This glossary of astronomy is a list of definitions of terms and concepts relevant to astronomy and cosmology, their sub-disciplines, and related fields. Astronomy is concerned with the study of celestial objects and phenomena that originate outside the atmosphere of Earth. The field of astronomy features an extensive vocabulary and a significant amount of jargon.
In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
A measure of a star's absolute brightness. It is defined as the apparent magnitude the star would show if it were located at a distance of 10 parsecs, or 32.6 light-years.
A roughly circular mass of diffuse material in orbit around a central object, such as a star or black hole. The material is acquired from a source external to the central object, and friction causes it to spiral inward towards the object.
A compact region in the center of a galaxy displaying a much higher than normal luminosity over some part of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars. A galaxy hosting an AGN is called an active galaxy.
A measure of the proportion of the total solar radiation received by an astronomical body, such as a planet, that is diffusely 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 a black 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).
A chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals. See metallicity.
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 the atmosphere. The brighter the object appears, the lower its magnitude.
In the orbit of a planetary body, one of the two extreme points of distance between the body and its primary – either the point of minimal distance, called the periapsis, or the point of maximal distance, called the apoapsis. The term may also be used to refer to the value of the distance rather than the point itself. All elliptical orbits have exactly two apsides.
An object that has been intentionally placed into orbit by humans, often around the Earth but also around other bodies within the Solar System. Contrast natural satellite.
The orbital node at which an orbiting object moves north through the plane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves away from the observer (in orbits outside of the Solar System). The position of the ascending node with respect to a reference direction, called the longitude of the ascending node, is used along with other parameters to describe an orbit. Contrast descending node.
aspect
The position of a planet or Earth's Moon with respect to the Sun, as viewed from Earth.[1]
Any pattern of stars recognizable in Earth's night sky. An asterism may form part of an official constellation or it may be composed of stars from more than one constellation.
A minor planet of the inner Solar System, i.e. one that orbits the Sun at a distance no greater than the orbit of 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 as comets; bodies less than one meter in diameter are known as meteoroids; very large asteroids are sometimes called planetoids or planetesimals; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as distant minor planets.
The circumstellar disc in the Solar System located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is often called the main asteroid belt or main 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 the observable universe that is a single, tightly bound, contiguous structure, such as a star, planet, moon, or asteroid. Though the terms astronomical "body" and astronomical "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 the observable universe but is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. 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 more astronomical objects, events, or theoretical constructs, e.g. those of the planets of the Solar System, the phases of the Moon, the zodiacalconstellations, and the solstices and equinoxes. Many of these symbols were commonly used historically, though in the modern era they are usually limited to almanacs and astrology, 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 for astronomical constants and in other forms of shorthand.
A unit of length used primarily for measuring distances within the Solar System or secondarily between the Earth and distant stars. Originally conceived as the semimajor axis of the Earth's orbit around the Sun, 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).
A gaseous envelope held in place by the gravity of a planet. 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 its equatorial plane and orbital plane. Axial tilt usually does not change considerably during a single orbital period; Earth's axial tilt is the cause of the seasons. Axial tilt is distinct from orbital inclination.
An angular measurement of an object's orientation along the horizon of the observer, relative to the direction of true north. When combined with the altitude above the horizon, it defines an object's current position in the spherical coordinate system.
B
Two bodies of similar mass orbit a common barycenter external to both, as usual in binary star systems
The common center of mass about which any two or more bodies of a gravitationally bound system orbit. The barycenter is one of the foci of the elliptical orbit of each body participating in the system; its location is strongly influenced by the mass of each body and the distances between them. For example, in a planetary system where the mass of the central star is significantly larger than the mass of an orbiting planet, 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 as baryons were generated in the early Universe, including the means by which baryons outnumber antibaryons.
The prevailing cosmological model for the origin of the observable universe. It depicts a starting condition of extremely high density and temperature, followed by an ongoing expansion that led to the current conditions.
A star system consisting of exactly two stars orbiting around their common barycenter. The term is often used interchangeably with double star, though the latter can also refer to an optical 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 the event horizon.
The surface velocity at which the centrifugal force generated by a rapidly spinning star matches the force of Newtonian gravity. At rotational velocities beyond this point, the star begins to eject matter from its surface.[2]
A star used for calibration of high-powered telescopes.
coudé spectrograph
This is a spectrograph placed at the Coudé focus of a reflecting telescope. The focus remains stationary as the telescope is re-oriented, which is advantageous for the stable mounting of heavy spectroscopic instruments.[3]
A diagram of the relationships between the Earth's axis of rotation, its celestial equator, and the plane of its orbit around the Sun, known as the ecliptic. Note that the Earth's rotational axis is not perpendicular to the ecliptic but rather is tilted; 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's sky at which a hypothetical indefinite extension of the Earth's axis of rotation "intersects" the celestial sphere, i.e. the two points in the sky that are directly overhead the terrestrial North and South Poles, around which all fixed stars appear to revolve during the course of a day. The celestial poles form the north and south poles of the equatorial coordinate system.
