and double stars, this distance is usually given in astronomical units. The actual separation of the two objects depends on the angle of the line between the two objects to the line-of-sight of the observer. |
proper motion | Space and Astronomy | 1 | The rate of angular motion of an object over an interval of time, usually years. For stars, this is typically given in milliarcseconds per year. |
protoplanet | Space and Astronomy | 1 | A large planetary embryo that originated within a protoplanetary disk and has since undergone internal melting to produce an interior of non-uniform composition. Protoplanets represent an intermediate step in the formation of a full-sized planet; they are thought to form out of smaller planetesimals as they collide with each other and gradually coalesce into larger bodies. |
protostar | Space and Astronomy | 1 | A concentration of mass formed out of the contraction of a collapsing interstellar cloud. Once sufficient mass has fallen onto this central core, it becomes a pre-main-sequence star. |
pseudo-synchronous rotation | Space and Astronomy | 1 | For an eccentric orbit, this is a near synchronization of revolution and rotation at periastron. |
pulsar | Space and Astronomy | 1 | A highly magnetized rotating neutron star or white dwarf that emits a beam of electromagnetic radiation. This beam is observed only when it is pointing toward Earth, making the object appear to pulse. |
quadratic field strength | Space and Astronomy | 1 | A method of computing the mean strength of a varying stellar magnetic field. It is determined by calculating the root mean square of a series of longitudinal magnetic field strength measurements taken at different times. |
quadrature | Space and Astronomy | 1 | A configuration in which two celestial bodies have apparent ecliptic longitudes that differ by 90 degrees as viewed from a third body, e.g. when a planet's elongation is perpendicular to the direction of the Sun as viewed from the Earth. The term is used especially to describe the position of a superior planet or the Moon at its first and last quarter phases. |
quasar | Space and Astronomy | 1 | An extremely distant and old celestial object outside of our galaxy. They appear star-like but have power outputs several thousand times that of our entire galaxy. They may be the centre of other galaxies, with their brilliance coming from stars and gases being sucked into a black hole at their nucleus. |
radial velocity | Space and Astronomy | 1 | The velocity of an object along the line of sight to the observer, which in astronomy is usually determined via Doppler spectroscopy. Positive values are used to indicate a receding object. An object such as a star can undergo changes in its radial velocity because of the gravitational perturbation of another body, or because of radial pulsations of its surface. The latter, for example, occurs with a Beta Cephei variable star. |
radio astronomy | Space and Astronomy | 1 | The subfield of astronomy that studies astronomical objects at radio frequencies, using large radio antennas known as radio telescopes. |
radio source | Space and Astronomy | 1 | Any astronomical object that emits strong radio waves into space. These objects are the basis for radio astronomy. |
red-giant branch | Space and Astronomy | 1 | A conspicuous trail of enlarged red stars found on the Hertzsprung–Russell diagram for a typical globular cluster. It begins at the main-sequence turnoff point and extends toward the higher luminosity and lower temperature range until reaching the red-giant tip. This branch consists of older stars that have evolved away from the main sequence but have not yet initiated helium fusion in their core region. |
redshift | Space and Astronomy | 1 | An increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation. |
regular moon | Space and Astronomy | 1 | A natural satellite following a relatively close and prograde orbit with little or no orbital inclination or orbital eccentricity. Regular moons are thought to form in situ about their primary, as opposed to irregular moons, which are thought to be captured. |
relativistic jet | Space and Astronomy | 1 | A beam of ionised matter accelerated close to the speed of light. Most have been observationally associated with central black holes of some active galaxies, radio galaxies or quasars. |
réseau | Space and Astronomy | 1 | A grid of fine lines or crosshatches engraved upon a transparent glass plate, which when placed in front of film during a photographic exposure produces a corresponding grid in the resulting photograph by creating permanent shadows on the film negative. These grids are used in some photographic telescopes to produce reference markers in photographs of distant stars, allowing precise and convenient measurement of astrometric positions. |
retrograde motion | Space and Astronomy | 1 | Orbital or rotational motion of an object in the direction opposite the rotation of the object's primary. The direction of rotation is determined by an inertial frame of reference such as the fixed stars. Contrast prograde motion. |
