Thisglossary of astronomy is a list of definitions of terms and concepts relevant toastronomy andcosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study ofcelestial objects and phenomena that originate outside theatmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of sophisticated vocabulary.
In theHarvard spectral classification system, a class ofmain-sequence star having spectra dominated by Balmer absorption lines of hydrogen.Main-sequence stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 timesthe mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.[1]
A roughly circular mass of diffuse material inorbit around a central object, such as astar orblack hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.[4]
A compact region in the center of agalaxy displaying a much higher than normalluminosity over some part of theelectromagnetic spectrum with characteristics indicating that the luminosity is not produced bystars. A galaxy hosting an AGN is called anactive galaxy.[5]
An airplane with an astronomical telescope, which relies upon altitude to reduceatmospheric absorption and improveseeing conditions. Drawbacks include the instability of the lifting platform and higher costs.
A measure of the proportion of the totalsolar radiation received by anastronomical body, such as aplanet, that isdiffusely reflected away from the body. It is a dimensionless quantity typically measured on a scale from 0 (indicating total absorption of all incident radiation, as by ablack body) to 1 (indicating total reflection). The albedo reported for an astronomical body may vary widely by the spectral and angular distribution of the incident radiation, by the "layer" of the body being measured (e.g. upper atmosphere versus surface), and by local variation within these layers (e.g.cloud cover and geological or environmental surface features).
The boundary separating a star'scorona from thestellar wind, defined as the point at which the coronal plasma'sAlfvén speed equals the large-scale stellar wind speed.
Achemically peculiar star belonging to the more general class ofA-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. They are generally slow rotators, and are often found inbinary systems wheretidal braking has slowed the rate of spin.[6] Seemetallicity.
A measure of the brightness of a celestial body as seen by an observer on Earth, adjusted to the value it would have in the absence of theatmosphere. The brighter the object appears, the lower its magnitude.
In theorbit of aplanetary body, one of the two extreme points of distance between the body and itsprimary – either the point of minimal distance, called theperiapsis, or the point of maximal distance, called theapoapsis. The term may also be used to refer to the value of the distance rather than the point itself. Allelliptical orbits have exactly two apsides.
An object that has been intentionally placed intoorbit by humans, often around the Earth but also around other bodies within theSolar System.[8] Contrastnatural satellite.
Theorbital node at which an orbiting object moves north through theplane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of theSolar System). The position of the ascending node with respect to a reference direction, called thelongitude of the ascending node, is used along with otherparameters to describe an orbit. Contrastdescending node.
aspect
The position of aplanet or Earth'sMoon with respect to theSun, as viewed from Earth.[9]
Any pattern ofstars recognizable in Earth'snight sky. An asterism may form part of an officialconstellation or it may be composed of stars from more than one constellation.
Aminor planet of the innerSolar System. They are primarily found orbiting theSun between Jupiter and Mars, but canapproach the Earth or occupytrojan orbits with Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects:small Solar System bodies primarily composed of dust and ice instead of mineral and rock are known ascomets; bodies less than one meter in diameter are known asmeteoroids; very large asteroids are sometimes calledplanetoids orplanetesimals; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known asdistant minor planets.
Thecircumstellar disc in theSolar System located roughly between the orbits ofMars andJupiter that is occupied by numerous irregularly shapedsmall Solar System bodies ranging in size from dust particles toasteroids andminor planets. The asteroid belt is often called themain asteroid belt ormain belt to distinguish it from other asteroid populations in other parts of the Solar System.
A field that studies the geology of solidified bodies such as the planets and their moons, asteroids, comets, and meteorites. Investigations are centered around the composition, structure, processes, and history of these objects.
A type of naturally occurring physical entity, association, or structure within theobservable universe that is a single, tightly bound, contiguous structure, such as astar,planet,moon, orasteroid. Though the terms astronomical "body" andastronomical "object" are often used interchangeably, there are technical distinctions.
A list of astronomical objects, typically grouped together because they share a common type, morphology, origin, means of detection, or method of discovery.
A type of naturally occurring physical entity, association, or structure that exists within theobservable universe but is a more complex, less cohesively bound structure than anastronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as aplanetary system,star cluster,nebula, orgalaxy. Though the terms astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.
Any abstract pictorial symbol used to represent one or moreastronomical objects, events, or theoretical constructs, e.g. those of theplanets of theSolar System, thephases of the Moon, thezodiacalconstellations, and thesolstices andequinoxes. Many of these symbols were commonly used historically, though in the modern era they are usually limited toalmanacs andastrology, and their appearance in scientific literature has become increasingly infrequent. Exceptions include the symbols for the Sun (☉), the Earth (🜨), and the Moon (☾), which are sometimes used forastronomical constants and in other forms of shorthand.
A unit of length used primarily for measuring distances within theSolar System, or secondarily between the Earth and distant stars. Originally conceived as thesemi-major axis of theEarth's orbit around the Sun,[3] the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10−6 parsecs; 1.5813×10−5 light-years).
An envelope of gases surrounding an astronomical body such as a planet and held in place by its gravity. This shell of gas has no clearly defined exterior boundary, but instead grows increasingly tenuous with altitude. The term can also be applied to a stellar atmosphere, referring to the visible outer layers of a star.
The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between itsequatorial plane andorbital plane. Axial tilt usually does not change considerably during a singleorbital period; Earth's axial tilt is the cause of theseasons. Axial tilt is distinct fromorbital inclination.
The imaginary central line around which a compact body such as a star or planet undergoes circular rotation. On Earth, the points where this axis intersect the surface define thegeographical poles. A rotational axis can "wobble" due toprecession andnutation.
An angular measurement of an object's orientation along the horizon of the observer, relative to the direction oftrue north. When combined with thealtitude above the horizon, it defines an object's current position in thespherical coordinate system.
The commoncenter of mass about which any two or more bodies of a gravitationally bound systemorbit. The barycenter is one of thefoci of theelliptical orbit of each body participating in the system; its location is strongly influenced by themass of each body and the distances between them. For example, in aplanetary system where the mass of the centralstar is significantly larger than the mass of an orbitingplanet, the barycenter may actually be located within the radius of the star, such that the planet appears to orbit the star itself, though both bodies actually orbit the shared barycenter.
The process by which the class of subatomic particles known asbaryons were generated in the early Universe, including the means by which baryons outnumber antibaryons.
The prevailingcosmological model for the origin of theobservable universe. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
Astar system consisting of exactly twostars orbiting around their commonbarycenter. The term is often used interchangeably withdouble star, though the latter can also refer to anoptical double star, a type of optical illusion which is entirely distinct from true binary star systems.
A concentration of mass so compact that it creates a region of space from which not even light can escape. The outer boundary of this region is called theevent horizon.