An imaginary sphere that encompasses the Earth's entire sky and is stationary with respect to the background stars. It is the basis for spherical astronomy.
A parameter indicating the magnetic activity in a star's chromosphere. One measure of this activity is log R′HK, where R′HK is the ratio of the equivalent width of a star's singly ionized calcium H and K lines, after correction for photospheric light, to the bolometricflux.[4] Schröder et al. (2009) divide solar-type stars into four groups depending on their activity index: very active (log R′HK above −4.2), active (−4.2 to −4.75), inactive (−4.75 to −5.1), and very inactive (below −5.1).[5]
A numeric value that is used to compare the brightness of a star measured from different frequency bands of the electromagnetic 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.
A relatively small, icy body that displays extended features when it approaches the Sun. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun.
A property of two objects orbiting the same body whose orbital periods are in a rationalproportion. For example, the orbital period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.
common proper motion
A term used to indicate that two or more stars share the same motion through space, within the margin of observational error. That is, either they have nearly the same proper motion and radial velocity parameters, which may suggest that they are gravitationally bound or share a common origin,[6] 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 two astronomical objects or spacecraft have either the same right ascension or the same ecliptic 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 the celestial sphere surrounding a specific and identifiable grouping of stars. The names of constellations are assigned by tradition and often have an associated folklore based in mythology, while the modern demarcation of their borders was established by the International Astronomical Union in 1930. Compare asterism.
An aura of plasma that surrounds cooler stars such as the Sun. It can be observed during a solar 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.
Dust which exists in outer space or has fallen on Earth, generally composed of fine particles of solid matter far smaller than those found in terrestrial dust.
A type of radiation consisting of high-energy protons and atomic nuclei which move through space at nearly the speed of light, and which may originate from the Sun or from outside the Solar System. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
The apparent movement of an astronomical object (e.g. the Sun, the Moon, a planet, a star, a constellation, etc.) across the observer's local meridian. During each day, the Earth's rotation causes every astronomical object to appear to move along a circular path on the celestial sphere, creating two points at which it crosses the meridian: an upper culmination, at which the object reaches its highest point above the horizon, and a lower culmination, at which it reaches its lowest point, nearly 12 hours later. When not otherwise qualified, the time of culmination typically refers to the time at which the upper culmination occurs.[8]
A ring-shaped circumstellar disc of dust and debris orbiting its host star. It is created by collisions between planetesimals. A debris disk can be discerned from an infrared excess being emitted from the star system, as the orbiting debris re-radiates the star's energy into space as heat.
Circumstellar debris disks as detected by the Hubble Space Telescope, and artist's impressions of the disks' orientations around their host stars
The orbital node at which an orbiting object moves south through the plane of reference (in geocentric and heliocentric orbits) or at which the orbiting object moves toward the observer (in orbits outside of the Solar System). Contrast ascending node.
The apparent motion of an astronomical object (e.g. the Sun, a planet, or a distant star) around the two celestial poles in the Earth's night 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 the diurnal circle.
Any pair of stars 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 a binary star system.
A hotter and more massive star, in contrast to late-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.
The plane of the ecliptic (grey) is defined by the Earth's orbit around the Sun, and is distinct from the plane of the celestial sphere's equator (green), which is permanently tilted 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 two equinoxes, the Sun's path appears to intersect the celestial equator, though the two planes are never in fact coplanar.
A type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless appearance. They are one of three main morphological classes of galaxy, along with spiral and lenticular galaxies.
A type of Kepler orbit with an orbital eccentricity of less than 1 (often inclusive of circular orbits, which have eccentricity equal to 0), or one with negative energy. Elliptical orbits take the shape of an ellipse, 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 an elliptical orbit, with the larger body located at one of the focal points of the ellipse
A list or table of the expected positions of astronomical objects or artificial satellites in the sky at various dates and times. Modern ephemerides are often provided by computer software.
A moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or orbital elements of an astronomical object, because such quantities are subject to perturbations 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 a planet that represents the intersection of the spheroid's surface with a plane perpendicular to its axis of rotation and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the celestial equator.
Either of the two precise times of year when the imaginary plane of the Earth's equator, extended indefinitely in all directions, passes through the center of the Sun (i.e. the two points at which this plane intersects the plane of the ecliptic); or, equivalently, when the Sun's apparent geocentric longitude is either 0 degrees or 180 degrees.[10] The two equinoxes, known as the vernal equinox and the autumnal equinox, occur on or near March 20 and September 22 each year. On the day of an equinox, the center of the visible Sun appears to be directly above the equator, and the durations of day and night are approximately equal all over the planet. Compare solstice.
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's kinetic energy and gravitational 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's center of mass. An object which has achieved escape velocity is neither on the surface nor in a closed orbit of any radius.
A curve on the Hertzsprung–Russell diagram that a solitary star of a particular mass and composition is expected to follow during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during part or all of its lifetime.[11]
Any astronomical object that exists outside the Solar System. The term is generally not applied to stars or any objects larger than a star or the Solar System itself, such as galaxies.