right ascension | Space and Astronomy | 1 | In the equatorial coordinate system, the celestial equivalent of terrestrial longitude. It divides the celestial equator into 24 hours, each of 60 minutes. |
ring system | Space and Astronomy | 1 | A disk- or ring-shaped accumulation of various solid material such as dust and moonlets that orbits an astronomical body such as a planet. Ring systems are common components of satellite systems around giant planets, as with the Rings of Saturn. See also circumplanetary disk. |
roche limit | Space and Astronomy | 1 | The distance from an astronomical object at which the tidal force matches an orbiting body's gravitational self-attraction. Inside this limit, the tidal forces will cause the orbiting body to disintegrate, usually to disperse and form a ring. Outside this limit, loose material will tend to coalesce. |
rogue planet | Space and Astronomy | 1 | Any planetary-mass object that orbits a galactic center directly rather than a star or substellar object. Such objects have often been ejected from the planetary system in which they formed, or otherwise have never been gravitationally bound to any star system. |
rosseland optical depth | Space and Astronomy | 1 | An extinction coefficient of an atmosphere, which describes the net opacity to radiation at a given depth. See optical depth. |
rotation period | Space and Astronomy | 1 | The time that an object takes to complete a single revolution about its own axis of rotation relative to the background stars. It is not necessarily the same as the object's synodic day or sidereal day. |
rotational modulation | Space and Astronomy | 1 | A phenomenon which causes the luminosity of a star to vary as rotation carries star spots or other localized activity across the line of sight. Examples include RS CVn and BY Dra variables. |
saber's beads | Space and Astronomy | 1 | A broken arc of illuminations seen at the limb of very young or very old lunar crescents. The visual similarity to the moments before and after a total solar eclipse was first noted by American astronomer Stephen Saber. |
satellite galaxy | Space and Astronomy | 1 | A smaller companion galaxy that orbits within the gravitational potential of a more massive and luminous host galaxy; e.g. the Large Magellanic Cloud is a satellite galaxy of the Milky Way. |
scattered disc | Space and Astronomy | 1 | A distant circumstellar disc in the Solar System that is sparsely populated by icy small Solar System bodies, which are a subset of the broader family of trans-Neptunian objects. |
scintillation | Space and Astronomy | 1 | Rapid variations in the apparent brightness, color, or position of a star (or any other distant luminous object) as viewed through a medium, such as the Earth's atmosphere, caused by the passing of light through layers of turbulence in the medium. Most terrestrial scintillation effects are the result of atmospheric refraction caused by small-scale fluctuations in air density, and are much more pronounced near the horizon, since light rays near the horizon must travel longer paths through the atmosphere before reaching the observer. |
secular | Space and Astronomy | 1 | Continuing, or changing in a non-periodic way, over a long period of time. |
secular motion | Space and Astronomy | 1 | Any change in movement that happens over a very long time period. Examples include the perihelion precession of Mercury, the tidal acceleration of the Earth–Moon system, and precession of the Earth's axis. |
selenocentric | Space and Astronomy | 1 | With reference to, or pertaining to, the geometric center of the Earth's Moon. |
semi-diameter | Space and Astronomy | 1 | The angle at the position of an observer subtended by the equatorial radius of the Sun, the Moon, or a planet. |
semi-major axis | Space and Astronomy | 1 | One half of the longest diameter (the major axis) of an ellipse. It is expressed in units of length and often used to give a physical dimension to a two-body elliptical Kepler orbit, such as for a binary star system or star–planet system. When the distance between the orbiting bodies is unknown, the semi-major axis may be given as an angle. |
september equinox | Space and Astronomy | 1 | The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending southward at each zenith passage. It represents the moment at which the North Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere. Contrast March equinox. |
sidereal day | Space and Astronomy | 1 | The rotation period of an object (e.g. the Earth) with respect to the distant fixed stars of its own celestial sphere (rather than to its primary star, e.g. the Sun), measured as the time it takes for the fixed stars, as viewed from a particular point on the object's surface, to return to the same position in the sky on consecutive nights. The Earth's sidereal day is equal to approximately 86,164.09 seconds (23 hours, 56 minutes, 4.09 seconds), about four minutes shorter than the solar day, which instead reckons time based on the Sun's position in the sky. |