The surface velocity at which thecentrifugal force generated by a rapidly spinning star matches the force ofNewtonian gravity. At higher rotational velocities, the star begins to eject matter from its surface.[11]
A diagram of the relationships between the Earth'saxis of rotation, itscelestial equator, and theplane of its orbit around the Sun, known as theecliptic. Note that the Earth's rotational axis is not perpendicular to the ecliptic but rather istilted; this means that the path of the Sun, as viewed from Earth, appears to move both above and below the celestial equator during the course of the year.
One of two coordinates in the Earth'ssky at which a hypothetical indefinite extension of the Earth'saxis of rotation "intersects" thecelestial sphere, i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which allfixed stars appear to revolve during the course of a day. The celestial poles form the north and south poles of theequatorial coordinate system.
A thin transition region of a star's outer atmosphere, positioned above the coolerphotosphere and below the hotcorona. For the Sun, the chromosphere is only visible during atotal eclipse, when it gives off a red glow due to BalmerHydrogen-alpha emission. Risingsolar spicules occur in this region ofmagnetic activity.
chromospheric activity index
A parameter indicating themagnetic activity in a star'schromosphere. One measure of this activity islogR′HK, whereR′HK is the ratio of theequivalent width of a star'ssingly ionized calciumH and K lines, after correction forphotospheric light, to thebolometricflux.[12] Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active (logR′HK above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1).[13]
Incelestial mechanics, clearing the neighborhood around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than itsnatural satellites or those otherwise under its gravitational influence.[15]
A numeric value that is used to compare the brightness of a star measured from differentfrequency bands of theelectromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.[3]
A relatively small, icy body that displays extended features when it approaches theSun. The energy from the Sun vaporizesvolatiles on a comet's surface, producing a visiblecoma around the cometary body. Sometimes a comet can produce a longtail radiating away from the Sun.
A property of two objects orbiting the same body whoseorbital periods are in arationalproportion. For example, the orbital period ofSaturn around the Sun is very nearly 5/2 the orbital period ofJupiter.
common proper motion
A term used to indicate that two or more stars share the same motion through space, within themargin of observational error. That is, either they have nearly the sameproper motion andradial velocity parameters, which may suggest that they aregravitationally bound or share a common origin,[16] or they are known to be gravitationally bound (in which case their proper motions may be rather different but average to be the same over time).
A phenomenon during which twoastronomical objects orspacecraft have either the sameright ascension or the sameecliptic longitude as observed from a third body (usually the Earth), such that, from the observer's perspective, the objects appear to closely approach each other in the sky.
A region on thecelestial sphere surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based inmythology, while the modern demarcation of their borders was established by theInternational Astronomical Union in 1930. Compareasterism.
An aura ofplasma that surrounds cooler stars such as theSun. It can be observed during asolar eclipse as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
An arch-like structure in the Sun'satmosphere made up of relatively denseplasma confined and isolated from the surrounding medium bymagnetic flux tubes.[17]
Dust which exists inouter space or has fallen on Earth, generally composed of fine particles of solid matter far smaller than those found in terrestrial dust.
A type ofradiation consisting of high-energyprotons and atomic nuclei which move through space at nearly the speed of light, and which may originate from theSun or from outside theSolar System. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
A type ofoptical spectrometer placed at theCoudé focus of areflecting telescope. The focus remains stationary as the telescope is re-oriented, which is advantageous for the stable mounting of heavy spectroscopic instruments.[18]
The apparent movement of anastronomical object (e.g. theSun, theMoon, aplanet, astar, aconstellation, etc.) across the observer's localmeridian. During each day, the Earth's rotation causes every astronomical object toappear to move along a circular path on thecelestial sphere, creating two points at which it crosses the meridian: anupper culmination, at which the object reaches its highest point above thehorizon, and alower culmination, at which it reaches its lowest point, nearly 12 hours later. When not otherwise qualified, thetime of culmination typically refers to the time at which the upper culmination occurs.[20]
A ring-shapedcircumstellar disc of dust and debris orbiting its host star. It is created by collisions betweenplanetesimals. A debris disk can be discerned from aninfrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.
Circumstellardebris disks as detected by theHubble Space Telescope, and artist's impressions of the disks' orientations around their host stars
Theorbital node at which an orbiting object moves south through theplane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of theSolar System). Contrastascending node.
The apparent motion of anastronomical object (e.g. theSun, aplanet, or a distantstar) around the twocelestial poles in the Earth'snight sky over the course of one day. Diurnal motion is caused by Earth's rotation about its own axis, such that every object appears to follow a circular path called thediurnal circle.
Any pair ofstars which appear near each other on the celestial sphere, either because the two stars coincidentally lie along nearly the same line of sight from the Earth, though they are in fact physically distant from each other, or because the two stars are actually located in physical proximity to each other, by which they may form a co-moving pair or abinary star system.
A hotter and more massive star, in contrast tolate-type stars that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually cooled off as they aged. It may be used to refer to the higher-temperature members of any particular population or category of stars, rather than of all stars in general.
Theplane of the ecliptic (grey) is defined by the Earth's orbit around the Sun, and is distinct from the plane of thecelestial sphere'sequator (green), which is permanentlytilted 23.4 degrees with respect to the ecliptic. To an observer on Earth, this means that the path the Sun appears to follow upon the celestial sphere is not a straight line (i.e. the ecliptic does not appear "flat"). Twice during each complete orbit, at the twoequinoxes, the Sun's path appears to intersect the celestial equator, though the two planes are never in fact coplanar.
A type ofgalaxy with an approximatelyellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along withspiral andlenticular galaxies.
A type ofKepler orbit with anorbital eccentricity of less than 1 (often inclusive ofcircular orbits, which have eccentricity equal to 0), or one with negativeenergy. Elliptical orbits take the shape of anellipse, and are very common in two-body astronomical systems.
A relatively small body (such as a planet) orbiting a larger one (such as a star) in anelliptical orbit, with the larger body located at one of the focal points of the ellipse
A list or table of the expected positions ofastronomical objects orartificial satellites in thesky at various dates and times.[3] Modern ephemerides are generally provided by computer software.
A moment in time used as a reference point for some time-varying astronomical quantity,[7] such as thecelestial coordinates ororbital elements of anastronomical object, because such quantities are subject toperturbations and change over time. The primary use of astronomical quantities specified by epochs is to calculate other relevant parameters of motion in order to predict future positions and velocities. In modern usage, astronomical quantities are often specified as a polynomial function of a particular time interval, with a given epoch as the temporal point of origin.
The imaginary line on a gravitationally rounded spheroid such as aplanet that represents the intersection of the spheroid's surface with aplane perpendicular to itsaxis of rotation and equidistant from itsgeographical poles. The plane of the Earth's terrestrial equator is the basis for thecelestial equator.