A bright spot on a star's photosphere formed by concentrations of magnetic field lines. For the Sun in particular, faculae (see solar facula) are most readily observed near the solar limb. An increase in faculae as a result of a stellar cycle increases the star's total irradiance.
Any galaxy that does not belong to a larger cluster of galaxies and is gravitationally isolated.
field star
A randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a star cluster. These field stars are important to identify in order to prevent them from contaminating the results of a study.[12]
A term used to classify the brightest stars in the night sky, with apparent magnitudes lower (i.e. brighter) than 1.50. There are 22 stars that are classified as first magnitude stars.
The "background" of astronomical objects in the night 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 the Solar System which do. The fixed stars are typically taken to include all stars other than the Sun, as well as all other extrasolar and deep-sky objects.
The branch of astronomy that studies objects and phenomena within the Milky Waygalaxy, as opposed to everything outside of the Milky Way, which is the domain of extragalactic astronomy.
The region at the center of a galaxy, usually home to a very dense concentration of stars and gas. It almost always includes a supermassive black hole which, when active, can generate a much higher luminosity in a compact region than its surroundings. This excess luminosity is known as an active galactic nucleus, and the brightest such active galaxies are known as quasars.
A large-scale structure consisting of hundreds or thousands of galaxies bound together by gravity. Galaxy clusters are distinct from similarly named galactic clusters and other types of star clusters and from smaller aggregates of galaxies known as galaxy groups. Galaxy groups and galaxy clusters can themselves cluster together to form superclusters.
A gravitationally bound aggregation of up to 50 galaxies, each at least as luminous as the Milky Way Galaxy. Larger aggregations may be called galaxy clusters, and galaxy groups and clusters can themselves cluster together to form superclusters.
A cataclysmic event that generates a brief but intense outburst of gamma ray radiation which can be detected from billions of light-years away. The source of most GRBs is theorized to be supernova or hypernova explosions of high-mass stars. Short GRBs may also result from the collision of neutron stars.
With reference to, or pertaining to, the geometric center of the Earth;[14] centered upon the Earth, e.g. a geocentric orbit.
geocentric zenith
The point projected upon the celestial sphere by a straight line that passes through the geocenter and an observer; i.e. the observer's zenith as defined with respect to the center of the Earth.[14]
The ratio of the brightness of an astronomical body at a phase 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 the center 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 for light-time, aberration, etc.[14]
A circulargeosynchronous orbit, which maintains a constant altitude of 35,786 kilometres (22,236 mi) directly above Earth's equator in the same direction as Earth'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 of stars which are gravitationally bound to each other and which orbit a galactic core as a satellite. They differ from open 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 a galactic cluster, which can bend passing light from a distant source by a noticeable degree. The effect, known as gravitational lensing, can make background objects appear to an observer to take on a ring or arc shape.
A luminous red galaxy (LRG) acting as a gravitational lens, distorting the light from a much more distant blue galaxy into an Einstein ring
An ionized nebula powered by young, massive O-type stars. Ultravioletphotons from these hot stars ionize gas in the surrounding environment, and the nebular gas shines brightly in spectral 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 of spiral galaxies and in star-forming irregular galaxies.
heliocenter
The precise geometric center of the Earth's Sun, i.e. the arithmetic mean position of all points within the approximate spheroid that is the shape of the Sun.
The vast, bubble-like cavity in the interstellar medium which surrounds and is created by the plasma emanating from the Earth's Sun. The heliosphere encompasses the entirety of the Solar 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 of luminosity versus effective temperature for a population of stars; depending on the usage, the star's absolute magnitude may be substituted for luminosity, and its color index or spectral type for temperature. Single stars of known mass and composition follow predictable tracks across this chart over the course of their evolution. Hence, knowing a star's mass and metallicity allows its age to be estimated. Stars of similar types are also found grouped together in specific regions of the chart, including main-sequence, red giant, and white dwarf stars.
The approximate region around an astronomical object within which its gravitational attraction dominates the motions of satellites. It is computed with respect to the next most gravitationally attractive object, such as the nearest star or the galactic core. Satellites moving outside this radius tend to be perturbed away from the main body.[15]
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, the plane perpendicular to a line from an observer to the zenith that passes through the point of observation.[14]
For a given celestial object, the angular distance on the celestial sphere measured westward along the celestial equator from the observer's local meridian to the hour circle that passes through the celestial object;[14] or, equivalently, the angle between the plane containing Earth's rotational axis and the zenith, and the plane containing Earth's rotational axis and the object of interest. Analogous to right ascension, the hour angle is one of many ways commonly used to specify the longitudinal position of an object upon the celestial sphere.
Any imaginary great circle drawn upon the celestial sphere that passes through both of the celestial poles and is therefore perpendicular to the celestial equator.[14] Similar to a meridian but additionally taking into account the terrain and the depth to the geocenter 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.