sidereal period | Space and Astronomy | 1 | The orbital period of an object within the Solar System, e.g. the Earth's orbital period around the Sun. The name "sidereal" implies that the object returns to the same position relative to the fixed stars of the celestial sphere as observed from the Earth. |
sidereal time | Space and Astronomy | 1 | The calculation of the passage of time based on the diurnal motion of the fixed stars in the Earth's sky. The fundamental unit of sidereal time is the sidereal day, i.e. the time interval between two successive returns of the fixed stars to the local meridian, as viewed from a given location on the Earth's surface. |
sidereal year | Space and Astronomy | 1 | The time that Earth or another planetary body takes to orbit the Sun once with respect to the fixed stars. |
sky | Space and Astronomy | 1 | Everything that lies above the surface of the Earth, including the atmosphere and outer space. In the context of astronomy, the term "sky" is also used as another name for the celestial sphere. See also night sky. |
small solar system body | Space and Astronomy | 1 | An object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The SSSBs are: the comets; the classical asteroids, with the exception of the dwarf planet Ceres; the trojans; and the centaurs and trans-Neptunian objects, with the exception of the dwarf planets. |
sssb | Space and Astronomy | 1 | An object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The SSSBs are: the comets; the classical asteroids, with the exception of the dwarf planet Ceres; the trojans; and the centaurs and trans-Neptunian objects, with the exception of the dwarf planets. |
solar day | Space and Astronomy | 1 | A synodic day on Earth, i.e. the rotation of the Earth with respect to the Sun, measured as the time it takes for the Sun, as viewed from a particular point on the Earth's surface, to return to the same position in the sky (e.g. to cross the same meridian) on consecutive days. Because the Earth's orbit around the Sun affects the angle at which the Sun is seen from the Earth, the Sun appears to take slightly longer to return to the same position than do the fixed stars, which results in the solar day being on average about four minutes longer than the sidereal day. The length of the solar day is also not constant, but rather changes over the course of the year because the Earth's orbit is not perfectly circular and because its rotational axis is not perpendicular to its orbital plane. One mean solar day (averaged over the Earth's orbital period) is currently equal to 86,400 seconds, or exactly 24 hours. |
solar eclipse | Space and Astronomy | 1 | An occultation of the Sun by the Earth's Moon, in which a portion of the Earth passes through the shadow cast by the Moon, temporarily blocking sunlight, fully or partially, from reaching that portion of the Earth's surface. A solar eclipse occurs when the Moon is precisely aligned between the Sun and the Earth. Because all three bodies are continuously moving, the shadow of the Moon traces out a narrow path across the Earth's surface, and from any given location within or very close to this path, the eclipse is visible only for a short duration. Depending on the observer's location and on the apparent sizes of the solar and lunar disks in the sky, an eclipse may appear to be total, partial, or annular. |
solar facula | Space and Astronomy | 1 | Solar faculae are bright spots in the photosphere of the Sun that form in the canyons between solar granules. They are produced by concentrations of magnetic field lines. The Sun's faculae are most readily observed near the solar limb. Faculae occur on other stars (see facula). |
solar flare | Space and Astronomy | 1 | A surface of the Sun event that is seen as an intense variation in brightness on or near its surface. It occurs as a result of the sudden release of magnetic energy that has built up in the solar atmosphere. |
solar granule | Space and Astronomy | 1 | A convection cell in the Sun's photosphere |
solar jet | Space and Astronomy | 1 | Transient, collimated flows of plasma in the Sun's atmosphere. |
solar mass | Space and Astronomy | 1 | A standard unit of mass equal to the mass of the Earth's Sun, or approximately 1.98847×1030 kg. It is commonly used to express the masses of other stars and astronomical objects relative to the Sun. |
solar moss | Space and Astronomy | 1 | A feature in the Sun's atmosphere that appears as bright, "sponge-like" patches in extreme ultraviolet light, occurring above the Sun's visible surface at the base of hot coronal loops in active regions. |
solar prominence | Space and Astronomy | 1 | A large, bright, transient feature, often in the shape of a loop, consisting of plasma extending outward from the Sun's photosphere into the corona. Prominences may be hundreds of thousands of kilometers long. |
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