Either of the two precise times of year when the imaginary plane of the Earth'sequator,extended indefinitely in all directions, passes through the center of theSun (i.e. the two points at which this plane intersects theplane of the ecliptic); or, equivalently, when the Sun's apparent geocentriclongitude is either 0 degrees or 180 degrees.[22] The two equinoxes, known as thevernal equinox and theautumnal equinox, occur on or nearMarch 20 andSeptember 22 each year. On the day of an equinox, the center of the visible Sun appears to bedirectly above the equator, and the durations of day and night are approximately equal all over the planet. Comparesolstice.
The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object'skinetic energy andgravitational potential energy is equal to zero. It is a function of the mass of the body and of the distance between the object and the body'scenter of mass. An object which has achieved escape velocity is neither on the surface nor in a closedorbit of any radius.
A curve on theHertzsprung–Russell diagram that a solitarystar of a particular mass and composition is expected to follow during the course of itsevolution. This curve predicts the combination of temperature andluminosity that a star will have during part or all of its lifetime.[23]
Anyastronomical object that exists outside theSolar System. The term is generally not applied tostars or any objects larger than a star or the Solar System itself, such asgalaxies.
A bright spot on astar'sphotosphere formed by concentrations ofmagnetic field lines. For theSun in particular,solar faculae are most readily observed near thesolar limb. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.
Anygalaxy that does not belong to a larger cluster of galaxies and is gravitationally isolated.
field star
A randomly situatedstar that lies along theline of sight to a group of physically associated stars under study, such as astar cluster. These field stars are important to identify in order to prevent them from contaminating the results of a study.[24]
Theangular extent of the observable world that can be seen from a given place at a given moment. In astronomy, the field of view is theangular area viewed by an instrument such as a telescope, usually expressed insquare degrees, or for higher magnification instruments, in squarearc-minutes.
A term used to classify the brightest stars in the night sky, specifically those having anapparent magnitude lower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars.
The "background" ofastronomical objects in thenight sky which are so distant from observers on Earth that they do not appear to move relative to each other, as opposed to the "foreground" of objects within theSolar System which do. The fixed stars are typically taken to include allstars other than theSun, as well as all otherextrasolar anddeep-sky objects.
The branch of astronomy that studies objects and phenomena within theMilky Waygalaxy, as opposed to everything outside of the Milky Way, which is the domain ofextragalactic astronomy.
The region at the center of agalaxy, usually home to a very dense concentration of stars and gas. It almost always includes asupermassive black hole which, when active, can generate a much higherluminosity in a compact region than its surroundings. This excess luminosity is known as anactive galactic nucleus, and the brightest such active galaxies are known asquasars.
A large-scale structure consisting of hundreds or thousands ofgalaxies bound together by gravity. Galaxy clusters are distinct from similarly namedgalactic clusters and other types ofstar clusters and from smaller aggregates of galaxies known asgalaxy groups. Galaxy groups and galaxy clusters can themselves cluster together to formsuperclusters.
A gravitationally bound aggregation of up to 50galaxies, each at least as luminous as theMilky Way Galaxy. Larger aggregations may be calledgalaxy clusters, and galaxy groups and clusters can themselves cluster together to formsuperclusters.
A cataclysmic event that generates a brief but intense outburst ofgamma ray radiation which can be detected from billions oflight-years away. The source of most GRBs is theorized to besupernova orhypernova explosions of high-mass stars. Short GRBs may also result from the collision ofneutron stars.
With reference to, or pertaining to, thegeometric center of theEarth;[26] centered upon the Earth, e.g. a geocentric orbit.[3]
geocentric zenith
The point projected upon thecelestial sphere by a straight line that passes through thegeocenter and an observer; i.e. the observer'szenith as defined with respect to the center of the Earth.[26]
The ratio of the brightness of an astronomical body at aphase angle of zero to an idealized flat, fully reflecting,diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.
geometric position
The position of an object (celestial or otherwise) with respect to thecenter of the Earth or to the position of an observer, i.e. as defined by a straight line between the center of the Earth (or the observer) and the object at a given time, without any corrections forlight-time,aberration, etc.[26]
Acirculargeosynchronous orbit, which maintains a constant altitude of 35,786 kilometres (22,236 mi) directly above Earth'sequator in thesame direction asEarth's rotation such that, to an observer on Earth's surface, the orbiting object appears motionless, in a fixed position in the sky.Artificial satellites are often placed in geostationary orbit so that antennas on Earth do not have to rotate to track them.
A tight, spherical conglomeration of many thousands ofstars which are gravitationally bound to each other and whichorbit agalactic core as asatellite. They differ fromopen clusters in having a much higher combined mass, with a typical lifespan extending for billions of years.
Any very large distribution of mass, such as agalactic cluster, which can bend passing light from a distant source by a noticeable degree. The effect, known asgravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.
A luminous red galaxy (LRG) acting as agravitational lens, distorting the light from a much more distant blue galaxy into anEinstein ring
An ionizednebula powered by young, massiveO-type stars.Ultravioletphotons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly inspectral lines of hydrogen and other elements. Because O-type stars have relatively short lifetimes (typically a few million years), the presence of an H II region indicates that massive star formation has taken place recently at that location. H II regions are often found in the arms ofspiral galaxies and in star-formingirregular galaxies.
heliocenter
The precisegeometric center of the Earth'sSun, i.e. the arithmetic mean position of all points within the approximatespheroid that is the shape of the Sun.
The vast, bubble-like cavity in theinterstellar medium which surrounds and is created by theplasma emanating from the Earth'sSun. The heliosphere encompasses the entirety of theSolar System and a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.
A plot ofluminosity versuseffective temperature for a population ofstars; depending on the usage, the star'sabsolute magnitude may be substituted for luminosity, and itscolor index orspectral type for temperature. Single stars of known mass and composition follow predictabletracks across this chart over the course of theirevolution. Hence, knowing a star's mass andmetallicity allows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, includingmain-sequence,red giant, andwhite dwarf stars.
The approximate region around anastronomical object within which its gravitational attraction dominates the motions ofsatellites. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or thegalactic core. Satellites moving outside this radius tend to be perturbed away from the main body.[27]
The apparent boundary between the surface of a celestial body and its sky when viewed from the perspective of an observer on or near that body's surface; more specifically, theplane perpendicular to a line from an observer to thezenith that passes through the point of observation.[26]
For a givencelestial object, the angular distance on thecelestial sphere measured westward along thecelestial equator from the observer's localmeridian to thehour circle that passes through the celestial object;[26] or, equivalently, the angle between theplane containing Earth'srotational axis and thezenith, and the plane containing Earth's rotational axis and the object of interest. Analogous toright ascension, the hour angle is one of many ways commonly used to specify the longitudinal position of an object upon the celestial sphere.