A critical mass below which an astronomical object cannot sustain its surface luminosity through nuclear fusion. This mass limit, equal to about 7% of the mass of the Sun, forms the dividing line between brown dwarfs and hydrogen-fusing stars.[17]
An archaic term that is sometimes used to refer to the planets Mercury and Venus. The name originated from the fact that these planets orbit closer to the Sun than the Earth and hence, in the geocentriccosmology of Ptolemy, both appear to travel with the Sun across the sky. This is in contrast to the so-called superior planets, such as Mars, which appear to move independently of the Sun.
The matter that exists in the space between the stars in a galaxy. This medium mainly consists of hydrogen and helium, 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 as extinction. 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 of redshift.
Also Laplace's invariable plane or the Laplace plane.
The imaginary plane passing through the barycenter of a planetary system and perpendicular to its angular momentum vector, and which may be regarded as the weighted average of all planetary orbital and rotational planes comprising the system.
A curve on the Hertzsprung–Russell diagram that represents the evolutionary positions of stars having the same age but differing masses. This is in contrast to an evolutionary 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 an interstellar 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 86,400 SI 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 the tropical year. It is therefore widely used as the basis for defining the standard astronomical epoch and the light-year.
The motion of one orbiting body relative to another, as an ellipse, parabola, or hyperbola, which forms a two-dimensional orbital 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 orbital perturbations that may exist in reality.
Any of a set of points near two large bodies in orbit 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 the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in certain scenarios) the Coriolis 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 labels L1 to L5. The phenomenon is the basis for the stable orbits of trojan satellites and is commonly exploited by man-made satellites.
A slight oscillating motion of the Moon as seen from the Earth, a result of the Moon's elliptical orbit. It can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.
A unit of length used to express astronomical distances that is equivalent to the distance that an object moving at the speed of light in vacuum would travel in one Julian 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 measure galactic-scale distances in non-specialist publications, the unit of length most commonly used in professional astrometry is the parsec.
An optical effect seen in stars (including the Sun), where the center part of the disk appears brighter than the edge or limb of the image.
line of apsides
The imaginary line connecting the two apsides (the periapsis and the apoapsis) of an elliptical orbit, and which therefore represents the distance of the orbit's longest axis.
The angle between a specified reference direction, called the origin of longitude, and the direction of an orbit's ascending node, as measured on a specified plane 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 canonical orbital elements used to characterize an orbit.
The shape of the portion of the Moon 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 a synodic month: as the orbital positions of the Moon around Earth and Earth around the Sun change, the visibility of the side of the Moon that constantly faces Earth alternates between completely illuminated (known as a full moon) and completely darkened by the Moon's own shadow (known as a new 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 be waxing; when the illuminated portion is becoming smaller, it is said to be waning. The phase of the Moon at any particular time appears the same from every point on Earth.
The phases 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 numerical logarithmic scale indicating the brightness of an astronomical 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 the naked eye, although this can vary depending on sky conditions and eyesight.
The precise time of year on Earth when the Sun appears to cross the celestial equator, while generally trending northward at each zenith passage. It represents the moment at which the North Pole of the Earth begins to tilt toward the Sun, and typically occurs on or near March 20 each year. It is the vernal equinox in the Northern Hemisphere and the autumnal equinox in the Southern Hemisphere. Contrast September equinox.
The fraction of an elliptical orbit's period that has elapsed since the orbiting body passed periapsis, expressed as the angular distance from the pericenter which a fictitious body would have if it moved in a perfectly circular orbit in the same orbital period as the actual body in its elliptical orbit. Unlike the true 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 the two-body problem mathematically convenient.
A line running north–south across the sky and passing through the point directly overhead known as the zenith.
meridian astronomy
The measurement of positions of celestial objects based on observation of the times of their transit across the meridian and of their zenith distance at those times, with the intention of obtaining accurate star positions which are self-consistent over large areas of sky.[19]
One of a set of 110 "nebulous" astronomical objects, 103 of which were catalogued as non-comets by French comet hunter Charles Messier between 1771 and 1781. The Messier catalogue includes most of the deep-sky objects easily visible from the Northern Hemisphere.
The visible passage of a glowing meteoroid, micrometeoroid, comet, or asteroid through the Earth's atmosphere, usually as a long streak of light produced when such an object is heated to incandescence 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 a meteor that originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.
A series of meteors that seemingly radiate from a single area in the night sky. These are produced by debris left over from a larger body, such as a comet, and hence they follow roughly the same orbit. This makes many meteor showers predictable events, as they recur every year.
A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.
A very small meteorite that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50 μm to 2 mm. Micrometeorites are a major component of cosmic dust.
A very small meteoroid, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed a micrometeorite.
microvariable
A stellar object such as a variable star that undergoes very small variations in luminosity, in which the amplitude of the fluctuations amounts to just a few thousandths of a magnitude. Detecting microvariability typically requires a sufficient number of observations to rule out random error as a source.[20]
1. The barred spiralgalaxy that includes the Earth's Solar System. The name describes the galaxy's appearance from the Earth: a hazy band of light visible in the night sky, formed from billions of stars that cannot be individually distinguished by the naked eye. The Milky Way Galaxy has a diameter of 100,000–200,000 light-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 the Galactic Center, which the Sun 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 direct orbit around the Sun that is neither a dominant planet nor originally classified as a comet. A moon is not a minor planet because it orbits another body instead of the Sun.
The solid, rocky body that orbits the Earth as its only natural satellite, completing a full orbit every 27.3 days. The Moon's gravitational influence is responsible for tides on Earth; because of tidal 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 the night sky, though its orbital position with respect to the Earth and the Sun causes its visibility to change in a regular cycle of phases when viewed from the Earth. The adjectival lunar is often used specifically to describe the orbit, gravity, and other properties of the Earth's Moon.
A loose grouping of stars which travel together through space. Although the members were formed together in the same molecular cloud, they have since moved too far apart to be gravitationally bound as a cluster.
A type of astronomy based on the acquisition of information about astronomical objects through the coordinated observation and interpretation of four disparate classes of "messenger" signals with extrasolar origins: electromagnetic radiation, gravitational waves, neutrinos, and cosmic 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 for active galaxies that had the visual appearance of a galaxy with a particularly bright, star-like nucleus. As a group, they are intermediate between Seyfert galaxies and Quasar. Most are giant ellipticals that are radio sources and display prominent emission lines.[24]
The point on the celestial sphere exactly opposite from the zenith. Thus, where the zenith is directly above an observer, the nadir is underfoot. The zenith and nadir form the two poles of the horizon 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 of compact star that is composed almost entirely of neutrons, which are a type of subatomic particle with no electrical charge. Typically, neutron stars have a mass between about 1.35 and 2.0 times the mass of the Sun, 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's sky at nighttime, when the Sun is below the horizon, and more specifically when clear weather and low levels of ambient light permit visibility of celestial objects such as stars, planets, and the Moon. The night sky remains a fundamental setting for both amateur and professional observational astronomy.
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.
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
O–C diagram
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.[25]
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 planetPluto 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.
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.
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.
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.[27]
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.
P
The parallax shift of a star at a distance of one parsec as seen from the Earth (not to scale)
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.
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.
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.
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.
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.[28]
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 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.
The scientific study of planets, moons, and planetary 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 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.
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.
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.
Also gravitational primary, primary body, or central body.
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.
The observed physical separation between two astronomical objects, as determined from their angular separation and estimated distance.[29] 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.
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.
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.
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.
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.
A moon or planet is said to be in quadrature 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 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.
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.
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 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.
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.[33]
S
Saber's beads
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.
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
Continuing, or changing in a non-periodic way, over a long period of time.[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 ellipticalKepler 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.
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.
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.
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.
The calculation of the passage of time based on the diurnal motion of the fixed stars in the Earth's sky.[14] 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.
A synodic day on Earth, i.e. the rotation period 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.
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.[14]
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).
A standard unit of mass equal to the mass of the Earth's Sun, or approximately 1.98847×1030kg. It is commonly used to express the masses of other stars and astronomical objects relative to the Sun.
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.
A standard unit of distance equal to the radius of the Earth's Sun (typically measured from the Sun's center to the layer in the photosphere at which the optical depth equals 2/3), or approximately 695,700 kilometres (432,300 mi). It is commonly used to express the radii of other stars and astronomical objects relative to the Sun.
The calculation of the passage of time based on the diurnal motion of the Sun in the Earth's sky.[14] The fundamental unit of solar time is the solar day, i.e. the time interval between two successive returns of the Sun to the local meridian, 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 from mean solar time. Solar time and sidereal time were employed by astronomers as time reckoning systems before the introduction of ephemeris time.
A stream of charged particles, primarily protons, electrons, and alpha particles, released from the Sun's corona and flowing outwards at up to 900 kilometres per second (2,000,000 mph) into interplanetary space.[1] Phenomena influenced by the solar wind include aurora, geomagnetic storms, and the plasma tails of comets.
Either of the two precise times of year when the Sun reaches its most northerly or most southerly point in the sky as seen from Earth; or, equivalently, when the Sun's apparent geocentric longitude is either 90 degrees or 270 degrees. The solstices occur on or near June 20 and December 21 each year. The June solstice, called the summer 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 the winter solstice.
A type of binary star system where the individual components have not been resolved with a telescope. Instead, the evidence for the binarity comes from shifts observed in the spectrum. This is caused by the Doppler effect as the radial velocity of the components change over the course of each orbit.
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.
A branch of observational astronomy which is used to locate the positions of astronomical objects on the celestial sphere as they would appear from a particular date, time, and location on Earth. It relies on the mathematical methods of spherical geometry and the measurements of astrometry.
The nominal gravitational acceleration of an object in a vacuum near the surface of the Earth, as a result of Earth's gravity and, less importantly, the centrifugal force generated by its rotation. It is by definition equal to 9.80665 m/s2 (approximately 32.17405 ft/s2).