Any imaginarygreat circle drawn upon the celestial sphere that passes through both of thecelestial poles and is therefore perpendicular to thecelestial equator.[26][7] Similar to ameridian but additionally taking into account the terrain and the depth to thegeocenter at a ground observer's particular location, the concept of the hour circle is employed to describe the longitudinal position of a celestial object relative to the observer's local meridian.
hybrid-chromosphere star
These hybrid stars are G and K giant and supergiant stars that display the spectra of a hotcoronae found in more massive giants and the cool stellar winds of M-type giants.[28] They can be a source of X-ray emission.[29]
A critical mass below which an astronomical object cannot sustain its surfaceluminosity through nuclear fusion. This mass limit, equal to about 7% of themass of the Sun, forms the dividing line betweenbrown dwarfs and hydrogen-fusing stars.[31]
An archaic term that is sometimes used to refer to the planetsMercury andVenus. The name originated from the fact that these planets orbit closer to theSun than the Earth and hence, in thegeocentriccosmology ofPtolemy, both appear to travel with the Sun across the sky. This is in contrast to the so-calledsuperior planets, such asMars, which appear to move independently of the Sun.
Thematter that exists in the space between thestars in agalaxy. This medium mainly consists ofhydrogen andhelium, but is enhanced by traces of other elements contributed by matter expelled from stars.
An effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, known asextinction. This effect causes more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon ofredshift.
AlsoLaplace's invariable plane or theLaplace plane.
The imaginary plane passing through thebarycenter of aplanetary system and perpendicular to itsangular momentum vector, and which may be regarded as the weighted average of all planetaryorbital and rotational planes comprising the system.
A curve on theHertzsprung–Russell diagram that represents theevolutionary positions of stars having the same age but differing masses. This is in contrast to anevolutionary track, which is a plot of stars having the same mass but differing ages. In fact, multiple evolutionary tracks can be used to build isochrones by putting curves through equal-age points along the tracks. When the mass of a star can be determined, an isochrone can be used to estimate the star's age.
A physical state in which aninterstellar cloud of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.
A unit of time defined as exactly 365.25 days of 86,400SI seconds each. Because these are units of constant duration, the Julian year is also constant and does not vary with a specific calendar or with any of the other means of determining the length of a year, such as thetropical year. It is therefore widely used as the basis for defining the standard astronomicalepoch and thelight-year.
The motion of oneorbiting body relative to another, as anellipse,parabola, orhyperbola, which forms a two-dimensionalorbital plane (or sometimes a straight line) in three-dimensional space. Kepler orbits are idealized mathematical constructions which consider only the point-like gravitational attraction of two bodies, neglecting more complex orbitalperturbations that may exist in reality.
Any of a set of points near two largebodies inorbit at which a smaller object will maintain a constant position relative to the larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies, but at the Lagrangian points thegravitational forces of the large bodies, thecentripetal force of orbital motion, and (in certain scenarios) theCoriolis acceleration all align in a way that causes the small object to become "locked" in a stable or nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically identified with the labelsL1 toL5. The phenomenon is the basis for the stable orbits oftrojan satellites and is commonly exploited byman-made satellites.
A slight oscillating motion of theMoon as seen from the Earth, a result of theMoon's elliptical orbit. It can allow normally hidden parts of the Moon'sfar side to be visible along thelimbs of the lunar disk.
A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at thespeed of light in vacuum would travel in oneJulian year: approximately 9.46 trillion kilometres (9.46×1012 km) or 5.88 trillion miles (5.88×1012 mi). Though the light-year is often used to measuregalactic-scale distances in non-specialist publications, the unit of length most commonly used in professionalastrometry is theparsec.
limb
The apparent circumferential edge of any celestial body with a detectable disk, e.g. the Sun, the Moon, a planet, or a small Solar System body.[26]
An optical effect seen in stars (including theSun), where the center part of the disk appears brighter than its edge orlimb.
line of apsides
The imaginary line connecting the twoapsides (theperiapsis and theapoapsis) of anelliptical orbit, and which therefore represents the distance of the orbit's longest axis.
AnX-ray optics design with an ultra widefield of view, based on the structure of the eyes of alobster. It allows X-ray light to enter from multiple angles, thereby capturing more X-rays from a larger area than otherX-ray telescopes.
The angle between a specified reference direction, called theorigin of longitude, and the direction of anorbit'sascending node, as measured on a specifiedplane of reference. The angle is typically measured eastwards from the reference direction to the ascending node (i.e. counterclockwise as seen from the north). It is one of six canonicalorbital elements used to characterize an orbit.
The shape of the portion of theMoon that is illuminated by direct sunlight as viewed from Earth. This shape is referred to as a phase because it gradually changes in a regular cycle over the course of asynodic month: as the orbital positions of the Moon around Earth and Earth around theSun change, the visibility of the side of the Moon thatconstantly faces Earth alternates between completely illuminated (known as afull moon) and completely darkened by the Moon's own shadow (known as anew moon). There are also intermediate phases, during which the visible side may be only partially sunlit, e.g. when the Moon appears as a crescent. During the part of the lunar cycle in which the illuminated portion is growing larger, the Moon is said to bewaxing; when the illuminated portion is becoming smaller, it is said to bewaning. The phase of the Moon at any particular time appears the same from every point on Earth.
Thephases of the Moon are caused by the visible side of the Moon being alternately illuminated by sunlight and immersed in shadow during its orbit around the Earth.
A numericallogarithmic scale indicating the brightness of anastronomical object, where the lower the value, the brighter the object. By convention, a first magnitude star is 100 times as bright as a sixth magnitude star. Magnitude 6 is considered the lower limit of objects that can be seen with thenaked eye, although this can vary depending on sky conditions and eyesight.
The precise time of year on Earth when the Sun appears to cross thecelestial equator, while generally trending northward at eachzenith passage. It represents the moment at which theNorth Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is thevernal equinox in the Northern Hemisphere and theautumnal equinox in the Southern Hemisphere. ContrastSeptember equinox.
The fraction of anelliptical orbit'speriod that has elapsed since the orbiting body passedperiapsis, expressed as the angular distance from thepericenter which a fictitious body would have if it moved in a perfectlycircular orbit in the sameorbital period as the actual body in its elliptical orbit. Unlike thetrue anomaly, the mean anomaly does not correspond to a real geometric angle but is instead a contrived parameter used to make calculating the position of the orbiting body in thetwo-body problem mathematically convenient.
An imaginary line running north–south across thesky and passing through the point directly overhead known as thezenith.
meridian astronomy
The measurement of positions of celestial objects based on observation of the times of theirtransit across themeridian and of theirzenith distance at those times, with the intention of obtaining accuratestar positions which are self-consistent over large areas of sky.[33]
One of a set of 110 "nebulous"astronomical objects, 103 of which were catalogued as non-comets by French comet hunterCharles Messier between 1771 and 1781. The Messier catalogue includes most of thedeep-sky objects easily visible from the Northern Hemisphere.
The visible passage of a glowingmeteoroid,micrometeoroid,comet, orasteroid through the Earth'satmosphere, usually as a long streak of light produced when such an object is heated toincandescence by collisions with air molecules in the upper atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also shedding material in its wake.