A massive, luminous spheroid of plasma held together by its own gravity which, for at least a portion of its life, radiates energy into outer space due to the thermonuclear fusion of hydrogen into helium within its core. Astronomers can determine the mass, age, platemperature, chemical composition, and many other properties of a star by observing its motion through space, its luminosity, and its emission spectrum.
Any small number of stars that orbit each other, bound by gravitational attraction, such as a binary star system. In the broadest sense, very large groups of stars bound by gravitation such as star clusters and galaxies are also star systems. Star systems are distinct from planetary systems, which include planets and other bodies such as comets.
Any galaxy 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 a merger.
starfield
Any set of stars visible in an arbitrarily sized field of view of a telescope, usually in the context of some region of interest within the celestial sphere.[35][36] For example, the starfield surrounding the stars Betelgeuse and Rigel could be defined as encompassing some or all of the Orion constellation.
The outermost region of a star, 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 the stellar atmosphere; or another name for the stellar atmosphere itself.
One of a class of very large black holes which possess masses ranging from hundreds of thousands to many billions of times the mass of the Sun. These are typically found at a galactic core, where they can have a profound effect upon the evolution of the surrounding galaxy.
The time it takes for an object to rotate once about its own axis (i.e. its rotation period) relative to the primary it is orbiting (rather than to the much more distant fixed stars). The synodic day may be described as the time between two consecutive sunrises (in the case where the primary is a star), which is not necessarily the same as the sidereal day. As it does on Earth, an object's synodic day may change slightly in duration over the course of the orbital period due to eccentricity and axial tilt; Earth's synodic day is often called a solar day.
The time it takes for a body visible from another body (often the Earth) to complete a cycle with respect to the background stars visible in the second body's celestial sphere. Synodic period is most commonly used to indicate the elapsed time between a given body's consecutive appearances in the same location in the night 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 the orbital 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 (the Sun).
The calculation of the passage of time based on successive conjunctions of an astronomical object, such as a planet (i.e. successive returns of the object to the same aspect in the Earth's sky).[14]
The straight-line configuration of three celestial bodies in a gravitational system.
T
tangential velocity
The component of the velocity of a star or other astronomical body that is perpendicular to the line of sight of the observer (i.e. in the tangent plane). This component can be computed from the body's observed proper motion and its measured distance from the observer.[19]
A star with nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapor in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to more accurately measure the spectrum.[38]
The boundary within the heliosphere, approximately 75 to 90 AU from the Sun, beyond which the solar wind slows to subsonic speeds (relative to the Sun) as a result of interactions with the local interstellar medium.
The line that divides the illuminated side of a moon or planet from its dark side. The line moves as the object rotates with respect to its parent star.
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 the Milky 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 the galactic plane. Stars belonging to this population generally follow orbits that lie close to this plane.[39] This is in contrast to members of the thick disk population and halo stars.
The transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will gradually recede from its primary while slowing the rotation rate of both bodies.
The net result of continued tidal braking such that, over the course of an orbit, there is no net transfer of angular momentum between an astronomical body and its gravitational partner. When the orbital eccentricity is low, the result is that the satellite orbits with the same face always pointed toward its primary.[40] An example is the Moon, which is tidally locked with the Earth.
tidal stream
A stream of stars and gases which are stripped from gas clouds and star clusters because of interaction with the gravitational field of a galaxy such as the Milky Way.[41]
tilt erosion
The gradual reduction of the obliquity of an orbiting satellite due to tidal interactions.[42]
A measure of the orbital motion of a relatively small body (e.g. an asteroid or comet) with respect to a larger, perturbing body (e.g. a planet), used for restricted three-body problems in which the three bodies all differ greatly in mass. The parameter is calculated from the orbital elements of each body, including the small body's semimajor axis, eccentricity, and inclination, 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.[19]
topocentric
With reference to, or pertaining to, a point on the surface of the Earth.[14]
2. An astronomical event during which a celestial body or object passes visibly across the face of a much larger body. An example is the transit of Venus across the face of the Sun, which was visible from Earth in 2004 and 2012. Because a transit results in a decrease in the net luminosity from the two objects, the transit method can be used to detect extrasolar 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 an occultation or eclipse.
The angle between the direction of periapsis and the current position of an orbiting body as it moves along an elliptical orbit, as measured from the nearest focus of the ellipse. The true anomaly is one of three angular parameters that define a position along an orbital path, the other two being the eccentric anomaly and the mean anomaly, and also one of six canonical orbital 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, including astronomical, civil, and nautical twilight.[14]
1. The entirety of space and time and their contents, including galaxies, stars, planets, all other forms of matter and energy, and the physical laws and constants that describe them. When not otherwise qualified, "the Universe" usually refers to the entire Universe, whose spatial extent is unknown because it is not directly measurable; this is distinguished from the observable universe, whose size it is possible to measure.
2. One of many hypothetical parallel universes which exist as causally disconnected constituent parts of a larger multiverse, which itself comprises all of space and time and their contents.