A solid piece of debris from ameteor that originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.
A series ofmeteors that seemingly radiate from a single area in thenight sky. These are produced by debris left over from a larger body, such as acomet, and hence they follow roughly the sameorbit. This makes many meteor showers predictable events, as they recur every year.
A measure of the abundance of elements other thanhydrogen andhelium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.
A very smallmeteorite that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50μm to 2mm. Micrometeorites are a major component ofcosmic dust.
A very smallmeteoroid, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed amicrometeorite.
microvariable
A stellar object such as avariable star that undergoes very small variations inluminosity, in which the amplitude of the fluctuations amounts to just a few thousandths of amagnitude. Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.[34]
1. Thebarred spiralgalaxy that includes the Earth'sSolar System. The name describes the galaxy's appearance from the Earth: a hazy band of light visible in thenight sky, formed from billions ofstars that cannot be individually distinguished by the naked eye. The Milky Way Galaxy has a diameter of 100,000–200,000light-years and is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located on the inner edge of one of the Milky Way's spiral arms, about 27,000 light-years from theGalactic Center, which theSun orbits with a period of 240 million years.
2. The hazy band of light itself, which from Earth appears as a band because the galaxy's disk-shaped structure is viewed side-on from within.
An object in directorbit around theSun that is neither a dominantplanet nor originally classified as acomet. Amoon is not a minor planet because it orbits another body instead of the Sun.
The solid, rocky body that orbits the Earth as its onlynatural satellite, completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible fortides on Earth; because oftidal locking, only one side of the Moon is ever visible from the Earth. Sunlight reflected from its surface makes the Moon appear very bright in thenight sky, though its orbital position with respect to the Earth and theSun causes its visibility to change in a regular cycle ofphases when viewed from the Earth. The adjectivallunar is often used specifically to describe the orbit, gravity, and other properties of the Earth's Moon.
Acluster of closely related stars which share a common motion in space (e.g. theHyades) such that, from the perspective of a distant observer, theirproper motions all appear to be directed toward a single convergent point. If the linear velocity of the cluster is known, then the distance of each star can be estimated from the total proper motion, a technique known asmoving cluster parallax.[33]
A loose grouping ofstars which travel together through space. Although the members were formed together in the samemolecular cloud, they have since moved too far apart to be gravitationally bound as acluster.
A type ofastronomy based on the acquisition of information aboutastronomical objects through the coordinated observation and interpretation of four disparate classes of "messenger" signals withextrasolar origins:electromagnetic radiation,gravitational waves,neutrinos, andcosmic rays. Because these four extrasolar messengers are created by different astrophysical processes, their presence or absence during a celestial event can reveal useful information about their sources.
An early classification foractive galaxies that had the visual appearance of a galaxy with a particularly bright, star-like nucleus. As a group, they are intermediate betweenSeyfert galaxies andquasars. Most aregiant ellipticals that are radio sources and display prominentemission lines.[38]
The point on thecelestial sphere that is exactly opposite thezenith.[7] Thus, where the zenith is directly above an observer, the nadir is underfoot. The zenith and nadir form the two poles of thehorizon line.
The human eye as used without any magnifying or light-collecting optical aid, such as a telescope, nor any eye protection. Many astronomical objects emit or reflect visible light that is sufficiently bright to fall within the limits of normal human visual perception, allowing observers to see them from the Earth's surface without any special equipment. Vision corrected to normal acuity using eyeglasses or contact lenses is still considered unaided.
A type of elementary particle, electrically neutral and with an extremely small rest mass, that interacts with other particles only via the weak interaction and the gravitational interaction. Neutrinos therefore typically pass through normal matter unimpeded and undetected.
A type ofcompact star that is composed almost entirely ofneutrons, which are a type ofsubatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of theSun, but with a radius of only 12 km (7.5 mi), making them among the densest known objects in the universe.
The appearance of the Earth'ssky atnighttime, when theSun is below thehorizon, and more specifically when clear weather and low levels of ambient light permit visibility ofcelestial objects such asstars,planets, and theMoon. The night sky remains a fundamental setting for both amateur and professionalobservational astronomy.
The quantity of some specified particle or object class per unit volume. For atoms, molecules, or subatomic particles, the volume is typically expressed in cm3 or m3. With stars, cubic parsecs (pc3) are often used.
A continuous, gravity-induced change in the orientation of anastronomical body'saxis of rotation which results from the combined effects of small, short-term variations. Nutation is distinguished fromprecession, which is a similar but longer-term change in axial orientation.
A diagram ofobserved minuscalculated values over time, showing how observed data differ from theoretical values which have been calculated according to a particularscientific model. It is often used as a diagnostic tool to determine the accuracy of the model. With avariable star, it is typically used to compare phase differences over time.[39]
A group of massivestars 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 ofstellar classifications O and B.
The duration of time between the earliest and latestobservations made by astronomers of an object within theSolar 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 ofasteroids andcomets, 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 itsorbit around the Sun (or otherprimary), and therefore result in a high degree ofuncertainty when estimating the shape and characteristics of the object's orbit.
The practice and study of directly observingastronomical objects with the use oftelescopes and other astronomical instruments. It is concerned with recording data about theobservable universe, as opposed totheoretical astronomy, which is concerned with calculating the measurable implications of astronomical models.
A celestial event that occurs when a distantastronomical body orobject is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view.Solar andlunar eclipses are specific types of occultations.
A vast theoretical cloud of predominantly icyplanetesimals hypothesized to surround theSun at distances ranging from 2,000 to 200,000AU. It is thought to be divided into two regions: a disc-shapedinner Oort cloud and a spherical outer Oort cloud. The outer limit of the Oort cloud is often considered the cosmographical boundary of theSolar System.
A measure of the resistance of a medium to the radiative transmission of energy. Within a star, it is an important factor in determining whetherconvection occurs.
The positioning of two celestial objects on opposite sides of the sky, as seen 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.
Thegravitationally curved trajectory of anobject, such as the trajectory of aplanet moving around astar or anatural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximatelyelliptical orbits around a commoncenter 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.
A parameter that determines how much anorbit deviates from a perfect circle. For anelliptical orbit, the eccentricity ranges from greater than zero to less than one.[3]
The tilt of an object'sorbit around an astronomical body, expressed as the angle between theorbital plane or axis of direction of the orbiting object and a specifiedplane of reference.[7][3]
The situation that occurs when two or moreorbiting bodies exert regular, periodic gravitational influences on each other such that one or more of theirorbital 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 tomean-motion orbital resonance, in which the bodies'orbital periods are related by a ratio of small integers. For example, thedwarf planetPluto exists in a stable 2:3 resonance withNeptune, such that Pluto completes two orbits around theSun in the same time it takes Neptune to complete three. Resonance may act on any time scale, from short-term tosecular, and often leads to either long-term stabilization of the orbits or their eventual destabilization.
A planetary system showing a 1:2 orbital resonance between the orbital periods of two planets (small bodies), both of which are orbiting a large central star. The inner planet completes two revolutions in the time it takes the outer planet to complete one.
Thespeed at which an astronomical body or objectorbits around abarycenter, 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 entireorbital period, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs atperiapsis.
The hypothetical, idealizedKepler orbit that an orbiting object would follow around itsprimary if allperturbations were absent, i.e. the orbit that coincides with the instantaneousorbital state vectors at a given moment in time.[41]
The vast,nearly empty expanse that exists beyond the Earth and between allcelestial bodies, characterized generally by extremely low densities of particles, extremely low temperatures, andminimal gravity. Most of the volume of theuniverse is intergalactic space, and even galaxies and star systems consist almost entirely of empty space.
1. The angular difference between an object’s apparent direction as seen from two different points of observation. In astronomy, when not otherwise qualified, the term is usually taken to mean the difference in the apparent position of a celestial object (relative to more distant background objects) as seen from opposite sides of the Earth’s orbit, known as thetrigonometric parallax.[42]
2. The angular distance between two points as seen from a third point in space, such as the radius of the Earth’s orbit as seen from a star. By extension, the term can also be used to mean a star’s distance, even if measured by some indirect method such asphotometry orspectroscopy.[42]
Theparallax shift of a star at a distance of oneparsec as seen from the Earth (not to scale)
A unit of length defined as the distance at which a star would show aparallax shift of exactly onearcsecond as observed from Earth's orbit. It is equal to 3.2616 light-years or 206,265 astronomical units. The word "parsec" is aportmanteau of the wordsparallax andsecond.
The brightestabsolute magnitude achieved during the periodic rise in luminosity that characterizes avariable star. This data point can provide useful distance information for acataclysmic variable and can be determined from alight curve of the stellar variability.
The complex motion of an astronomical body that is subject to forces other than the gravitational attraction of itsprimary alone, or any force which complicates theorbital characteristics of the body such that the idealizedKepler orbit of thetwo-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/oratmospheric resistance.
Theelongation or angle between an orbiting body and the Sun as viewed from a particular perspective, such as from the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow.Inferior planets such asVenus generally have low phase angles as seen from Earth, so they often appear as a slim crescent;superior planets such asMars andJupiter usually have high phase angles, so that little of the shadowed side is visible.
An imaginary plane that is perpendicular to the line of sight. Typically this is used as a reference plane for determining theinclination of an orbital plane of a distant star system.[43][3]
The process of separating out different constituents of a planetary body, causing it to develop compositionally distinct layers (such as a metallic core).
A type ofemission nebula formed from a glowing shell of expanding plasma that has been ejected from ared giant star late in its life. The name derives from their resemblance to aplanet. An example is theRing Nebula.
The scientific study ofplanets,moons, andplanetary systems, with the aim of understanding their formation, composition, topography, dynamics, and interactions with other bodies.
Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of aplanet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gasdrag. The term is most commonly applied to small bodies thought to exist inprotoplanetary disks anddebris disks during the process of planet formation, but is also sometimes used to refer to various types ofsmall Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and aprotoplanet.
Anorbit in which the orbiting object passes directly over or nearly over bothpoles of the body being orbited during each revolution. It therefore has aninclination equal or nearly equal to 90 degrees to the body'sequator.
Any slow change in the orientation of an object'saxis of rotation. For the Earth in particular, this phenomenon is referred to as theprecession of the equinoxes.Apsidal precession refers to a steady change in the orientation of an orbit, such as the precession in the orbit ofMercury that was explained by the theory ofgeneral relativity.
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'sbarycenter.
The observed physical separation between two astronomical objects, as determined from theirangular separation and estimated distance.[44] For planets anddouble stars, this distance is usually given inastronomical units. The actual separation of the two objects depends on the angle of the line between the two objects to theline-of-sight of the observer.
A largeplanetary embryo that originated within aprotoplanetary 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-sizedplanet; they are thought to form out of smallerplanetesimals as they collide with each other and gradually coalesce into larger bodies.
A concentration of mass formed by the contraction of a collapsinginterstellar cloud. Once sufficient mass has fallen onto this central core, it becomes apre-main-sequence star.
A highly magnetized rotatingneutron star orwhite 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.
A configuration in which two celestial bodies have apparentecliptic longitudes that differ by 90 degrees as viewed from a third body, e.g. when a planet'selongation is perpendicular to the direction of theSun as viewed from the Earth. The term is used especially to describe the position of asuperior planet or theMoon at its first and last quarter phases.
A moon or planet is said to be inquadrature when its position as viewed from Earth is at a right angle to the direction of the Sun. In such cases the moon or planet appears to be in its quarter phase (or nearly so), where half of the body is illuminated by the Sun and the other half is in shadow.
The velocity of an object along the line of sight to the observer, which in astronomy is usually determined viaDoppler 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 thegravitational perturbation of another body, or because of radial pulsations of its surface. The latter, for example, occurs with aBeta Cephei variable star.
A conspicuous trail of enlarged red stars found on theHertzsprung–Russell diagram for a typicalglobular cluster. It begins at themain-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 initiatedhelium fusion in theircore region.
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.
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.
The distance from an astronomical object at which thetidal 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 aring. Outside this limit, loose material will tend tocoalesce.
A phenomenon which causes theluminosity of a star to vary as rotation carriesstar spots or other localized activity across the line of sight. Examples includeRS CVn andBY Dra variables.[48]
Rapid variations in the apparentbrightness, color, or position of astar (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 ofturbulence in the medium. Most terrestrial scintillation effects are the result ofatmospheric refraction caused by small-scale fluctuations in air density, and are much more pronounced near thehorizon, since light rays near the horizon must travel longer paths through the atmosphere before reaching the observer.
secular
Continuing, or changing in a non-periodic way, over a long period of time.[9]
The movement or distortion of a telescopic image as a result of turbulence in the Earth’s atmosphere. It is thought to be caused by undulations between separate layers of air, which disturb the path of light. The scale of these undulations is often regarded as producing "cells" of seeing, each typically around 100–150 mm across at sea level, and larger at higher altitudes. A small telescope may look through individual cells, whose movements give rise to a sharp but wandering image, while a larger one may look through several at once, producing multiple images. The "boiling" sometimes visible at thelimb of the Moon or the Sun is another manifestation of poor seeing. TheAntoniadi scale is widely used by amateur astronomers to evaluate seeing conditions.[42]
selenocentric
With reference to, or pertaining to, the geometric center of the Earth'sMoon.[26]
One half of the longest diameter (the major axis) of anellipse. It is expressed in units of length and often used to give a physical dimension to a two-bodyellipticalKepler orbit, such as for abinary 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.