Any star 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's radial velocity or its profile of spectral lines.
A reference to the faintness of the spectral lines for a star compared to standard stars with the same stellar 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.[43]
A correlation between the width of the singly ionized calcium K-line (Ca II K) at 3933 Å and the absolute visual magnitude of the emitting late-type stars. This linear relation makes it useful for determining the distances of G, K, and M-type stars.[44]
X
XBONG
An acronym of X-ray bright optically normal galaxy.
The area of the sky that extends approximately 8 degrees north or south (in celestial latitude) of the ecliptic, the apparent path of the Sun across the celestial sphere over the course of the year as observed from Earth. The Sun, Moon, and visible planets appear to travel across a band of twelve Zodiac constellations within this belt as the Earth orbits the Sun.
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Meat of domestic sheep Mutton redirects here. For goat meat, which is what 'mutton' primarily refers to in South Asia and the Caribbean, see goat meat. For other uses, see Mutton (disambiguation). Lamb Mutton Lamb, hogget, and mutton, generically sheep meat,[1] are the meat of domestic sheep, Ovis aries. A sheep in its first year is a lamb and its meat is also lamb. The meat from sheep in their second year is hogget. Older sheep meat is mutton. Generally, hogget and sheep meat are not...
لمعانٍ أخرى، طالع أدوات (توضيح). أدوات توجيه المروحية (بالإنجليزية: Helicopter flight controls) هي أدوات تستخدم لمساعدة الطيار بتحريك المروحية للحصول على سيطرة أفضل بإيروديناميكية الطيران. فعند أي تغيير بأدوات التحكم فإنها ترسل إشعارا ميكانيكيا للدوار مع تأثير ديناميكا الهواء
This article is part of the history of rail transport by country series Interactive map of the Bolivian rail network (2014) The history of rail transport in Bolivia began in the 1870s after almost three decades of failed efforts to build railroads to integrate the country, mining was the driving force for the construction of railways. The need to transport saltpeter to the coast triggered the first railway lines in Bolivia. It was the silver mining, however, that drove the construction of a r...
Artikel ini sebatang kara, artinya tidak ada artikel lain yang memiliki pranala balik ke halaman ini.Bantulah menambah pranala ke artikel ini dari artikel yang berhubungan atau coba peralatan pencari pranala.Tag ini diberikan pada Januari 2023. Masayo TakahashiLahir23 Juni 1961 (umur 62)Osaka, JepangKebangsaanJepangAlmamaterKyoto UniversityDikenal atasPenelitian mengenai aplikasi klinis dari Sel IPSsel punca pluripotenKarier ilmiahBidangSel punca OftalmologiInstitusiRiken InstituteKyoto ...
Italian businessman (1963–2011) Pietro Ferrero Jr.Born11 September 1963Farigliano, ItalyDied18 April 2011 (aged 47)Cape Town, South AfricaOccupationBusinessmanKnown forCEO of Ferrero SpA (1997–2011)ParentMichele Ferrero (father)RelativesPietro Ferrero (grandfather) Giovanni Ferrero (brother) Pietro Ferrero Jr. (Italian pronunciation: [ˈpjɛːtro ferˈrɛːro]; 11 September 1963 – 18 April 2011)[1][2] was the joint CEO (with his brother Giovanni) of Italian ...
The Comcast and Time Warner Cable logos On February 13, 2014, Comcast Corporation announced its intent to acquire Time Warner Cable. The deal was proposed to take the form of a stock swap, estimated at the time of announcement to be worth about $45.2 billion.[1] The two companies argued that the merger would increase their overall scale, allowing the company to become more competitive, improve customer service quality, and quicken innovation. The companies also argued that the deal wo...
30°3′N 31°13′E / 30.050°N 31.217°E / 30.050; 31.217 مصر السلطنة المصرية محمية 1914 – 1922 السلطنة المصريةعلم السلطنة المصريةشعار النشيد : ســـــلام أفندينــــا الأخضر: سلطنة مصرأخضر فاتح: السودان الإنجليزي المصري (سيادة مشتركة) أخضر خفيف: مناطق تنازل السودان عنها إلي شمال ...
Gedung Kebangkitan NasionalNama sebagaimana tercantum dalamSistem Registrasi Nasional Cagar BudayaMuseum Kebangkitan Nasional (2013). Cagar budaya IndonesiaPeringkatNasionalKategoriBangunanNo. RegnasCB.7LokasikeberadaanJakarta Pusat, DKI JakartaTanggal SK1983 & 2013Pemilik IndonesiaPengelolaMuseum Kebangkitan Nasional Museum Kebangkitan Nasional adalah sebuah gedung yang dibangun sebagai monumen tempat lahir dan berkembangnya kesadaran nasional dan juga ditemukannya organisasi perger...
Political coalition in Taiwan (Republic of China) For the political party practicing Green politics in Taiwan, see Green Party Taiwan. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.Find sources: Pan-Green Coalition – news · newspapers · books · scholar · JSTOR (December 2017) (Learn how and when to remove this template ...