The precise time of year on Earth when the Sun appears to cross thecelestial equator, while generally trending southward at eachzenith passage. It represents the moment at which theNorth Pole of the Earth begins to tilt away from the Sun, and typically occurs on or near September 22 each year. It is theautumnal equinox in the Northern Hemisphere and thevernal equinox in the Southern Hemisphere. ContrastMarch equinox.
Therotation period of an object (e.g. the Earth) with respect to the distantfixed stars of its owncelestial sphere (rather than to its primary star, e.g. theSun), 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 thesky 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 thesolar day, which instead reckons time based on the Sun's position in the sky.
Theorbital period of an object within theSolar 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 thefixed stars of thecelestial sphere as observed from the Earth.
The calculation of the passage of time based on thediurnal motion of thefixed stars in the Earth'ssky.[26] The fundamental unit of sidereal time is thesidereal day, i.e. the time interval between two successive returns of the fixed stars to the localmeridian, as viewed from a given location on the Earth's surface.
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.
Asynodic day on Earth, i.e. therotation of the Earth with respect to theSun, 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 thesky (e.g. to cross the samemeridian) 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 thefixed stars, which results in the solar day being on average about four minutes longer than thesidereal 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 isnot perfectly circular and because its rotational axis isnot perpendicular to its orbital plane. Onemean solar day (averaged over the Earth's orbital period) is currently equal to 86,400 seconds, or exactly 24 hours.
Anoccultation of theSun by the Earth'sMoon, 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 isprecisely 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 betotal,partial, orannular.[26]
A bright spot visible in thephotosphere of theSun that forms in the canyons betweensolar granules. They are produced by concentrations ofmagnetic field lines. The Sun's faculae are most readily observed near thesolar limb.Faculae can also occur on other stars.
A standard unit of mass equal to the mass of the Earth'sSun, or approximately1.98847×1030kg. It is commonly used to express the masses of otherstars and astronomical objects relative to the Sun.
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.
A large, bright, transient feature, often in the shape of aloop, consisting of plasma extending outward from theSun'sphotosphere into thecorona. Prominences may be hundreds of thousands of kilometers long.
A standard unit of distance equal to the radius of the Earth'sSun (typically measured from the Sun's center to the layer in thephotosphere at which theoptical depth equals 2/3), or approximately 695,700 kilometres (432,300 mi). It is commonly used to express the radii of otherstars and astronomical objects relative to the Sun.
The calculation of the passage of time based on thediurnal motion of theSun in the Earth'ssky.[26] The fundamental unit of solar time is thesolar day, i.e. the time interval between two successive returns of the Sun to the localmeridian, as viewed from a given location on the Earth's surface. Because the duration of this interval changes during the Earth's orbit around the Sun,apparent solar time is distinguished frommean solar time. Solar time andsidereal time were employed by astronomers as time reckoning systems before the introduction ofephemeris time.
A stream of charged particles, primarilyprotons,electrons, andalpha particles, released from theSun'scorona and flowing outwards at up to 900 kilometres per second (2,000,000 mph) into interplanetary space.[9] Phenomena influenced by the solar wind includeaurora,geomagnetic storms, and the plasma tails ofcomets.
Either of the two precise times of year when theSun reaches its most northerly or most southerly point in thesky as seen from Earth; or, equivalently, when the Sun's apparent geocentriclongitude is either 90 degrees or 270 degrees. The solstices occur on or near June 20 and December 21 each year. TheJune solstice, called thesummer solstice in the Northern Hemisphere, is the annual date featuring the longest duration of daylight and the shortest duration of nighttime for any given point in the Northern Hemisphere; the reverse is true in the Southern Hemisphere, where the June date is thewinter solstice.
A type ofbinary star system where the individual components have not been resolved with atelescope. Instead, the evidence for the binarity comes from shifts observed in thespectrum. This is caused by theDoppler effect as theradial velocity of the components change over the course of each orbit.
The measurement and interpretation of theelectromagnetic spectrum emitted by a radiating source. Manyphysical properties of a source can be deduced from features and changes in its spectrum.
The Earth's atmosphere permits certain wavelengths of electromagnetic energy to pass through but reflects or absorbs others, making it difficult or impossible to detect them from the surface. As a result,spectroscopic instruments are often placed in orbit, above the atmosphere, where detection of all parts of the spectrum is uninhibited.
The rate of travel ofelectromagnetic radiation through a medium. The speed of light in avacuum is a universalphysical constant, denoted byc.Massless particles andgravitational waves also travel at the speed of light. Light speed forms an upper limit for how fast information and matter can travel through space, while the large-scale expansion of space itself is not restricted.
A branch ofobservational astronomy which is used to locate the positions of astronomical objects on thecelestial sphere as they would appear from a particular date, time, and location on Earth. It relies on the mathematical methods ofspherical geometry and the measurements ofastrometry.
The nominalgravitational acceleration of an object in a vacuum near the surface of the Earth, as a result ofEarth's gravity and, less importantly, thecentrifugal force generated by its rotation. It is by definition equal to9.80665 m/s2 (approximately32.17405 ft/s2).
A massive, luminousspheroid ofplasma held together by its owngravity which, for at least a portion of its life, radiates energy intoouter space due to thethermonuclear fusion ofhydrogen intohelium within its core. Astronomers can determine the mass, age, temperature, chemical composition, and many other properties of a star by observing its motion through space, itsluminosity, and itsemission spectrum.
Any small number ofstars that orbit each other, bound by gravitational attraction, such as abinary star system. In the broadest sense, very large groups of stars bound by gravitation such asstar clusters andgalaxies are also star systems. Star systems are distinct fromplanetary systems, which includeplanets and other bodies such ascomets.
Anygalaxy that has an anomalously high rate of star formation. The criteria for a starburst is a star formation rate that would normally consume the galaxy's available supply of unbound gas within a time period shorter than the age of the galaxy. Most starbursts occur as a result of galactic interactions, such as amerger.
starfield
Any set ofstars visible in an arbitrarily sized field of view of atelescope, usually in the context of some region of interest within thecelestial sphere.[50][51] For example, the starfield surrounding the starsBetelgeuse andRigel could be defined as encompassing some or all of theOrion constellation.
The outermost region of astar, located above the stellar core, radiation zone, and convection zone. Although it constitutes only a small portion of the star's mass, for some evolved stars the stellar envelope can encompass a significant fraction of the radius.
1. The region within the volume of a star that transports energy from the stellar core to thestellar atmosphere; or another name for the stellar atmosphere itself.
Thestar occupying the center of theSolar System. It is a massive, nearly perfect sphere of hotplasma, heated toincandescence bynuclear fusion reactions in itscore. The physical properties of the Sun are used as a standard to describe the mass, radius, and luminosity of other stars.
One of a class of very largeblack holes which possess masses ranging from hundreds of thousands to many billions of times themass of the Sun. These are typically found at agalactic core, where they can have a profound effect upon the evolution of the surroundinggalaxy.
The time it takes for an object to rotate once about its ownaxis (i.e. itsrotation period) relative to theprimary it is orbiting (rather than to the much more distantfixed stars). The synodic day may be described as the time between two consecutivesunrises (in the case where the primary is astar), which is not necessarily the same as thesidereal day. As it does on Earth, an object's synodic day may change slightly in duration over the course of theorbital period due toeccentricity andaxial tilt; Earth's synodic day is often called asolar day.
The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to thebackground stars visible in the second body'scelestial sphere. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in thesame location in thenight sky as observed from Earth, but can in principle be calculated with respect to the sky as observed from any body. It is related to but distinct from theorbital period, a result of the fact that both the body being studied (e.g.Jupiter) and the body from which it is being observed (e.g. Earth) are independently orbiting a third body (theSun).
The calculation of the passage of time based on successiveconjunctions of an astronomical object, such as aplanet (i.e. successive returns of the object to the sameaspect in the Earth'ssky).[26]
The component of thevelocity of a star or otherastronomical body that is perpendicular to the line of sight of theobserver (i.e. in the tangent plane). This component can be computed from the body's observedproper motion and its measured distance from the observer.[33]
A device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. It employs an assembly of components that collect and focus the incoming radiation, providing an enlarged view with higherluminosity and betterangular resolution than can be observed with thenaked eye alone.
telluric star
Astar with nearly featureless continuum spectra that can be used to correct for the effect oftelluric contamination of theEarth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800Å. These features need to be corrected for in order to more accurately measure the spectrum.[53]
The boundary within theheliosphere, approximately 75 to 90AU from theSun, beyond which thesolar wind slows to subsonic speeds (relative to the Sun) as a result of interactions with the localinterstellar medium.
A branch of astronomy that uses analytical and computational models based on principles from physics and chemistry to describe, explain, and model the properties of astronomical objects and phenomena, with the ultimate goal of accurately predicting the observable or testable consequences of those models.
The layer of theMilky Way galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and centered on thegalactic plane. Stars belonging to this population generally follow orbits that lie close to this plane.[54] This is in contrast to members of thethick disk population andhalo stars.
The transfer of momentum between an astronomical body and an orbiting satellite as the result oftidal forces. This can cause changes in therotation periods for both bodies as well as modification of their mutual orbit. A satellite in aprograde orbit will gradually recede from its primary while slowing the rotation rate of both bodies.
The difference in gravitational attraction between different points in a gravitational field; the residual or differential force of gravity which causes different points in space to be affected by gravity unevenly, such that a body is stretched along the line connecting it to the center of mass of another body due to spatial variations in gravitational potential.
The net result of continuedtidal braking such that, over the course of an orbit, there is no net transfer ofangular momentum between an astronomical body and its gravitational partner. When theorbital eccentricity is low, the result is that thesatellite orbits with the same face always pointed toward itsprimary.[55] An example is theMoon, which is tidally locked with the Earth.
tidal stream
A stream ofstars and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of agalaxy such as theMilky Way.[56]
tilt erosion
The gradual reduction of theobliquity of an orbitingsatellite due to tidal interactions.[57]
A measure of the orbital motion of a relatively small body (e.g. anasteroid orcomet) with respect to a larger, perturbing body (e.g. aplanet), used forrestricted three-body problems in which the three bodies all differ greatly in mass. The parameter is calculated from theorbital elements of each body, including the small body'ssemimajor axis,eccentricity, andinclination, and is useful in specifically identifying small bodies observed before and after planetary encounters, as its numerical value remains largely constant throughout the body's lifetime. It is also used to distinguish between different kinds of orbits which are characteristic of different classes of bodies.[33]
topocentric
With reference to, or pertaining to, a point on the surface of the Earth.[26][3]
Asolar eclipse in which the disk of the Earth'sSun is completelyobscured by the Earth'sMoon. At totality, this formation allows the Sun'scorona andprominences to be directly observed.
2. An astronomical event during which a celestial body or object passes visibly across the face of a much larger body. An example is thetransit of Venus across the face of theSun, which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the netluminosity from the two objects, thetransit method can be used to detectextrasolar planets as they pass in front of their host stars. A transit by an object that appears roughly the same size or larger than the body it is transiting is called anoccultation oreclipse.
A small celestial body (mostly asteroids) that shares the orbit of a larger body, remaining in a stable orbit approximately 60° ahead of or behind the main body near one of itsLagrangian points.
The angle between the direction ofperiapsis and the current position of an orbiting body as it moves along anelliptical orbit, as measured from the nearestfocus of the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being theeccentric anomaly and themean anomaly, and also one of six canonicalorbital elements used to characterize an orbit.
The time period immediately before sunrise and after sunset during which, despite the Sun being completely below the horizon, the scattering of sunlight by the Earth's atmosphere supplies significant illumination to the ambient environment. Several definitions of twilight are commonly distinguished, includingastronomical,civil, andnautical twilight.[26]
1. The entirety ofspace andtime and their contents, includinggalaxies,stars,planets, all other forms ofmatter andenergy, and thephysical laws andconstants that describe them. When not otherwise qualified, "the universe" usually refers to theentire universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from theobservable universe, whose size it is possible to measure.
2. One of many hypothetical parallel universes which exist ascausally disconnected constituent parts of a largermultiverse, which itself comprises all of space and time and their contents.
Anystar that is observed to vary in brightness. This variation may be periodic, with one or more cycles that last hours, days, months, or even years. Some stars vary in an irregular manner, while others undergo cataclysmic changes in brightness. Other forms of variability are intrinsic changes to the star'sradial velocity or its profile ofspectral lines.
A reference to the faintness of thespectral lines for a star compared to standard stars with the samestellar classification. Since most absorption lines are caused by elements other than hydrogen and helium—what astronomers refer to as "metals"—these are sometimes called metal weak stars.[59]
A type ofstellar remnant composed mostly ofelectron-degenerate matter. A white dwarf lacks the mass needed to continue thenuclear fusion process with its constituent atoms, so the object's energy output normally comes from radiative cooling. See alsonova.
A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and theabsolute visual magnitude of the emittinglate-type stars. This linear relation makes it useful for determining the distances of G, K, and M-type stars.[60]
The area of thesky that extends approximately 8 degrees north or south (incelestial latitude) of theecliptic, the apparent path of theSun across thecelestial sphere over the course of the year as observed from Earth. The Sun,Moon, and visible planets appear to travel across a band of twelveZodiac constellations within this belt as the Earth orbits the Sun.[63]
A faint glow in the night sky fromsunlight reflected by aninterplanetary dust cloud. It is concentrated near the plane of thezodiac, orecliptic, particularly toward the Sun. The dust is mostly the result of cometary collisions, with a contribution from asteroids.[64]
The region of the sky obscured by theMilky Way’s disk, making it difficult for observers on Earth and telescopes within the Solar System to observe distant objects behind it.
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