Mosney Accommodation CentreMosney Accommodation CentreLocationMosneyCoordinates53°39′40″N 6°14′35″W / 53.66111°N 6.24306°W / 53.66111; -6.24306Subsequent namesButlin's Mosney (1948-1982)Mosney Holiday Centre (1982-2000) Mosney Accommodation Centre (2000 onwards) [Run under contract on behalf of the International Protection Accommodation Services within the Department of Children, Equality, Disability, Integration and Youth]Operated byMosney plcEstablished17...
Chicago City LeagueClassificationIndependentSportMinor League BaseballInaugural season1887Ceased1910PresidentUnknownNo. of teams8Country United StatesMost titles2 Leland Giants (1909, 1910) The Chicago City League was a minor league baseball league based in Chicago, Illinois. The Independent level league played with teams based entirely in Chicago. The league played in 1887, from 1890 to 1894 and 1909 to 1910 as a minor league. In the era of segregated baseball, the 1909 and 1910 leagues...
This article has multiple issues. Please help improve it or discuss these issues on the talk page. (Learn how and when to remove these template messages) This article relies excessively on references to primary sources. Please improve this article by adding secondary or tertiary sources. Find sources: The Adventures of Dr. McNinja – news · newspapers · books · scholar · JSTOR (February 2013) (Learn how and when to remove this template message) This art...
The rotunda of the Country Music Hall of Fame and Museum in Nashville, Tennessee. This is a list of the 152 inductees to the Country Music Hall of Fame and Museum, as of 2023[update], counting groups as a single inductee. Of these, 16 inductions are solo female performers, and 1 induction is a female duet. To date, Roy Rogers is the only artist to have been inducted twice: first in 1980 as a member of the Sons of the Pioneers and again in 1988 as a solo artist. The induction of Harold...
Il padrinoDon Vito Corleone riceve Amerigo BonaseraTitolo originaleThe Godfather Lingua originaleinglese, italiano Paese di produzioneStati Uniti d'America Anno1972 Durata175 min (versione cinematografica)179 min (versione restaurata) Rapporto1,85:1 Generenoir, drammatico, gangster RegiaFrancis Ford Coppola Soggettodal romanzo Il padrino di Mario Puzo SceneggiaturaMario Puzo, Francis Ford Coppola ProduttoreAlbert S. Ruddy Casa di produzioneParamount Pictures Distribuzione in italianoC...
Album by George Jones High-Tech RedneckStudio album by George JonesReleasedNovember 23, 1993 (1993-11-23)StudioMasterfonics and The Music Mill Nashville, TNGenreCountryLength30:46LabelMCAProducerBuddy CannonNorro WilsonGeorge Jones chronology Walls Can Fall(1992) High-Tech Redneck(1993) The Bradley Barn Sessions(1994) High-Tech Redneck is an album by American country music singer George Jones. It was released in 1993 on the MCA Nashville Records label and went Gold in 1994....
Nursing school in Philippines Institute of NursingMottoHominem Non Morbum Cura (Treat the Man, Not the Disease)EstablishedJune 1955[1]DeanMa. Belinda Buenafe, RN, MAN, PhDLocationManila, Metro Manila, PhilippinesCampusFar Eastern University - ManilaColors PeachWebsitewww.feu.edu.ph/index.php/institutes/institute-of-nursing/FEU-IN 50th Pinning Ceremony The Far Eastern University – Institute of Nursing, or simply IN, is an academic institute for nurse education of the Far Easter...
This biography of a living person needs additional citations for verification. Please help by adding reliable sources. Contentious material about living persons that is unsourced or poorly sourced must be removed immediately from the article and its talk page, especially if potentially libelous.Find sources: David Brant – news · newspapers · books · scholar · JSTOR (December 2014) (Learn how and when to remove this template message) Brant as Director o...
Neighbourhood in Skåne County, Skåne, SwedenFridhemNeighbourhoodCoordinates: 55°35′53″N 12°57′58″E / 55.59806°N 12.96611°E / 55.59806; 12.96611CountrySwedenProvinceSkåneCountySkåne CountyMunicipalityMalmö MunicipalityBorough of MalmöVästra InnerstadenPopulation (1 January 2011)[1] • Total1,815Time zoneUTC+1 (CET) • Summer (DST)UTC+2 (CEST) Fridhem is a neighbourhood of Malmö, situated in the Borough of Västra In...
Salah satu kebun anggur di kota Vaduz. Negara Kepangeranan Liechtenstein adalah penghasil anggur. Negara ini memiliki iklim yang ideal untuk budidaya anggur dengan lereng gunung yang menghadap ke barat daya, tanah berkapur dan rata-rata 1.500 jam sinar matahari setahun. Angin kering yang panas selama bulan-bulan musim panas, dikenal sebagai alat bantu foehn dengan memberikan efek kesejukan. Ada lebih dari 100 petani anggur di Liechtenstein yang menghasilkan anggur merah dan putih dimana walau...
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